摘要:

QUARTERLY REVIEWS SEMICONDUCTIVITY OF THE PHTHALOCYANINES By CHARLES J. HOFFMAN (RESEARCH LABORATORIES LOCKHEED MISSILES AND SPACE COMPANY PALO ALTO CALIFORNIA USA) 1. Scope WITHIN the last few years a number of Reviews1 and Symposia2 have been devoted to the discussion of the electrical conductivity of organic mole- cules. Of the organic semiconductors the phthalocyanines excite particular interest since they were some of the first organic materials found to display intrinsic semiconductor properties. The present Review is concerned with the phthalocyanines the phthalocyanine polymers and the corresponding thermal activation energies for conduction. 2. Phthalocyanines Several excellent Reviews on phthalocyanine chemistry are available. These discussions range from brief lucid and authoritative introductions3 to a number of more specialized works.4 The metal phthalocyanines are formed by a vigorous reaction initiated by the heating of a metal or metal salt with aromatic ortho-dinitriles or related substances.Phthalocyanines are related to porphyrin the funda- mental nucleus of chlorophyll and of hamoglobin. The phthalocyanine nucleus an azaporphyrin may be derived from the parent porphyrin nucleus by replacement of the methine groups by nitrogen atoms. The two hydrogen atoms attached to the central nitrogen atoms of the phthalo- (a) Eley Research 1959 12 293; (b) Garrett “Semiconductors,” ed. Hannay Reinhold Publ. Corp. New York 1959; (c) Inokuchi and Akamatu “Solid State Physics,” ed. Seitz and Turnbull Academic Press New York 1961 Vol. XII; ( d ) Boldebuck “Organic Semiconductors; Literature Survey,” Report No.56-RL-1490. General Electric Co. U.S.A. 1956; (e) Brown and Aftergut “Investigation of Organic Semiconductors,” Report No. 59-469 General Electric Co. U.S.A. 1959. a “Proceedings of the Princeton University Conference on Semiconduction in Mole- cular Solids,” ed. Pohl Ivy-Curtis Press Philadelphia 1960; “Symposium on Electrical Conductivity in Organic Solids,” eds. Kallmann and Silver Interscience Publ. Inc. New York 1961 ; ‘‘Organic Semiconductors,” eds. Brophy and Buttrey Macmillan New York 1962. (a) Dahler Ind. Eng. Chem. 1939 31 839; (b) Linstead “Thorpe’s Dictionary of Applied Chemistry,” 4th edn. Longmans Green and Co. London 1949 Vol. IX pp. 61 7-620; (c) Johnson “Encyclopedia of Chemical Technology,” eds. Kirk and Othmer Interscience New York 1953 Vol.X pp. 607-611. (a) Venkataraman “The Chemistry of Synthetic Dyes,” Academic Press New York 1952 Vol. 11 pp. 11 18-1 142 ; (b) Robertson “Organic Crystals and Molecules,” Cornell University Press Ithaca New York 1953; (c) Lubs “The Chemistry of Synthetic Dyes and Pigments ”Reinhold Publ. Corp. New York 1955 pp. 577-624; (d) Gerstner “Die Chemie der Applikation von Komplexfarbstoffen,” Akademie-Verlag Berlin 1959 pp. 187-195. 1 113 114 QUARTERLY REVIEWS cyanine molecule are replaceable by various metals. The phthalocyanine molecule is rigidly coplanar and tetradentate with the four co-ordinating nitrogen atoms occupying the corners of a square. All the carbon-nitrogen bonds within the large 16-membered central ring are to within a few hundredths of an Angstrom approximately equal.In metal-free phthalo- cyanine the carbon-nitrogen distance is 1-34 A and in nickel phthalo- cyanine the distance is 1-38 A4b. Detailed studies of the geometrical configurations of the phthalocyanines can be found el~ewhere.~~ 9 5 . -C-F- y /c=c- - f-c$+% N I - c-c fj 5-c- HY=& /C=FH -“( $-CH -C C-N (1) -c-c H (2) -c-c- H$-kN ,&Ui 11 ‘N-ce-N/ I -c-c !J t-c- -c-s‘ JJ‘ bc- -&c- HC-C y=h+ (3) FIG. 1. Porphyrin ring structures; (I) chlorophyll nucleus; (11) phthalocyanine; (111) hmnoglobin nucleus. The existence of three polymorphic forms of phthalocyanine a 18 and y is supported by X-rays and infrared’ studies. Of the three forms the /3 is the most stable. Heating the other forms to 300” converts them to the /%form. With few exceptions all electrical measurements of the phthalo- cyanines have been made on the /%crystal form.3. Phthalocyanine Polymers The phthalocyanine unit has been polymerised and except for a few cases the polymerisations were made in attempts to prepare dyes dielectric films or materials having a high thermal stability. Because of the difficulties encountered in the isolation purification and identification of the phthalo- cyanine polymers interest in their conducting properties has been some- what limited.8~9J0 Barrett Dent and Lindstead J. 1936 1719. (a) Susich Analyt. Chem. 1950 22 426; (b) Karasek and Decius J. Amer. Chem. (a) Ebert and Gottlieb J. Amer. Chem. SOC. 1952,74,2806; (6) Sidorov and Kotlyar Lawton and McRitchie “Research on Thermostable Molecules and Polymers,” Soc. 1952 74 4716.Optika i Spektroskopiya 1961 11 175. Report No. 57-642 Battelle Memorial Institute Columbus Ohio 1957. * Epstein and Wildi J . Chem. Phys. 1960,32 324. lo Felmayer and Wolf J. Electrochem. SOC. 1958,105 141. HOFFMAN SEMICONDUCTIVITY OF THE PHTHALOCYANINES 1 15 (a) Dimers and Trimers.-Co-condensation of 3,3’,4,4’-tetracyanodi- phenyl ether and phthalonitrile with copper-bronze results in the formation of a product in which polymerisation proceeds no further than the dimeric or trimeric stage.ll A dimer of copper phthalocyanine is obtained by the reaction of pyromellitic dianhydride with phthalic anhydride. The extent of polymerisation of this material was estimated from probability con- siderations based on the ratio of reactants. (b) Linear Polymers.-Linear polymeric phthalocyanines are prepared by reacting phthalic anhydride and a copper salt with either pyromellitic acid or 3,3’4,4’-tetracarboxydiphenyl ether in a flux of urea.12 In both reactions the linear polymers obtained are soluble in solvents in which copper phthalocyanine monomer and planar polymers of phthalocyanine exhibit little or no solubility.Some linear polymers have been estimated to contain 7-8 phthalocyanine units.12a FIG. 2. Expected repeating unit of a linear polymeric copper phthalocyanine from phthalonitrile 3,3’,4,4‘-tetracyanodiphenyl ether and copper (ref. 1 1). (c) Planar Polymers.-The majority of the phthalocyanine polymers are extensions of the monomeric unit in two dimensions. These polymers are insoluble in organic solvents but are soluble in concentrated sulphuric acid.Reaction of a metal or metal salt and urea with pyromellitic acid,1° pyromellitic dianhydride,13 or with pyromellitonitrile8 yields planar polymers consisting of an estimated 10 units 3-34 units and 5 units respectively. A completely cross-linked copper phthalocyanine polymer is obtained from the reaction of phthalic anhydride pyromellitic dianhydride urea and copper chloride. Using starting materials of higher molecular weights it is possible to synthesise other planar polymers of phthalo- cyanine. Planar-type polymeric copper phthalocyanines are prepared by heating 3,3’,4,4’-tetracyanobiphenyl or biphenyl-3,3’,4,4’-tetracarboxylic acid with urea and a source of copper in the presence of a ~ata1yst.l~ l1 Marvel and Martin J. Amer. Chem. SOC. 1958,80,6600. l2 (a) Rassweiler “Polymeric Phthalocyanines,” Diss.Abs. 1958 18,403 ; (b) Marvel l3 Drinkard “Studies on the Oxidative and Thermal Stability of Complex Inorganic l4 “High Temperature Dielectric Film,” Final Report for Contract DA 36-039-sc-87 and Rassweiler J. Amer. Chem. SOC. 1958 80 1197. Compounds,” Diss. A h . 1957 17,499. Sprague Electric Co. North Adams Mass. U.S.A. 1952. 116 QUARTERLY REVIEWS t FIG. 3. Expected repeating unit of a planar polymeric copper phthalocyanine from pyromellitonitrile urea and cuprous chloride (ref. 8). 4. Conductivity of the Phthalocyanines In 1948 Eley15 and Vartanyanl'j independently discovered that the phthalocyanines had a lower electrical resistivity than most organic substances. The resistivity p of a phthalocyanine is a function of the temperature and is expressed by the equation P = PoexP (EIkT) (1) Equation (1) is typical of semiconducting materials possessing a thermal activation energy E for conduction.Although eqn. (1) represents the experimental data a detailed interpretation of the equation is not simple. Electrical conductivity of molecular crystals greatly depends on both the molecular and the intermolecular barriers. Hence the thermal activation energy appears to be a composite of both these effects.8 In general the thermal activation energy can be related to the energy necessary for the production of charge carriers in the molecule. The number of carriers is proportional to exp (-42kT) provided conduction is related to the mobile electrons which have been promoted to a level of energy greater by the energy interval E than the lower bound energy level.The approxi- mate formula for the conductivity 0 can be written as or in terms of the resistivity By convention the energy interval E is used instead of the activation energy E because it is somewhat analogous to the energy gap of the familiar band theory model for inorganic semiconductors. Because of the relatively little knowledge available on the intermolecular interactions in molecular crystals it is difficult to formulate a suitable 0 = o,exp (-42kT) (2) P = poexp (EPkT) (3) Eley Nature 1948 162 819. l6 Vartanyan Zhur. fiz. Khim. 1948,22,769. HOFFMAN SEMICONDUCTIVITY OF THE PHTHALOCYANINES 1 17 mechanism for the electronic conduction in organic solids. Inokuchi and Akamatulc present a brief but valuable summary of the proposed theorjes for the mechanism of charge transfer between molecules and discuss the interpretation of the activation energy E in the empirical eqn.(1) accord- ing to the various concepts. Of these theories only that of Eley and his co-workers17 will be outlined in this Review. According to Eley the conductivity and the energy interval may be regarded as being associated with the mobile n-electrons of the conjugated ring structure of the phthalocyaninesl” since substitution of the two central hydrogen atoms by various metals has little effect on the electrical con- ductivity and the magnitude of the energy interval.ls The n-electrons form an “electron gas” which may be envisaged as a cloud stretching along the carbon-nitrogen-carbon ring both above and below the plane of the phthalocyanine molecule.In the normal state each of the lower energy levels will be filled by two electrons the highest occupied level being determined by the number of electrons that are available. All higher energy levels will be empty. For conduction to occur in the phthalocyanine molecule a n-electron must be thermally or optically excited from the highest filled energy level to the lowest unfilled energy le~e1.l~ If n is the number of n-electrons constituting the “gas” the normal state has 4 2 lower energy levels each containing two electrons. Conduction is associated with the transfer of electrons from the highest occupied level n/2 to the lowest unoccupied level n/2 + 1. These two levels are separated by E the energy interval. The magnitude of the energy interval has been estimated17 using a treatment similar to that proposed by Kuhn19 for computing the light absorption of organic dyes.It is possible using certain simplifications to consider the mobile n-electrons of a conjugated path as electrons in a one-dimensional box and using the Schrodinger equation to calculate the energy values for a particular n-electron. In molecules where the n- electrons form an open path the energy interval E is and for a closed path hZ n 4 m l2 E = - where h is Planck’s constant n the total number of n-electrons in the path considered m the mass of the electron and I the total path length. Since the average carbon-nitrogen distance in the phthalocyanines is 1-36 A the plot in Fig. 4 of eqns. (4) and (5) was made using 1.36n for 2 the path length. l7 (a) Eley Parfitt Perry and Taysum Truns. Furuduy Suc.1953 49 79; (6) Eley and Parfitt Truns. Furaduy Suc. 1955,51,1529. l 8 Fielding and Gutman J . Chem. Phys. 1957 26,411. l9 Khun J. Chem. Phys. 1949 17 1198. 118 QUARTERLY \ $0.81 REVIEWS X d C I 0 I I I I I 0 20 40 60 80 100 120 140 Number of mobile 77- electrons FIG. 4. The curves A and B are calculated by means of eqns. (4) and (5) respectively. References a Eley Research 1959 12,,?93 ; Lawton and McRitchie "Research on Thermostable Molecules and Polymers Report No. 57-642 Battelle Memorial Insti- tute Columbus Ohio 1957; Epstein and Wildi J. Chem. Phys. 190,32 324; dFel- mayer and Wolf J. Electrochem. SOC. 1958 105 141. Although eqns. (4) and ( 5 ) appear to be simple they must be used with utmost caution since n the number of mobile r-electrons is not necessarily the same as the total number of .rr-electrons in the m o l e c ~ l e ~ ~ ~ ~ ~ .To use the equations some guide to the values of n and I is required. A value of I for the phthalocyanine molecule has been obtained by Lonsdale20 from the anisotropy of the diamagnetic susceptibility and from this value it is possible to infer the magnitude of n. Lonsdale's method gives a value of 18.8 A for I which is the circumference of the large central closed ring with the number of mobile n-electrons n equal to 14. This discussion of the conduction mechanism for the phthalocyanines has been restricted to the single molecule with no reference to the inter- molecular conduction process. On the molecular level highly conjugated molecules such as phthalocyanines and aromatic hydrocarbons may be regarded as excellent conductors since there is no dissipation mechanism to impede the electron flow around the molecular ring.lb Apparently the actual limiting steps in the bulk conduction process are the transfer of electrons from crystal to crystal and the mobility of the electrons from one molecule to the next.Values of the energy interval E obtained for various phthalocyanines are listed in Table 1. The discrepancy of the E value for a given phthalo- cyanine can be attributed to the sample purity and the measurement technique. Eley has attributed the higher energy interval values of his earlier rnea~urements~~ to the intercrystalline resistance of the lightly packed phthalocyanine powders,la and his more recent measurements were made using compacted samples to minimise this f a ~ t 0 r .l ~ ~ Since most of the resistivity measurements of the phthalocyanines are made on com- pacted powders efforts are made to reduce the intercrystalline resistance by using compressed samples. The resistivity of compacted phthalocyanine powders does not change appreciably after pressures of 80 kg. cm.-2 are reached17" and it has been tacitly assumed that this limiting value is the true resistivity of the material.lU Experimental c values and the Eley equations (afer Eley). 2o Lonsdale Proc. Roy. SOC. 1937 A 159 149. HOFFMAN SEMICONDUCTIVITY OF THE PHTHALOCYANINES TABLE 1. Energy interval values E of monomer phthalocyanines E (ev) 2.39 0.87 0.90 2.39 1.1 1.44-1.87 1 *40-2.0 1 -49 1 -73 1.3 1.71 1.8 1-50-1.7 0.5 2.6 1.2 0.82 1.85 2.1 1.2 1.64 1.64 1 -79 1.86 1 - 6 0 1 *60 1.52 Sample form Temp.range Metal-free phthalocyanine lightly packed crystals film rubbed on quartz film rubbed on quartz lightly packed crystals compact 80 kg./cm.2 single cryst. ; subl. film single cryst. ; film compact 0.5 kg./cm.2 film subl. film single crystal compact 80 kg./cm.2 (summary of measurements) compact a-form 300-470" 1 0 - 1 0 0 1 0 - 1 0 0 300-500 300-500 25-1 60 25-420 150-350 50-390 200-350 60-170 Copper phthalocyanine lightly packed crystals 300-470 film rubbed on quartz 10-100 film rubbed on quartz 1 0 - 1 0 0 lightly packed crystals 300-500 subl. film film single crystal 50-390 single cryst.; subl. film 25-1 60 lightly packed crystals 25-340 Copper phthalocyanine chlorinated lightly packed crystals Nickel phthalocyanine single crystal 50-390 Cobalt phthalocyanine single crystal 50-390 Platinum phthalocyanine lightly packed crystals Atm.H2 H2; N2 H2; N2 He vac. 0,-free vac. N2 *2 N2 vac. some 0 H2 vac. some O2 H2; N vac. 02-free N2 N2 N2 N2 N2 N2 119 Ref. a b b C c d e f g h i k i i a b b d h g i 1 C I i i 1 120 QUARTERLY REVIEWS TABLE 1 .-roritinued Sample form Temp. Atm. Ref. range Magnesium p hthalocyanine 0.78 film rubbed on quartz 10-100 vac. b 0.47 film rubbed on quartz 10-100 someO b 1.2 film; subl. film g Zinc phthalocyanine 1.51 film 0,-free g a Eley Nature 1948 162 819; Hey Parfitt Perry and Taysum Trans. Faraday Soc. 1953 49 79; Kleitman “Electrical Properties of Phthalocyanines,” Report PB 11 1,419 U.S. Dept. of Commerce Office of Tech. Service Washington 1953; Kleitman and Goldsmith Phys. Rev. 1954,98,1544; Eley and Parfitt Trans.Faraday SOC. 1955 51 1529; 9 Vartanyan and Karpovich Doklady Akad. Nauk. S.S.S.R. 1956 111 561 ; Baba Chitoku and Nitta Hokkaid6 Daigaku 6y6 Denki Kenky2jo Iho 1956,8 127 (Chem. Abs. l957,51,9297f); a Fielding and Gutman J. Chem. Phys. 1957 26 411; Wihksne and Newkirk J. Chem. Phys. 1961 34 2184; Eley Research 1959 12 293; Felmayer and Wolf J . Efecrro- cheni. Soc. 1958 105 141. Vartanyan Zhur. fiz. Khim. 1948 22 769; Vartanyan has withdrawn the E values which he first reported,’* on the basis that the films contained oxygen impurities.21 His early work has been criticised because the measurements were made over a low temperature range jn which impurity conduction may have been an important factor.la Since the specimen was a thin film rubbed into a quartz surface the im- purity centres may have been dislocations.With one exception values for the energy intervals of the phthalocya- nines have been obtained by measuring the resistivities of the stable ,&crystal form. From a wide range of experimental methods using various specimens it is concluded that the value for the energy interval of the phthalocyanine p-form lies between 1.5 and 1.7 ev.la A much lower value 0.5 ev is reported for the energy interval of the a-modification of the metal-free phthalocyanine.22 Eley and Pa~-fittl’~ report that the conductivity of the a-form is approximately ten times greater than the conductivity of the /3-form whereas Wihksne and Newkirk22 report that the electrical conductivity of the a-form is more nearly 100,000 times greater than that of the p-form at the transition temperature.The conductivity of phthalocyanine polymers has been investigated in attempts to reduce the values of the energy interval characteristic of the single phthalocyanine unit. In Table 2 are given the values of E the energy interval for a few phthalocyanine polymers. Difficulties associated with the purification of the polymers as well as the lack of practical methods for determining polymer size have complicated the interpretation of these results. Because of these difficulties the estimates of the extent of polymer- a1 Vartanyan and Karpovich Doklady Akad. Nauk. S.S.S.R. 1956,111,561. 24 Wihksne and Newkirk J. Chem. Phys. 1961 34 2184. HOFFMAN SEMICONDUCTIVITY OF THE PHTHALOCYANINES 12 1 TABLE 2. Energy interval values for polymeric copper phthalocyanine compacts Extent of polymerisation E (ev) Sample form Atm.Ref. monomer 1.5-1 ‘7 average value vac. a dimer 0.7- 1 ‘0 below 7,000 kg./cm.2 b five units 0.12- 0.26 1,400- 14,OOO kg./cm.z vac. c ten units 0.81-0.95 lightly packed cryst. N2 d Lawton and McRitchie “Research on Thermo- stable Molecules and Polymers,” Report No. 57-642 Battelle Memorial Institute Columbus Ohio 1957; Felmayer and Wolf J . Electrochem. SOC. 1958 105 141. isation are at best approximations. Lawton and McRitchie9 estimated the size of the polymeric unit from probability considerations based on the ratio of the relative amounts of reactants. Felmayer and Wolflo used the results of the elemental analyses and theoretical calculations to compute the extent of polymerisation. Using the measured E value and the Eley e q u a t i ~ n l ~ ~ l ~ ~ Epstein and Wildis estimated the polymer to contain five phthalocyanine units having 140 7-electrons. a Eley Research 1959 12 293; Epstein and Wildi J. Chem. Phys. 1960,32 324;

ISSN:0009-2681    

DOI:10.1039/QR9641800113

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

CHEMICAL ACTIVATION By B. S . RABINOVITCH and M. C. FLOWERS* DEPARTMENT OF CHEMISTRY UNIVERSITY OF WASHINGTON SEATTLE 5 WASHINGTON U.S.A. 1. Introduction Nature of Chemical Activation.-A species which is energised by other than thermal collisional processes displays behaviour characteristic of an energy distribution which is non-equilibrium relative to the condition of statistical equilibrium for that species at the ambient temperature. If the species is activated in the course of its production by chemical reaction this process will be called “chemical activation”. The further reaction of such species and their energy degradation whether by radiation or collisional deactivation can provide information relevant to energy-transfer processes and reaction-rate theory. The actual fate of the excited molecules depends on the relative rates of all possible processes that may ensue; these are a function of the nature and energy content of the molecules.The ability to provide different known energy distributions of excited species represents one advantage of the chemical activation technique. An important aspect is the opportunity presented for study as in the case of exchange reactions of the activation reaction itself and of the energy distribution between products. Limitations of this Review.-This Review brings together known examples of chemical activation systems which have been accumulating increasingly in recent years in order to clarify their nature and to exemplify their usefulness. The major emphasis is on activated molecules produced in electronic ground states; these are the simplest systems.Brief mention of some reactions involving electronic excitation will be given however since these include important examples of chemical activation. Some systems such as atom recombination have been studied in great detail in recent years1 and cannot be considered here at length. Ion- molecule activation reactions are arbitrarily omitted ; aspects of these have been reviewed recently.2 Many chemical activation systems are not amenable to useful discussion either because of their complexity or because insufficient data are available. Only some of these need be mentioned. This Review is not concerned with experimental techniques but rather with the information that has been or may be obtained by the use of chemical activation. For the sake of brevity much of the exposition is given in terms of a restricted number of illustra- tive examples.Faraday Soc. 1962 33 for several recent studies and references. G. Porter Vol. 1 Pergamon Press London 1961 p. 67. See Symposium on Inelastic Collisions of Atoms and Small Molecules Discuss. a F. W. Lampe J. L. Franklin and F. H. Field “Progress in Reaction Kinetics,” ed. 122 RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 123 2. General Characterisation of Chemical Activation A. Energy Characteristics of Activation Reactions.-Microscopic reversibility. Chemically activated species are energised either with respect to further reaction or to deactivation. As governed by the principle of microscopic reversibility the products of reaction are activated for the reverse reaction if for no other.Thus it would appear that every reaction results in chemical activation. However in some case such as exchange and decomposition reactions the products have energy of relative translation and move apart.3 In other cases the reverse reaction cannot be observed; it may occur to such a negligible extent or so slowly as to be of no practical concern. Thus in the elementary isomerisation of methyl isocyanide4 MeNC + MeCN + 15 kcal. the reverse reaction is not observed. Similarly for the thermal reaction5 CyClO-C3& -+ CH,CH :CH2 + 7.5 kcal. the reverse isomerisation of ground electronic state singlet propene is not definitely known.s Activation may result from exothermic reaction whereby energy of reaction appears as internal energy of the products. An instance is the “atom cracking’’ p r o c e ~ s ~ ~ H + C,H -+ C2H6” + 98 kcal.where an asterisk* will always signify vibrational excitation. Another example is9 Cl + Na -+ NaCl* + Na + 70 kcal. followed by NaCl* + Na(,S) -+ NaCl + Na? (,P) Exothermic processes. C2H6* -+ 2.CH3 Here collisional deexcitation of sodium chloride is manifested by fluoresc- ence of the resulting electronically excited Nat(2P) atoms. (A dagger? will always signify electronic excitation.) Some highly exothermic overall reactions do not result in significant 4 F. W. Schneider and B. S. Rabinovitch J. Amer. Chem. SOC. 1962,84 4215. 6T. S. Chambers and G. B. Kistiakowsky J. Amer. Chem. SOC. 1934 56 399; H. 0. Pritchard R. G. Sowden and A. F. Trotman-Dickenson Proc. Roy. SOC. 1953 217 A 563. 6 D. W. Setser B. S. Rabinovitch and D. W. Placzek J.Amer. Chem. SOC. 1963 85 862. 7 (a) E. W. R. Steacie “Atomic and Free Radical Reactions,” 2nd edn. Reinhold Publishing Co. New York 1954; (b) N. A. D. Parlee and E. W. R. Steacie Canad. J. Res. 1939 B 17 371. 8 C. A. Heller and A. S. GoTdon J. Chem. Phys.? 1962,36,2648. 9 ( a ) M. Polanyi “Atomc Reactions” Wilhams and Northgate Ltd. London 1932; (b) M. G. Evans and M. Polanyi Trans. Farauhy SOC. 1939 35 178. J. C. Polanyi J. Chem. Phys. 1959 31 1338. 124 QUARTERLY REVIEWS instances of chemical activation the nature of the products or their energy distribution is such that only inconspicuous collisional deactivation occurs. An example is the decomposition of di-t-butyl peroxidelo Me,C.O.OCMe -+ 2Me,CO + C2H + 39 kcal. Attention must be focused here on the elementary steps.The production of t-butoxy-radicals Me,C.O.O.CMe +- 2Me,CO* - 38 kcal. is not exothermic; the evidence is that these radicals must be activated by collision before undergoing (endothermicll) decomposition Me,CO* -+ Me* + Me,CO - 3 kcal.; E = 15 kcal. mole-l The overall exothermicity results from recombination of methyl to give ethane. The overall decomposition of acetyl peroxide is somewhat similar (Me-CO-O.) -+ C2H + 2C02 + 100 kcal. Actually the exothermicity here does not result exclusively from methyl combination but also in part from the decomposition of acetoxy-radicals with reorganisation energy12 of the products (MeCO.O.) -+ 2 Me.CO.0. - 37 kcal. Me-CO.0. -+ Me. + CO + 17.5 kcal. l- ie exo Some hermicity of the second reaction is shared between the products. elementary step in an overall exothermic reaction must produce species with excess of energy.But unless the latter exhibit some special reaction property or easily apprehended behaviour as in the chlorine atom-sodium system all that occurs is thermal equilibration possibly observable as an increase in temperature. Special efforts can however lead to more useful results in some cases. Thus in the reaction H + Clz -+ HC1* + C1 the hydrogen chloride molecule produced is vibrationally excited and heat- ing occurs ; but in addition spectroscopic measurement of vibrational emission leads to valuable data.13 Endothermic processes. Activated molecules may arise by an endo- thermic process. The reaction of iodine atoms with 1,2-disubstituted ole- lo J. H. Raley F. F. Rust and W. E. Vaughan J. Amer.Chem. Soc. 1948 70 88; L. Batt and S. W. Benson J. Chem. Phys. 1962,36 895. l1 H. Hershenson and S. W. Benson J . Chem. Phys. 1962,37,1889. la L. Jaffe E. J. Prosen and M. Szwarc J . Chem. Phys. 1957,27,416; cf. also J. E. Lennard-Jones Trans. Faraday SOC. 1934 30 70; R. E. W. Norrish and L. H. Long Proc. Rey SOC. 1946,187 A 337. l8 P. E. Charters and J. C. Polanyi Discuss. Faruduy Suc. 1962,33 107. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 125 finsl4~l5 to produce iodo-radicals is -9 kcal. mole-1 endothermic. The iodo-radicals necessarily are formed with sufficient energy for the reverse decomposition to occur and the olefin produced may be geometrically isomerised. Potential energy sucfaces. Schematic potential energy diagrams are given in Fig. 1 for some general exothermic schemes and in Fig.2 for endothermic processes. A By and C each represent one or more species. B will normally include the chemically activated product species. Fig. l(a) is the case where B undergoes no reaction of interest other than the reverse. Examples are16 and1’ cis-stilbene -+ trans-s tilbene 2.NF2 + N2F4 Cases (b) and (c) illustrate the occurrence of further reactions of B which are exothermic and endothermic respectively; this distinction is only of formal importance in most cases. Instances of (b) and ( c ) respect- ively arela :CH2 + cis- but-Zene -+ cis-dimethylcyclopropane* cis-dimethylcyclopropane* -+ pentenes and19 H + cis-C*H + *C*HQ* C4HQ* + CH + C3H6 In practice case ( d ) represents the situation where not all excitation levels of B constitute activation for a subsequent reaction to give C or where only very modest further thermal excitation of B must occur,2o H + trans-C2H2D2 -f *C2H3D2* + 38 kcal.*C2H,D2* -+ D + C2H,D - 40 kcal. A larger threshold energy for production of C would transform case (d) into case (a). Evidently the microscopic reverse processes of those just described B -+ A correspond to endothermic chemical activation; Fig. 2(a) is simply the reflection of Fig. l(a). The distinction between the two cases is not trivial however since reactions were described at the outset of Section 2 where only one of the “reversible’y processes can be studied l4 H. Steinmetz and R. M. Noyes J. Amer. Chem. SOC. 1952,74,4141. l6 M. H. Back and R. J. CvetanoviC Canad. J. Chem. 1963,41 1396. l6 G. B. Kistiakowsky and W. R. Smith J. Amer.Chem. SOC. 1934 56 638; 1936 58,766; M. Calvin and H. W. Alter J. Chem. Phys. 1951,19,768 ; A. Dyck and D. M. McLure J. Chem. Phys. 1962,36,2326. l7 F. A. Johnson and C. B. Colburn J. Amer. Chem. Sue. 1961 83 3043. l9 B. S. Rabinovitch and R. W. Diesen J. Chem. Phys. 1959 30 735. 2o B. S. Rabinovitch D. H. Dills W. H. McLain and J. H. Current J. Chem. Phy$. H. M. Frey Proc. Roy. Sue. 1959 A 251 575. 1960,32 493. 126 QUARTERLY REVIEWS directly. Fig. 2(a’) further illustrates such a case represented by the system21 trans-C,H,D + NO -+ DHe-C(NO)HD* The reverse process leads to isomerisation of the olefin. The stability of the nitrosoethyl radical is not known but quite probably the potential well at B is very shallow. This system cannot be studied in reverse with B as initial reactant since the radical cannot be isolated.Although in most systems the activated species B can suffer collisional deactivation the last system represents an instance where at the ambient temperature the average thermal energy of the species may exceed the depth of the potential- energy well characterising B. The excited species cannot be collisionally stabilised but may only relax into over-all chemical equilibrium with reactants. The formation of trimethylene from cyclopropaneZ2 cyclo-C3H6 -+ C‘H2CH,.CH may represent a case intermediate between the surfaces 2(a’) and 2(b). At sufficiently high temperature the average thermal energy of trimethylene may exceed the depth of the we11;22,23 investigation of the reverse reaction with trimethylene as initial reactant has been rep~rted,,~ although further work seems necessary.Further description of systems corresponding to the behaviour repre- sented by these surfaces will be presented in connection with the character- isation of the chemical activation reaction in terms of its molecularity. B. Reaction Types Giving Rise to Chemical Activation.-Table 1 lists the types of activation reactions to be described. Such consideration allows TABLE 1 . Reactants A Products B Reaction type (i) single molecule single molecule unimolecular isomerisation (ii) single molecule two or more species unimolecular decomposition (iii) two species single molecule bimolecular association (iv) two species two species bimolecular exchange or dis- (v) three species two species “trimolecular” reaction more precise description of the energy distribution of the activated species.Type (i). Activation by unimolecular isomerisation. A product molecule B formed by unimolecular isomerisation automatically has a minimum energy Emin equal to E If AH (Figs. l a and 2a). The energy distribution placement 21 B. S. Rabinovitch and F. S. Looney J. Chem. Phys. 1955 23 2439. 22 E. W. Schlag and B. S. Rabinovitch J. Amer. Chem. SOC. 1960,82,5996. 23 F. H. Seubold J. Chem. Phys. 1953 21 1616; 1954,22 945; E. W. Schlag Thesis 24 F. E. Blacet and A. Miller J. Amer. Chem. SOC. 1957 79 4327; M. C. Flowers University of Washington 1958. and H. M. Frey J. 1960 2758. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 127 function above Emin of species B as formed f(E) depends on that of the reacting molecules; the latter is given25 at high pressure (p + 00) by the Boltzmann distribution K(E) weighted by the specific reaction probability W) [f(E) dE] = k(E) K(E) dE; E > El (1) For reaction of A at low pressure ( p -+ 0) the reaction co-ordinate is the collision co-ordinate and At intermediate pressures where w is the specific collision rate.B do) I I C Reaction co-ordinate FIG. 1. Potential-energy curves for exothermic reactions. 25 R. C. Tolman “Statistical Mechanics,” Chemical Catalogue Company New York 1927. 128 QUARTERLY REVIEWS Fig. l(a) represents the case where B may only revert to A if not col- lisionally deactivated. The isomerisation of olefins1s~2s and azo-com- pounds2' are examples of such behaviour. At sufficiently high pressures only collisional deactivation is significant and subsequent reaction of B if any is merely by thermal activation.Illustrations of other type (i) systems are provided in principle by the low-pressure isomerisations of [2H2]~y~lopr~pane,22 methylcyclopropane,28 and of dimethylcyclopro- pane.29 Methylcyclopropane in all of its possible combinations of con- secutive reactions I I CH,CHCH,CH2 -f all butenes cis-but-2-ene -f trans-but-2-ene etc. 26 J. E. Douglas B. S. Rabinovitch and F. S. Looney J. Chem. Phys. 1955 23 315; B. S. Rabinovitch and K. W. Michel J. Amer. Chem. SOC. 1959,87,5065; R. B. Cundall and T. F. Palmer Trans. Faraday SOC. 1961 57 1936 2226. 27 G. S. Hartley J. 1938 638; E. Fischer and Y. Frei J. Chem. Phys. 1957 27 328. 28 J. P. Chesick J. Amer. Chem. SOC. 1960 82 3277. 29 M. C. Flowers and H. M. Frey Proc. Roy. SOC. 1960 257 A 122; 1961,260 A 424.RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 129 provides examples of all cases of Fig. 1. The dimethyl system provides a number of cases of Fig. l(d) and 2(d) depending on whether the initial reactant is the cis- or trans-isomer. The further reaction of products C to give "D" e.g. cis-pentene -+ trans-pentene might also occur in this instance Me H* H v M e H E l = 59.4 [ E l = 60.51 Me MeI*] L 7 #The parenthetic quantities are the heats of formation AH; kcal. mole-' at 25". The values for dimethylcyclopropane and dimethylcyclobutane were estimated from comparison of heats of formation of cyclopentane and 1,2-dimethylcyclopentane and of cyclohexane and 1,2-dimethylcyclohexane. EE21 E2 61.4 63-6 61.2 62.9 62-3 62-3 61 a 9 61-9 + c~~-CH~.CH~.CH=CH.CH~ (-6.7) t- + trans-CH,.CH,.CH=CH.CH (-7.6) t- -+ (CH3)&=CH*CH,( -10.2) f- - + CH3*CH2C(CH3)=CH2(-8.7) t- A related system is the geometrical isomerisation and decomposition of trans-l,2-dimethylcy~lobutane.~~ Here the activation energies for geo- metrical isomerisation (61.3 kcal.) and for the two modes of decomposi- tion (-60.4 and 63.0 kcal.) are very similar; these represent Figs.2(b) and 2(4* H H [E2 1 E2 61.6 60.4 63.4 63 *O -+ 2C3H6 (10) t- -+ C2H4 + cis- and trans-C,H (10) t- cis-l,2-Dimethylcyclobutane as initial reactant provides another example of l(d). The present type of activation reaction allows the interesting pos- sibility at low pressures of molecules B reacting at a rate characterised by the energy-distribution function of A. 30 H. R. Gerberich and W. D. Walters J.Amer. Chem. SOC. 1961 83 3935 4884. 130 QUARTERLY REVIEWS Type (ii). Activation by unimolecular decomposition. The products of reaction of type (ii) have Emin = El -J= AH, and a total energy-distribu- tion function which is the same as for type (i). For most chemical reactions it is not possible to predict precisely how this energy will be divided. Part at least must go into relative translational energy of the products. If B consists of two (electronically unexcited) atoms the energy is converted into relative translational energy and is divided between the two products in an inverse function of their masses as governed by momentum conser- vation. For two molecular products a qualitative picture of the internal energy distribution can be obtained in some cases. If a bond (or bonds) in one product becomes significantly shortened relative to its length in the transi- tion state such products will tend to contain excess of vibrational energy.In the thermal decomposition of diazomethane CH2N2 -+ :CH2 + N2* the N-N distance shortens from 1.13 A to 1.095 A;31 the nitrogen molecule would be expected to be vibrationally excited. This seems to be the case although the singlet methylene product also carries some of the excess energy.32 Further examples include the decomposition of hydrogen azide in the presence of recombining hydrogen atoms.33 The decomposition is stated to produce NH radicals in both singlet and triplet states which give rise to emission N,H(triplet) + M -f N2(lC) + NH(3n) + M NH(3m +- NH(3C) + hv The thermal decomposition of dimeth~ldiazirine~~ forms isopropylidene with sufficient energy to rearrange to propene When E is much larger than ( E + AH,) such that the collisional transition probability P(E,+AH,)+E = 0 then only deactivation of the products can occur and subsequent reaction other than the reverse is en- tirely by thermal activation.This is the case for most decomposition reactions which give rise to “stable” products. In general unimolecular activation reactions are not important cate- gories relative to bimolecular reactions. Type (iii). Birnolecular association.-The energy of a bimolecular associa- 31 Tables of Interatomic Distances and Configuration in Molecules and Ions Chemical 32 D. W. Setser and B. S. Rabinovitch Canad. J. Chern. 1962,40,1425. 33 H. Guenebaut and M. Latour J. Chirn. phys. 1962,59,970.34 H. M. Frey and I. D. R. Stevens J. 1962,3865. Society Special Publication No. 11 1958. RABINOVITCH AND FLOWERS CHEMICAL ACTlVATION 131 tion process (El & AH + thermal energy of reactants) is confined entirely to the product molecule. If E, AH1 and the heat capacity of the reactants are known the energy content of the excited intermediate is also known. The spread of energies depends on the thermal spread of the reactants. The distribution function is given by eqn. (l) assuming thermal equilibrium of the reactants ;19 failure of the latter condition scarcely occurs in practice even for association reactions of radicals which occur on virtually every collision.35 The product molecules will if not deactivated decompose again into the original reactants [Fig. l(a)] or if there is an alternative reaction path [Fig.l(b) or l(c)] into other products. The relaxation time for decomposition of an activated molecule depends on the number and type of its active degrees of freedom on its vibrational frequency pattern on its energy content and on the free energy of activa- tion for decomp~sition.~~ As examples consider the associations O(,P) + 0 + 03* + 24 k~a1.~' 2C2H -+ n-C4Hlo* + 81 k ~ a 1 . ~ ~ n-d3H + C H -+ n-C,H,,* + 82 k ~ a l . ~ ~ C3H8 + :CH + n-C,Hl0* +-89 kcal.,O n-C,H + H -+ n-C,H,,* + 97 kcal.'" Each of these vibrationally activated molecules has a different lifetime for decomposition. That of triatomic ozone is the shortest for a number of reasons including its low number of active degrees of freedom; that of the n-butane formed with lowest energy by ethyl recombination is longest.The extremely useful possibility of producing a molecule at different known average energy levels should be noted. A great variety of reactions under this general heading has been investi- gated in addition to the above. Examples of each type of system shown in Fig. 1 are known e.g. NH3 + BF + NH,*BF3* R* + NO $ R-O-NO" + 28 kcal., [Fig. l(a)] + 57 k ~ a l . ~ - 37 kcal. [Fig. l(c)] R-O*NO* + RO* + NO The C4H10* systems above correspond to Fig. l(c). The reactions of methy- lene radicals and hydrogen atoms with olefins cited in Sect. 2A are relevant here also as examples of Fig. l(b) l(c) and l(d). These systems are dis- cussed later in more detail. 35 R. D. Present J. Chem. Phys. 1959 31 747; B. H. Mahan ibid. 1960 32 362. 36 R. A. Marcus J.Chem. Phys. 1952 20 359. 37 (a) F. Kaufmann Proc. Roy. Soc. 1958 247 A 123; (b) L. Elias E. A. Ogryzlo 38 A. Shepp and K. 0. Kutschke J . Chem. Phys. 1957,26,1020. 3Q P. Ausloos and E. W. R. Steacie Canad. J. Chem. 1955,33 1062; C . A. Heller 40 J. A. Bell and G. B. Kistiakowsky J. Amer. Chem. SOC. 1962 84 3417. 41 F. T. Smith and G. B. Kistiakowsky J. Chem. Phys. 1959,31 621. 42 P. Gray Trans. Faraday SOC. 1955 51 1367. and H. I. Schiff Canad. J. Chem. 1959,37,1680. J. Chem. Phys. 1958,28 1255. 132 QUARTERLY REVIEWS Type (iv) Bimolecular exchange. In this type the processes of interest have been principally radiative emission or collisional deactivation to the ground state of one or both of the products [Fig. l(a)]. These have been studied mainly by the observation of emission (or absorption) spectra in flash phot~lysis~~ or diffusion-emitter flame3 systems.In a few alleged cases44s45 the ability of one of the products to initiate the decomposition of another species [Fig. l(b) or l(c)] has been described. An interesting example is provided by ozone. In the thermal decomposi- tion of O3 + 0 + O(3P) the exchange reaction O(3P) + O3 -+ 02* + 0 occurs; one molecule carries most of the vibrational excitation in the latter bimolecular reaction,46 which is the chemical activation step of interest. Although further decomposition of O3 by energy chains involving vibrationally excited 02* (v > 19) occurs in the photolytic (blue light) decomposition of ~ ~ ~ n e ~ ~ ~ s ~ [in which O(l0) atoms are produced] the evidence is against their effective participation in the thermal decomposi- tion.In the latter system the chemically activated 02* molecules suffer collisional deactivation as their principal fate. It would be expected that very many exothermic reactions would exhibit non-equilibrium infrared emission (Table B Appendix). However unless the emission is intense information on the initial energy states of the pro- ducts is difficult to obtain. Even in favourable cases the interpretations have not been unambiguous.13 The energy of reaction may be distributed in various ways. For a simple three-centre exchange reaction P + QR 3 PQ + R Polanyi3 used a simplified valence-bond argument to demonstrate that in suitable cases almost the entire heat of reaction may appear as vibrational energy of the product PQ. Smith4' found by a dynamical analysis of a colinear system that the maximum fraction of the energy of reaction retained in the newly formed bond is limited by the kinematic factor sin2 /3 where /3 is the angle of rotation required to take a co- ordinate system describing the reactants into one describing the products and tan2 /3 = (Mp/Mo) + (Mo/MR) + (M~/MPMR); M is the mass of the particle.By taking non-colinearity into account Bunker48 concluded that the maximum fraction of the energy of reaction which can appear in the newly-formed bond is larger than that previously predicted. This removed 4s N. Basco and R. G. W. Norrish ( a ) Canad. J. Chem. 1960,38,1769; (b) Proc. Roy. SOC. 1961 260 A 293. 44 M. A. A. Clyne and B. A Thrush Trans. Faraday SOC. 1961,57 2176; F. P. Del Greco and F. Kaufman Discuss.Faraday SOC. 1962,33 128. 46 (a) F. Kaufman and J. R. Kelso J. Chem. Phys. 1958 28 510; (b) L. F. Phillips and H. I. Schiff ibid. 1962 36 1509 3283; (c) J. E. Morgan L. F. Phillips and H. I. Schiff Discuss. Farahy SOC. 1962,33 118; (d) J. E. Morgan and H. I. Schiff Cunud. J. Chem. 1963 41 903. 46 (a) S. W Benson and A. E. Axworthy jun. J. Chem. Phys. 1957 26 1718; E. Castellano and H. J. Schumacher ibid. 1962,36,2238; (b) W. D. McGrath and R. G. W. Norrish Pruc. Roy. Suc. 1960,254 A 317; (c) L. F. Phillips and H. I. Schiff J. Chem. Phys. 1962,37,924. 48 D. L. Bunker Nature 1962 194 1277; N. C. Blais and D. L. Bunker J. Chem. Phys. 1962,37,2713; 1963,39,315. F. T. Smith J. Chem. Phys. 1959,31,1352. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 133 some disagreement with experimental results which had previously been inter~reted~~ as contravening Smith’s model.Type (v). TrimoZecuZar reactions. This reaction type is the limiting case of bimolecular associations which may be described as occurring by three- body collisions;49 it should be understood that this terminology is simply convenient formalism. Three types of mechanism have been proposed for atomic association p r o c e s ~ e s ~ ~ ~ ~ ~ (4 (b) ! (4 x + x + x2* X2* + M -+ X2 + M X + M +XM* XM* + X 3 Xz + M X + M + X M * XM* + M + XM + M XM + X + X + M where the excited species B are in this case the complexes X2* or XM*. Both have very short lifetimes if M is atomic and decompose again unless collision with M or X occurs in a time which may be not much longer than the order of the period of molecular vibrations.Common reactions of this type includel~~~ 31 -+ I2 + I 21 + He -+ I + He 2 0 + Ar -+ O2 + Ar If as in case (c) XM is a complex of particular ~tability,~~ the system is similar to the bimolecular association considered previously. The energy released on recombination of atoms or small molecules may be very large. A product molecule which is stabilised against decomposition may still have sufficient energy to initiate the decomposition of another substance as alleged for the induced decomposition of hydrogen azide in the presence of recombining H or as suggested in the combina- tion of oxygen atoms with carbon monoxide:53 0 + CO + C02* (singlet) + 127 kcal. c o 2 * + 0 -+ co2 + 2 0 ( 3 ~ ) although the details of the combination are not certain.54 Or it may bring about some other process such as well-known instances55 of electronic excitation by collisions of the first kind 49 E.Rabinowitch Trans. Faruduy SOC. 1937 33 283. 5 0 G. Porter Discuss Faruduy SOC. 1961 33 198. :i M. I. Christie J. Amer. Chem. SOC. 1962 84 4066. (a) D. L. Bunker and N. Davidson J. Amer. Chem. SOC. 1958 80 5085 5090; (b) R. L. Strong J. C. W. Chien P. E. Graf and J. W. Willard J. Chem. Phys. 1957,26 1287; (c) G. Porter and J. A. Smith Nature 1959 184 446; Pruc. Roy. Soc. 1961 261 A 28; ( d ) R. R. Reeves G. Mannella and P. Harteck J. Chem. Phys. 1960,32,632. 6s B. H. Mahan and R. B. Solo J. Chem. Phys. 1962,37,2669. M. A. A. Clyne and B. A. Thrush Proc. Roy. SOC. 1962,269 A 404. 65 L. F. Phillips and T. M. Sugden Trans. Faraday Soc. 1961,57 2188. 134 QUARTERLY REVIEWS 2 H + M -+ H,* + M H2* + Pb -+ Pbt + H Pbt + Pb + hv Since energy removal by M is a function of the nature of the particles involved only broad generalisations on the energy level of the association product can be made.In general for the association of atoms or diatomic molecules in the presence of another atom or diatomic molecule deactiva- tion of excited product B is to levels only a little below the dissociation limit. If M is a larger species or one which interacts strongly with the nascent molecule the immediate deactivation energy level will be signific- antly lower. C. Chemical Activation involving Electronic Excitation.-For effective understanding of reactions involving excited electronic states detailed correlations between the various potential surfaces of the reactants and products are necessary.In most cases adequate information is not available about the topography and topology of the excited surfaces. Difficulties in the interpretation of mechanisms and in the calculation of rates usually arise when complex electronically excited species are involved. Even the ostensibly simple case of recombination of nitrogen atoms in the ground state has provided an extraordinarily complex system involving unstable intermediate m01ecules.~~ Ground state reactants which produce electronically excited products. Oxygen atoms in the ground state react with ethylene (olefins in to give acetaldehyde and ethylene oxide as important primary products O(3P> + CH,=CH - intermediatest C S 3 4 2 CH,.CHO The reaction is interpreted in terms of the initial formation of an electronic- ally excited triplet intermediate.Collisional deactivation gives rise to ethylene oxide. Uncertainty exists whether in forming the epoxide the spin change occurs before vibrational deactivation or after and whether migration of hydrogen occurs in the electronically excited intermediate or in a ground-state intermediate. The relative positions of the various sur- faces are not known. This typifies the difficulties encountered in non- adiabatic systems. An analogous system is that in which :CH2(3Zi) adds to olefins to form the corresponding electronically excited triplet cyclopropane :58 If spin angular momentum is not conserved in a reaction the rate may be considerably reduced.59 There are instances however where the rate of 66 G. G. Manella Chem. Rev. 1963 63 1.67 (a) R. J. CvetanoviC J. Chem. Phys. 1955,23 1375; 1959,30,19; (b) S. Sat0 and 68 F. J. Duncan and R. J. CvetanoviC J. Amer. Chem. SOC. 1962,84 3592. 69 K. J. Laidler and K. E. Shuler Chem. Rev. 1951,48,153. R. J. CvetanoviC Candad. J. Chem. 1958,36 1668. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 135 :CH + R'HC=CHR~ - R'HC-CHR 2 t L C H a such reactions is not governed by the step involving the change of spin as in the Hg(3P1) photosensitised isomerisation of olefins such as b~t-2-ene,~O and in the overall reaction of oxygen atoms with carbon monoxide via the C02(3B2) states3ss4 o(3~) + cope+) -+ co,(lq Electronically excited reactants producing ground-state products. The reactions of the products are usually easier to interpret in this case. We restrict our illustration to a few reactions of the versatile oxygen atom.The O(l0) state lies 45 kcal. above the triplet ground state. The atoms should reacts7 with ethylene to give a vibrationally-excited electronic ground state product since the excess of energy of the reaction estimated at 129 kcal. mole-l is considerably less than the vertical difference between the ground state and the first excited singlet state of ethylene oxide (167 kcal. mo1e-1).61 Subsequent reactions of the excited ethylene oxide may be treated as ground state adiabatic processes. O(l0) also reacts with HCl and H20 to give vibrationally-excited ground state OH radicals O(l0) + HCl -f .0H*(2n) + Cl(2P3/2) O(l0) + H 2 0 + .0H*(217> + OH(2n) + 45 kcal. + 29 kcal. The reaction of O(3P) with HCl and H 2 0 is endothermic and this reaction would not be expected to produce electronically or vibrationally excited OH radicals.In fact it appears that O(3P) atoms do not react with H20 at all since the addition of water to ozone photolysed by red light i.e. involving O(3P) does not affect the decompo~ition.~~ Cases in which electronically excited reactants give rise to electronically excited products will not be considered. D. Characteristics of Chemical Activation.-From the above description of reaction types giving rise to chemical activation some conclusions and deductions regarding the characteristics of chemical activation may be made. It is possible to produce excited molecules which in suitable cases contain known amounts of energy. This energy can be varied by utilising different reactions to produce the same molecule.Information about. the distribution of the energy of reaction among two or more products can be obtained. In many instances only molecules in their ground electronic state are involved; the reactions that occur are adiabatic are well characterised mechanistically and are readily amenable to comparison with rate theories. eo R. J. CvetanoviC H. E. Gunning and E. W. R. Steacie J. Chem. Phys. 1959 31 573. T. Liu and A. B. F. Duncan J. Gem. Ph-ys. 1949 17 241. 136 QUARTERLY REVIEWS 3. Survey of Occurrence of Chemical Activation Chemi1uminescence.-One of the first phenomena studieds2 which in- volves chemical activation was bio-chemiluminescence which is usually associated with enzymic oxidation. The reactions are well characterised only in a few case^.^^,^^ Summaries have been given.65 Many ordinary oxidations in solution are chemil~minescent.~~~~~ Some solid-state reactions such as decomposition of sodium azide emit ultra- violet light;6s many of these systems involve elementary reactions of free radicals and atoms.No example of ultraviolet emission in bio-luminescence has been observed. Emission has long been observed in gaseous flames. Hydrocarbon- oxygen flames are the most common and have received much experimental attention. The vibrational and rotational distributions of OH in various flames have been measured and in several cases non-thermal equilibrium distribution The mechanism of production of the various species in hydrocarbon flames is largely speculative and in any case complex. The luminescent reaction of sodium atoms with halogen^,^,^^ and with inorganic71 and organic72 halides are well known and better under- stood.Detailed discussions of flames have been gi~en.’~,’~ In many cases emission from a discharge is due to direct electronic excitation. However in some cases the emission results from reaction of atoms or molecules in their ground electronic states as in the nitrogen afterglow ;75 the species responsible appear to include electronically excited nitrogen molecules formed by the combination of nitrogen atoms :7s M + N + N -+ M + N,t +N2 + hv 62 R. Boyle Phil. Trans. 1667,2 No. 31,581. 63 E. H. White F. McCapra G. F. Field and W. D. McElroy J. Amer. Chem. SOC. 64 W. D. McElroy and H. H. Seliger Scientific American 1962,207,76. 66 E. N. Harvey “The Nature of Animal Light,” J. B. Lippincott Philadelphia 1920 66 M.Trautz 2. phys. Chem. 1905,53 1. 67 R. Audubert Compt. rend. 1936 202 131,406. R. Audubert J. Chim. phys. 1936,33. 507; Trans. Faruduy SOC. 1939,35 197. 6B A. G. Gaydon and H. G. Wolfhard Proc. Roy. SOC. l948,194,A 169; H. P. Broida J . Chem. Phys. 1951,19 1383; K. E. Shuler and H. P. Broida ibid. 1952,20 1389. 70 H. Beutler and M. Polanyi 2. phys. Chem. 1928,1 B 3; S. v. Bogdandy and M. Polanyi ibid. 1928 1 B 21. 71 M. Polanyi and G. Schay 2. phys. Chem. 1928,1 B 30; W. Heller and M. Polanyi Trans. Faraday SOC. 1936,32,633. 72 C. E. H. Bawn and W. J. Dunning Trans Faraday SOC. 1939 35 185; E. D. Kaufman and J. F. Reed J. Phys. Chem. 1963,67,896. 73 A. G. Gaydon “The Spectroscopy of Flames,” Chapman and Hall Ltd. London 1 ac7 1961 83 2404. “Bioluminescence,” Academic Press New York 1952.1 / J I . 74 K. E. Shuler J. Phys. Chem. 1953 57 396. 76 R. J. Strutt Proc. Roy. SOC. 1911,85 A 219; 1911,86 A 56; 1913 88 A 539; 76 H. Sponer Z . Physik 1925 34 622; K. R. Jennings and J. W. Linnett Quart. 1915,91 A 303; E. P. Lewis Astrophys. J. 1900,12,8. Rev. 1958 12 116. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 137 Recombination Reactions.-The recombinations of atoms and radicals produce activated products as in 2H 3 Hz*. Recent considerations of the mechanism of recombination of atoms and small molecule^^^-^^ (discussed above) have had their counterparts much earlier. Some forty years ago P01anyi~~ and H e r ~ f e l d ~ ~ pointed out that atoms only combine if the nascent molecule suffers a collision with a third body which removes some of its energy Boden~tein~~ supported such “dreirstoss” restriction since he calculated that triple collisions could adequately account for the reaction rate.This was an alternative to the ideas of Trautzso who had proposed an “intermediate complex’’ in explanation of the third-order reaction be- tween nitric oxide and oxygen. At sufficiently low pressure the combination of atoms must always be of the second order the excited molecule being stabilised by emission. This process is many-fold slower than the termolecular collision rate at all normal pressures. Bimolecular second-order recombination of atoms with emission of light has been observed experimentally in favourable casesB1 M2P3/2) + Br(2Pl/2) + Brd3n,,) Br,(317,,) -f Br2(l2;) + hv The first mention that combination of any two species might not occur unless a third body were present seems to have been made (for hydrogen atom and ethylene) by Taylor and This idea governed the general conception of methyl and radical recombination for some time.Energy Chain Theory.-An early theory of unimolecular reactions by Christiansen and K r a m e r ~ ~ ~ used the concept of chemical activation. They suggested that reaction products could carry excess of energy El + AH, and activate other reactant molecules by collision. The Boltzmann distribu- tion would thus not be depleted by reaction. The theory as suggested failed for several reasons. But distantly related ideas were used some time later to explain isotope exchange,84 “atom ~racking”,~ and h~t-radical~~ reactions. The postulate of energy chains in connection with solid-phase radiation-induced polymerisations has recently been re-introduced by Semenov.86 Summary.-The detailed discussion of this section has been confined to some early important systems.That chemically activated molecules and radicals may be important intermediates of reaction is now generally 7 7 M. Polanyi Z . Physik 1920 1 337. 78 K. F. Herzfeld 2. Physik 1922 8 132. 7 9 M. Bodenstein Z . phys. Chem. 1922,100,68. M. Trautz Z . Electrochem. 1916 22 104. 81 H. C. Urey and J. A. Bates Phys. Rev. 1929,34 1541 ; G. Herzberg Astrophys. J. 82 H. S. Taylor and D. G. Hill J. Amer. Chem. SOC. 1929,51,2922. 83 J. A. Christiansen and H. A. Kramers 2. phys. Chem. 1923,104,451. 84 N. R. Trenner K. Morikawa and H. S. Taylor J. Chem. Phys. 1937 5 203. 86 B. S. Rabinovitch S. G. Davis and C.A. Winkler Canad. J . Res. 1943 B 21 251. 86 N. N. Semenov Pure and Applied Chem. 1962 5 353. 1939 89 290; H. B. Palmer J . Chem. Phys. 1957,26 648. 138 QUARTERLY REVIEWS recognised. A representative collection (although not complete) of various studies involving chemical activation is given in the Tables of the Appendix. Investigations which explicitly use the features of chemical activation have been made for only about a decade and it is on a few representatives of these studies that attention will be focused in the remainder of this Review. 4. Specific Studies of Chemical Activation A. Bimolecular Exchange or Displacement Reactions.-In this type of study information can be obtained about the nature of the activation reaction. Specifically for the reaction P + QR -+ PQ + R the relative rates of formation of different vibrational states of the product PQ can be determined.87 For the reaction P + QRS -+ PQ + RS the vibrational excitation of RS relative to PQ can also be found.Other important aspects of these studies include the nature of collisional deactivation of the excited molecules produced and the question of promotion of reaction by product molecules due to activation. Four reactions have been chosen to illustrate these matters. ( i ) Reaction between hydrogen atoms and molecular chlorine. H + C1 + HC1* + Cl + 45 kcal.; Es E 2.5 kcal. This system has been studied in detail by J. C. Polanyi and his c o - w o r k e r ~ ~ ~ ~ ~ at pressures down to mm./Hg. At the lowest pressure where collisional deactivation is less im- portant an observed infrared emission arises from all vibrational levels of HCl up to v = 5 (38.1 kcal.).The intensities of the individual rotational lines of various bands of the dv = 1 and dv = 2 sequences were summed and the vibrational and rotational population distribution of the HCl molecule was calculated. Now at thermal equilibrium the population of the vth level N, is N Qv N = - exp [ -G(v)hc/kT] where N is the total population Qv is the vibrational partition function and G(v) is the energy of the vth level. A plot of log (Nv/Nv’) against hc[G(v’) - G(v)] is a straight line of slope 1/T if vibrational equilibrium exists. If the value of T thus obtained corresponds to the ambient tempera- ture then equilibrium presumably exists between vibration and translation. Fig. 3 reveals that the HCl molecules produced possess a distinctly non- equilibrium vibrational distribution.Calculation shows that there are appreciable initial rates of formation of HCl into all vibrational states up to v = 5 but each rate decreases as ZI increases. (a) H. S. Heaps and G. Herzberg Z . Physik 1952 133 48; (b) J. D. McKinley D. Garvin and M. J. Boudart J. Chem. Phys. 1955,23 784; (c) T. M. Cawthorn and J. D. McKinley ibid. 1956 25 585; ( d ) F. Kraus 2. Naturforsch. 1957 12 A 479; ( e ) D. Garvin H. P. Broida and H. J. Kostkowski J. Chem. Phys. 1960,32,880; 1962 37 193 ; (f) H. P. Broida ibid. 1962,36,444. 88 (a) J. K. Cashion and J. C. Polanyi J. Chem. Phys. 1958,29,455; 1959,30,1097; 1959,31 1338; (b) J. K. Cashion and J. C. Polanyi Proc. Roy. Soc. 1960,258 A 529. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 139 2 4 6 8 10 12xIC G (v’) - G (I) (cm.?) FIG.3. Stationary-state distribution among vibrational states of HCl* produced by the [A Boltzmann distribution of vibrators at the temperatures indicated would give points lying along the solid lines.] 0 Represents points obtained at ca. 0.2 mm. Hg. (after Cashion and Polanyi ref. 88b). 0 represents points obtained at ca. 0.02 m. Hg. (after Charters and Polanyi ref. 13). At higher pressures -10-1 mm. the vibrational distribution was found to be of Boltzmann type with T = 2700”~ although surprisingly rota- tional states were not in an equilibrium distribution. The highest vibra- tional level observed was z = 6; the fact that z = 6 was not observed at lower pressures suggests that vibration-vibration transitions were import- ant in the work at higher pressure; a ten-fold reduction in pressure reduces collisions of HC1 with all other molecules ten-fold but reduces HCI-HCl collisions by a factor of 100.Vibrational-translational transitions are not efficient and the transition probability Pv,v-l is less than even if the entire energy of reaction went initially into the HC1 molecules approxi- mately lo5 collisions do not remove more than 90% of the energy. This follows from the observation that at 1 mm. pressure the HCI molecules were approximately in a Boltzmann distribution equivalent to 3500”~. The total vibrational energy at this temperature is equivalent to 10% of the available energy. A plot can be made for rotational distributions in vibrational levels ZI = 1-4 observed at the lowest pressures.It was found that for J N 1-7 the rotors in each vibrational level are in thermal equilibrium which is characterised by the same “temperature” (-100”c) for all u. For higher J values the distribution deviates toward those characteristic of higher temperatures. The total excess of energy available from this reaction (E + AH) is 45 + 2.5 = 47.5 kcal. There is thus sufficient energy to reaction of hydrogen atoms with chlorine molecules. 140 QUARTERLY REVIEWS excite the J = 20 rotational level of the v = 4 state which corresponds to 31-2 + 11-7 kcal. The highest observed level (estimated here from plots in the reference) was J = 17. Now HCl molecules suffer approximately lo3 collisions on the average before emission and it might be expected that the distribution would have relaxed from its initial value; for low rotational states a change of AJ = 1 is promoted efficiently by collisions; at high J values the rotational spacings approach in magnitude the values for vibrational jumps and collisional exchanges may not be as effi~ient.*~ It appears probable that the observed rotational distributions are a result of an initial non-Boltzmann equilibrium distribution.On almost every vibrational-vibrational radiative transition the rotational quantum number will change also; this effect although small will tend to maintain a non-equilibrium rotational distribution. At higher pressures for rota- tional excitation at the v = 1 level (higher vibrational levels did not yield accurate rotational data) a Boltzinann distribution characteristic of room temperature was observed up to J = 3.At still higher J values deviations were observed which corresponded to higher temperatures. The distinct differences in rotational and vibrational distributions in the two sets of work show further that collisional deactivation is an important effect at the pressures studied. This work admirably demonstrates the presence of chemical activation and the information of various kinds that may be learned about the mole- cule PQ. To obtain information on the excitation of RS reactions of the type P + QRS -+ PQ + RS must be investigated. Although reactions such as H + O3 -+ OH + OZ8’ and 0 + NO -+ NO + OZg0 have been examined selection of the present system is desirable for several reasons. This reaction has been studied by Cashion and Polanyigl in the same manner as the earlier investigations of H + C1 at higher pressures.Both product molecules are heteronuclear and infrared-active and may readily provide information regarding the excitation of the released species NO as well. Cashion and Polanyi concluded that the major infrared emission re- sulted from the reaction (ii) Reaction between hydrogen atoms and nitrosyl chloride. H + NOCl+ HCl* + NO + 65 kcal. Emission consisted of the fundamental first and second .overtone vibra- tional sequences of the electronic ground state of HCl. The highest vibrational level definitely observed (v = 9) is 63.6 kcal. above the zero state. Although the v = 9 and lower states may have arisen from vibra- 8 9 T. L. Cottrell and J. C. McCoubrey “Molecular Energy Transfer in Gases,” Butterworths London 1961; K.F. Herzfeld and T. A. Litovitz “Absorption and Dispersion of Ultrasonic Waves,” Academic Press New York 1959. ( a ) F. J. Lipscomb R. G. W. Norrish and B. A. Thrush Proc. Roy. Soc. 1956 233 A 455; (6) R. A. Kane J . J . McGarvey and W. D. McGrath J . Chem. Phys. 1963 39 840. 91 J. K. Cashion and J. C. Polanyi J . Chem. Phys. 1961 35 600. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 141 tional-vibrational energy transfer as occurred in the H + C1 system at similar pressures the initial distribution is definitely a non-equilibrium one. The calculation made in this paper of the relative rates of chemical reaction into each vibrational level may be somewhat in error but probably not the semi-quantitative conclusion that the relative rates of formation into each decreases as z) rises.Such behaviour may not be general however.4s Emission was also observed from vibrationally excited NOCl (v = 1,2) and a weak emission from vibrationally excited NO (v = 1). Emission from NO* was approximately one-tenth that of HCl under all flow condi- tions. This suggests that the production of excited RS is not an important process. Because the NO and HCI emissions varied in a similar manner with conditions they may well be formed by the same chemical reaction. Excitation of NO (and of NOCl) by a collisional process whilst acting as deactivators may also have occurred. The rotational level distribution for HCl could be described as an equili- brium distribution at a temperature of 1970"~ except for the first three rotational states which corresponded to a much lower effective tempera- ture.(iii) Reaction of oxygen atoms with chlorine dioxide. The use of infrared emission in these studies suffers from the restriction that the emitting species undergoes 1 03-l O6 collisions before emitting. The measurement of absorption spectra immediately after the formation of a species can yield more direct information on the initial energy distribution. Measure- ments at progressively longer delay times then give rates of relaxation of the excited species. Such a technique developed by Norrish and his co-workers has been applied to the reaction of O(3P) atoms with C102:90 o(3~) + c1o2 -+ 02* + CIO Flash photolysis of ClO was used to produce the 0 atoms; + 61 kcal. C102 + hv + ClO + 0 The vibrationally excited O2 was studied by means of the Schumann- Runge absorption between 2200 and 2600 A.The photolyses were carried out with 250 to 3000-fold excesses of inert gases present. Decay of C10 and of excited O2 was measured at varying delay times. In all experiments the maximum concentration of excited O2 was observed at the shortest delay time (10 psec.) after the beginning of the photolytic flash. This time is shorter than the radiative lifetimes of excited 02 since dipole emission is forbidden and only magnetic dipole or quadrupole emission occurs. The vibrational distributions for 0 levels beIow 'u = 5 could not be measured. In levels ZI = 5 6 and 7 it appeared that approximately equal numbers of molecules were present but appreciably less in 'u = 8 the highest level observed (34 kcal.). Rotational distributions could not be 142 QUARTERLY REVIEWS accurately measured but appeared to follow a Boltzmann distribution corresponding to the ambient room temperature.The excitation of C10 was difficult to detect owing to the small disper- sion of its spectrum. Measurements of the half-life of excited 0 were made with different inert gases. For higher levels the half-lives were slightly shorter and those for the lower ones slightly longer. The number of collisions required to deactivate the v = 6 level were calculated from the data (Table 2). TABLE 2. Collisional deactivation efficiencies for 0 2 * ( v = 6 ) Deactivation species Number of collisions for deactivation N2 > 107 co2 7000 ClO 2000 ClO -2000 Ar > 107 - ~ 0 frequency intervals 7 to 6 6 to 5 Near vibrational frequencies (cm.-l) 2359 1286 1388 868 945 1106 1417 1 440 Included in Table 2 are the vibrational frequencies nearest to the dv' = 7-6 and 6-5 intervals.Removal of energy evidently occurs most readily for near-resonance of the energy levels. The analysis of relaxation data in terms of half-lives becomes complex when more than two levels are involved in the relaxation process;92 it is necessary to follow the relaxation of several and preferably all energy levels. Basco and Norrishg3 have shown however that for systems in which the vibrational populations decrease towards higher levels and where the change of relative populations is small over one or two apparent half-lives measurement of the changes in relative populations can provide accurate relaxation data. On applying this to the 0 + C102 reaction where the half- lives of v = 5 6 and 7 levels were not greatly different it was found that the fifth sixth and seventh levels were probably populated to a decreasing extent but that the relaxation results may not be in error by more than a factor of two.(iv) Reaction of nitrogen atoms with nitric oxide. The reaction of N atoms with NO in the presence of ozone may provide a c ~ s ~ ~ ~ ~ J where the further reactivity of the chemically activated product is promoted N + NO +N2* + 0 + 75 kcal. (d) Reaction (d) is faster (k = 2.2 x 10-l1 ~ m . ~ molecule-l sec.-l) than sub- sequent reactions between NO and N with 0 92 K. E. Schuler J. Phys. Chem. 1957,61,849. 93 N. Basco and R. G. W. Norrish Discuss. Faraday SOC. 1962 33 99. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 143 NO + 0 + M + NO + M (4 NO + 0 +NO + hv ( f ) N + O + M + N O + M + h v (g) In the presence of excess of N atoms or NO chemiluminescence results [reactions c f ) and ( g ) ] .When reaction (d) occurs between stoicheiometric amounts of N and NO the reaction is non-chemiluminescent and (h) may be studied without interference from cf) and (g) N * + 0 3 + N z + 0 2 + O (4 Vibrational excitation from reaction (d) was first suggested by Kaufmann and K e l s ~ . ~ ~ ~ Morgan Phillips and S ~ h i f f ~ ~ ~ l ~ carried out a calorimetric investigation and determined that the average energy of the chemically- produced nitrogen molecules is 21 & 5 kcal. mole-l i.e. substantially below the exothermicity. Phillips and S ~ h i f f ~ ~ on investigating the reac- tion of active nitrogen with ozone observed that the addition of nitrous oxide (an efficient deactivator of vibrationally excited nitrogen) reduced the decomposition of the ozone.They suggested that nitrogen molecules were produced with sufficient energy (> 23.4 kcal. mole-l) to decompose ozone. This conclusion has been supported by later in which N atoms were titrated with nitric oxide. Ozone was also added and its rate of consumption was determined. In this way the percentage of nitrogen molecules formed with energy equal to or greater than 23.4 kcal. (v > 4) was evaluated as 62-75 %. Comparison of this percentage with the results from the calorimetric study indicated that nitrogen molecules are produced with a non-Boltzmann distribution of energies having a maximum popula- tion near v = 4. The rates of collisional deactivation of excited nitrogen molecules by N and N20 were also obtained (k = 3.5 x 10-l6 for N,; k = 1.3 x 10-l6 ~ m .~ molecule-l sec.-l for N,O) by the titration of N,* (v > 4). It is of interest to compare the collisional efficiency of nitrogen molecules for deactivation of N,* below v = 4 as measured chemically and the efficiency for degradation of vibrational into translational energy as measured calorimetrically. The latter efficiency is estimated as some fifty times lower than the former. This fits in with theoretical considerations that because of the matching of the energy levels of nitrogen resonance transfer would be expected to be very efficient; also the Landau-Teller theory predicts inefficient conversion of vibrational energy into tran~lation.~~ B. Bimolecular Association Reactions.-The principal features of systems involving bimolecular association reactions [Section 2B type (iii)] will be illustrated in detail in terms of two examples chosen from a number of similar reactions (Appendix Table B) these are the addition of H atoms to alkenes to give radicals and the reactions of methylene radicals with alkenes to produce hot molecules.(i) Reaction ofhydrogen atoms with butene. The reaction of H atoms with I44 QUARTERLY REVIEWS but-1-ene and but-Zene has been investigated in detail by Rabinovitch and his co-workers with use of a diffusion flame type of experimental arrangement - 94 . (0 The collision yield for H addition to the double bond is approximately The chemical relaxation time is therefore much longer than momen- tum relaxation times and a Boltzmann distribution of reactants is main- tained.The effect of multiple isotopic deuterium substitution of butene has also been investigated.94a Variation of energy of the activated radical. The energy of the activated radical may be varied by starting with cis- or trans-but-2-ene or with but-1-ene and by adding either hydrogen or deuterium atoms. The thermochemistry of these systems is quite well known. Some average internal energy parameters of s-butyl radicals formed in the various ways are given in Table 3. Since the activation energy for reaction (i) is small the system can be studied at -78"c as well as at 25"c which reduces both the energy spread of the radicals and their average energy. Dependence of rates of decomposition on energy. The activated s-butyl radical may undergo the following reactions H + C4H -+ s -C4H9*; Ea = 2 kcal.mole-1 kL ( j ) k E (D) ( k ) w . ( S ) (0 s-d,H,* -+ C4H8 + H -+ *CH3 + C3H6 -+ s-CgHg TABLE 3. Energy (kcal. mole-I) of excited s-butyl and C2H1] butyl radicals in various systems. System Min. energy Min. excess energy Average excess energy En,,, = E + AHl E2i,,. = E,,,i,,. - E of formed radicals @+) H + trans 38.7 H + cis 40.0 H + but-1-ene 41-1 D + trans 40.5 D + cis 41.8 D + but-1-ene 42.9 - 7 8 " ~ 25 O C 5.7 7.2 8.6 7.0 8.5 9.9 8.1 9.6 11.0 7.5 9-0 10.5 8.8 10.3 11-8 9.9 11.4 12-9 $Refers to reaction ( k ) in the text. Since reaction ( j ) is the reverse of the formation reaction (i) E:,. = 0 and <E+> is only a few kilocalories for this process. The lifetime of an 94 ( a ) J. W. Simons D. W. Setser and B. S.Rabinovitch J. Amer. Chem. SOC. 1962 84 1758; (b) B. S. Rabinovitch R. F. Kubin and R. E. Harrington J . Chem. Phys. 1963 38 405. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 145 activated species depends on its complexity and on its excess of energy. For the former reason as comparison with the H + C2H4 system shows,20 and for the latter reason as shown by calc~lation,~~ reaction ( j ) is negligible compared with the decomposition (k) for which E:in. is 6-10 kcal. mole-l (Table 3). An alternative fate of the activated radical is collisional deactivation. The relative amounts of stabilisation S (reaction Z) and decomposition D (reaction k) of the activated radicals are measured as a function of pressure. This system provides well-controlled conditions and furnishes a clear test of unimolecular reaction theory and the predicted variation of rate with excitation energy.Since the radicals are formed with some energy spread the measured rate constant is not kE but an average rate constant defined as k = w.D/S. This is related to kE as follows cn Emin- where f ( E ) is given by eqn. (1); k ; is negligible and can be neglected. The limiting high and low pressure forms which this equation takes are k, = (ICE) and k, = (l/kE>-l. The ratio ka,/kao provides a measure of the effective energy spread or dispersion described byf(E) and is unity for monoenergetic activation; this ratio for butenes was found to be less than 2 at 25'c and less than 1.3 at -78" which is orders of magnitude smaller than the comparable ratio i.e. energy dispersion for thermal activation.This demonstrates the relatively monoenergetic character of the excited butyl radicals especially at -78"c. The M a r c ~ s - R i c e ~ ~ ~ ~ ~ form of kE has been used to calculate theoretical values of k, and kao for each system. This involves the computation of kE on a quantum statistical rather than classical basis. Good agreement of both absolute rate magnitudes and their variation with energy was ob- tained between observed and calculated values (Fig. 4) of k, and kao. This supports the validity of the Marcus-Rice (RRKM) model for the calculation of unimolecular reaction rates and the postulate of the theory regarding efficient intramolecular relaxation of energy between the active degrees of freedom of the molecule. This agreement was found for these systems and others,* only when all vibrational and internal rotational degrees of freedom were taken as active together with those overall rotations which interact with vibrational motions.These activation systems thus assist materially in establishing the best detailed a priori 96 R. A. Marcus and 0. K. Rice J. Phys. Colloid Chem. 1951,55,894. 2 146 QUARTERLY REVIEWS DBRT HBRT A A T v A- A FIG. 4. k as a function of (E+)4 the average energy of the reacting radicals. Observed values 0 p = 0; A p = a; calculated values filled symbols. The system designation employs the letters H or D for the atom used B C T for but-l-ene cis- and trans-but-2-ene reactant 78 for -78"c (195"~) and RT for 27"c (300"~). The relation between observed and calculated values is avertical one in the Figure (taken from ref.97). theoretical model for the calculation of unimolecular reaction rates. Predictive tests e.g. .predicted and measured secondary isotopic rates for [2H,]butyl from H + [2H,]cis-but-2-ene have also supported the theory.94a An interesting confirmation of the occurrence of internal energy random- isation is provided by a comparison of the behaviour of the s-butyl radical formed from but-2-ene (H atom addition to a non-terminal carbon atom is followed by rupture of the attached C-C bond) H + C-C=C-C += C-C-C+C I H and from but-1-ene (addition is to the terminal carbon atom and rupture of the most remote C-C bond takes place) On initial formation the excess of energy is obviously distributed in different ways in these two cases. The fact that the radicals in both pre- parations decompose at comparable ratesg6 means that the relaxation time for internal energy migration is less than that for decomposition.H + C-C-C=C + C+C-C-CH 96 R. E. Harrington B. S. Rabinovitch and H. M. Frey J. Chem. Phys. 1960 33 1271. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 147 Intermolecular energy transfer. The butyl radical system has also been used for the measurement of collisional transition probabilities. 97 a The effect of the addition of various inert gases on the value of k for s-butyl was studied. The principle of the experiment assumes a very simple form in these non-equilibrium systems radicals are formed directly without collisional activation having energy in excess of Emin. (Fig. 5 ) ; if they i&Multi-step Single-step -I I ~ deactivation deactivation Stabilization ( S ) FIG.5. Representation of decomposition and stabilisation processes for a model system with activation at level E n s E,,,in. (taken from ref. 94b). experience an energy cascade on collision rather than efficient deactiva- tion below the critical level E on each collision they may undergo further decomposition at each successive level entered above E,; the ratio of S/D especially at low pressures is sensitive to the details of the vibrational cascade i.e. where /3 is the effective collision factor w is the collision frequency n is the number of transitions in the active region and ki refers to the specific rate of decomposition at each intermediate energy level. The average amount of energy removed per collision by the rare gases and diatomic molecules H2 D2 N2 was found to be 1.2-3 kcal.mole-l. The average energy transferred per collision to poly-atomic deactivators such as butene SF, CH,C1 and CD3F was >9 kcal. mole-l with inter- mediate values of 5 kcal. mole-l found for smaller molecules such as C02 and CH4 ; collisional deactivation cross-sections conform well to the Lennard-Jones potential. Energy transfer even to rare-gas atoms and small molecules is thus surprisingly efficient ((LIE) > kT). Refined measure- ment can also lead to information about the form of the collision transition probabilities as a function of energy; this is the problem ahead. The collisional deactivation efficiencies of the rare gases increases some- 97 ( a ) G . H. Kohlmaier and B. S. Rabinovitch J. Cheni. Phjls. 1963 38 1692 1709; (b) D. W. Setser B. S.Rabinovitch and J. W. Simons J. Chem. Phys. 1964,40 1751. 148 QUARTERLY REVIEWS what with increasing atomic weight and with decreasing bath temperature. The theories of Landau and Teller and of Schwartz Slawsky and H e r ~ f e l d ~ ~ for energy exchange applied usually to the collisions of diatomic or small molecules at low energies as mentioned earlier predict much smaller probabilities for the transfer of such large amounts of energy a positive temperature coefficient and a decreased efficiency with increase in mass. These items are contrary to the findings in the activated s-butylg7 and ethyl radica120 systems. These theories may not be used to predict the extent of energy exchange between polyatomic molecules at high energies. (ii) Reaction of methylene with but-2-ene.The reaction of singlet methylene with olefins and alkanes gives rise to an energised molecule which then undergoes a variety of non-radical reactions. The reaction of methylene with cis- and trans-but-2-ene has been studied in greatest detail by Frey18 and by Setser and Rabin~vitch.~~ Methylene was produced by the photo- lysis (4100 A) and thermal decomposition of diazomethane and by photolysis of ketene at various wavelengths centered at 3100 A. The addi- tion to the double bond is stereospecific; trans-but-2-ene gives excited trans- 1,2-dimethylcyclopropane initially Me* This is characteristic for reactions of singlet methylene. 98 The excited dimethylcyclopropane has more than sufficient energy to react and gives the same products that are formed in the thermal isomerisation of 1,2- dimethylcyclopropaae (Sec.2B) for which Eo - 60 kcal. mole-l. Methylene insertion into carbon-hydrogen bonds gives rise directly to 2-methylbut-2- ene and trans-pent-2-ene -+trans-CH,CH =CH-CH2CH3* + CH3*CH=C(CH3)CH3* :CH2 + trans-CH3CH=CHCH3 Energy variation of activated dimethylcyclopropane. The methylene is invariably produced with an ill-defined excess of vibrational energy. The relative magnitude of this excess may be varied by 5-10 kcal. by pyrolysis of diazomethane at different temperatures or photolysis at different wave- lengths of the photolysing light. From experimental evidence of the effect of inert gases on the reaction it appearsla that methylene radicals may have excess of translational energy arising from their genesis which is removed after a few collisions with inert-gas molecules.The yield of the stabilised dimethylcyclopropane increases with pressure as exp(-constantlp) when p is sufficiently large and tends to a limiting value. This value is independent of the mode of formation of methylene but the yield at lower pressures varies with the mode of formation. This 98 P. S. Skell and R. C . Woodworth J. Amer. Chem. SOC. 1956 78,4496. RABINOVITCH AND FLOWERS CHEMICAL ACTIVATION 149 reflects the different excesses of energy carried by methylene. A plot for trans-but-2-ene as initial reactant of the form y = b + mx i.e. is found to be a straight line; S refers to total stabilised dimethylcyclo- propane. The reciprocal of the slope gives the rate of geometrical isomerisa- tion of activated trans- dimethylcyclopropane. Since this value is found to be constant (differing for different sources of methylene however) it follows that the activated molecules are formed in a distribution that is closely monoenergetic.The average lifetime of thermally activated dimethylcyclopropane molecules reacting (average energy -79 kcal. mole-l) is as compared with the chemically activated species (average energy -1 10 kcal. mole-l) which has an average lifetime of sec. The molecules formed by carbon-hydrogen insertion reactions also possess sufficient energy to undergo further reaction at low pressures. Thus cis-pent-2-ene* + trans-pent-2-ene* and cis-pent-2-ene* -+ decomposition products Relation to reaction theory. Application of a theoretical formulation to the unimolecular isomerisation rate of the excited dimethylcyclopropane has been made in a manner similar to that described for the butyl Since the total energy is not known apriori the theoretical calculations were reversed a value of the energy (-105-110 kcal.mole-l) was found which gave the best fit of the calculated values to the experimental k,. The method has been used to find the fraction of excess photolysis energy retained by ethylene on photolysis of ketene (50-85 %) and diazomethane Mention should be made of another system. Butler and KistiakowskylOO produced energised methylcyclopropane by reaction of methylene with propylene or cyclopropane (30-50 %).32 The difference in energy content of the methylcyclopropane formed by these reactions is known to be 7.9 kcal. mole-l. The experimental rate ratio found for the two systems was 3.8.The theoretical formulation is in good agreement this energy increment is predicted3a to cause an increase in rate of approximately 3-4. Comparison of the two modes of production of methylcyclopropane sv M. C. Flowers and H. M. Frey J. 1962 1157. loo J. N. Butler and G. B. Kistiakowsky J. Amer. Chem. SOC. 1960,82,759. 150 QUARTERLY REVIEWS and the proportions of butene structural isomer products demonstrates as pointed out by Butler and Kistiakowsky that intramolecular energy transfer must be more rapid than the rates of reaction of the energised molecules. Studies of energy transfer. The effect of added inert gases on the rate of deactivation of the excited cyclopropane species in methylene systems has been studied.la~lol Use of added oxygen removes any effects due to the collisional conversion of singlet methylene into the ground triplet state.lo2 Large amounts of energy (-10-20 kcal.mole-l) on the average are removed per collision on deactivation of cyclopropane by ethylene and of dimethylcyclopropane by butene (Table 4); the inert gases helium and argon have less but fairly high efficiency (0.15-0.3 on a pressure basis). (E)-105 kcal. mole-l; E,-60 kcal. mole-l. TABLE 4. Deactivation of various cyclopr~panes.~~ Species Deactivator Average energy decrement per collision Dime t hylcyclo- Methylcyclo- 450" 400" 325" 300" 150" 25" propane butene 10 12 propane cyclopropane 3307 Cyclopropane ethylene 7 10 12 10-30iJ SCalculated on the basis of a simple step-ladder energy cascade model; other models do not alter the average decrement appreciably. YEstimated from the data of reference 101b.IlConflicting estimates from reference 97b and unpublished data of J. W. Simons. 5. Summary Bimolecular activation reactions offer the most useful and poten tially fruitful systems although the study of the recombination of atoms has also been of great value.' A. Bimolecular Exchange Reactions.-The systems discussed previously are fairly complex even though they involve small molecules and some interpretations are probably subject to change. Nonetheless in general the following conclusions apply. PQ product molecules may be vibrationally excited up to the maximum possible value but when P is an H atom and in some cases when it is heavier,g0b entry into lower vibrational states is favoured. The level of excitation has been discussed by Basco and Norrish;43a it follows more or less the earlier mentioned theories of P~lanyi,~ Smith,*' and Bunker.48 The significance of this for the reverse reaction of the product PQ is that vibrational excitation of these species as reactants may enhance reactivity ; lol ( a ) H.M. Frey and G. B. Kistiakowsky J . Amer. Chem. SOC. 1957 79 6373; (b) J. N. Butler and G. B. Kistiakowsky ibid. 1961 83 1324; (c) D. W. Setser J. W. Simons and B. S. Rabinovitch Bull. SOC. chim. befges 1962 71 662. lo* H. M. Frey J . Amer. Chem. SOC. 1960 82 5947. RARINOVITCH AND FLOWERS CHEMICAL ACTIVATION 151 but the favoured path in reaction phase space at least when P is a light atom and in some cases when it is not is the one which has usually been assumed in bimolecular processes namely that involving a high degree of excitation of relative motion of the reaction partners.Exceptions occur enhanced cross-section for excitation of the ZI = 4 state of OH* in the reactions c s H+O3-+O27 +OH* corresponds to resonance excitation of 02(1C,+). This implies a special efficiency of the reverse process at this level of vibrational excitation of OH. Blais and Bunkel"L8 have calculated the behaviour expected in these types of systems for particular interaction potentials. For certain conditions PQ is highly excited if P is heavy. RS is not highly excited as based on the evidence from the reaction of H atoms with NOCl. In other reactions of this type little or no evidence has been obtained for excitation of RS. This is also in accord with the $re- viously mentioned theories. Vibration-vibration energy transfer collisional transition probabilities may be measured and have been found to be relatively high especially at near-resonance.Vibration-translation and rotation-translation probabili- ties may also be determined. The results for small molecules support the predictions of the Landau-Teller theory and variants These studies provide a valuable and direct method for obtaining information about specific collision-induced transitions. Reactivity of products may be enhanced as a consequence of their excitation but to date this is not a well-established or important pheno- menon. Various future studies of many types would be useful. Enough data for good test of theory of reaction are not even available for the system P + QR. The careful study of more systems of the type P + QRS with production of RS would be useful in order to provide more information regarding the reaction hypersurface in such systems.The study of the de- activation efficiencies of inert gases for a molecule in various vibrational states (e.g. the deactivation of 02* which may be produced in different energy distributions using different activation reactions) would be valuable. B. Bimolecular Association Reactions.-These systems provide a method of forming excited species with very small dispersion of energy. The minimum excess of energy EL,,,. may be varied depending upon whether or not the same bond is broken in reaction as is formed in activation. Thus for the system H + C2H4 + *C2H6* E:,,. = 0 the same bond is broken. While for the reaction E:,,, = 7 kcal. mole-' H + CH,CH=CH*CH + s -CpHg +*CH + C3H6, 152 QUARTERLY REVIEWS a different bond is broken; E;,.is a function of the two bonds involved thus H + CH3.CH=CHCH,Br -+ CH,CH,kHCH,Br* -+ but-1-ene + Br I?;". - 25 kcal. mole-' Considerable flexibility of energetics and bond type is possible in such systems. Where more than one type of bond rupture may occur comparative dissociation processes may be accurately studied . H + C-C-C=C-C-C -+ C-C-6-C-C-C + + C=C-C-C-C or +c-6 + c=c-c-c Daintonlo3 has summarised some of the useful characteristics of chemical activation techniques. A Marcus-Rice model for the calculation of unimolecular reaction rates coupled with the assumption that all vibrational and internal rotational degrees of freedom are active has been found to yield good agreement with experiment.Following from this is the conclusion that intramolecular energy relaxation in highly excited anharmonic species is very rapid. These systems are well adapted to detailed study of isotope and quantum- statistical effects. Careful studies should yield information on the nature of the distribution of collisional transition probabilities. The assumption of unit collisional deactivation efficiency in the case of complex molecules may not be much in error for practical purposes. This activation technique possibly provides the best experimental conditions for the study of energy transfer by highly vibrationally excited complex species ; they complement the information that can be obtained for small radicals and molecules from exchange reaction systems. This work was supported by the U.S.Office of Naval Research. lo8 F. S . Dainton Pure and Applied Chem. 1962 5 311. lo4 S. W. Benson J. Chem. Phys. 1964,40 105. @ G APPENDIX 8 TABLE A. Unimolecular isomerisations and decompositions. 2 c1 9 Reaction Activation with respect to Evidence for Refs. 1 Methylcyclopropane - butenes* cis-trans Isomerisation of but-Zene 28 29 29 s 30 $ 16 26 27 chemical activation z 2 cis- 3 trans- 1,2-Dimethylcyclopropane* Reverse reaction # + pentenes* Structural isomerisation # 3 cis- -+ trans-l,2-Dimethylcyclobutane* Reverse reaction # 4 cis-olefin 3 trans-olefin* Reverse reaction # 5 cis-RN=NR -+ trans-RN=NR* Reverse reaction # 7 Dimethyldiazirine + Me2C:* + N Rearrangement to CH,CH = CH proposed mechanism 34 g 9r .. 33 8 6 N3H - NHt(3n) + N2 Emission emission from NH(3n) 8 Ethylene oxide -f CH3-CHO* Decomposition of acetaldehyde decomposition products 104 0 9 1 Italicised references in the Appendix refer to citations in the main text.# Cases where it would be predicted that further reaction would occur if reaction were carried out at sufficiently low pressure. Reactions5 1 H + 0 2 - H 0 2 * 2 H t *OH-H20* H $- .OH -+ H2O + h~ 3 H + N O - + H N O t -+ HNO* 4 H + RNH -+ RNH2* 5 H + .CH3-+CH4* n TABLE B . Bimolecular combinat ions. $ cn P -AH+' Evidence for chemical Reactions of activated Refs. (kcal./mole) activation molecule (other than deactivation or reverse reaction) 47 Decomposes unless deactivated (d.u.d.). 1.r. emission 1 18 d.u.d. EIectronic excitation of sodium lead and thallium 1 18 Either radiative combination or emission from H20 49 d.u.d.Emission in red and i.r. 85 d.u.d. 102 d.u.d. Deuterium exchange 1 55,2 3 A 4 C2H5-NH2* 4C,H5+*NH2 F 3 E Decomposition e.g. 5 4 C H 3 + *CH2-N H -+ C2H5N + H2 6 2 s! vl $ Isotopic labelling is not specifically designated but was used in many cases cited. 0 Reactant atoms are in ground electronic state unless otherwise indicated. # Approximate heat of reaction; El must be added to obtain level of activation of products. ( a ) D. E. Hoare and A. D. Walsh Trans. Furuday Soc. 1957 53 1102; (b) L. I. Avramenko and R. V. Kolesnikova Proc. Acad. Sci. U.S.S.R. (Phys. Chern.) 1961,140,750; R. R. Baldwin and C. T. Brooks Trans. Faraday SOC. 1962,58 1782; R. N. Dixon and B. F. Mason Nature 1963 197 1198. (a) 0. Oldenberg and F. F. Rieke J. Chern. Phys. 1939 7,485; E. M. Bulewicz and T.M. Sugden Trans. Faraday SOC. 1958 54 1855; (b) P. J. Padley and T. M. Sugden Proc. Roy. SOC. 1958,248 A 248. P. J. Padley Trans. Faraday SOC. 1960 56,449. J. K. Cashion and J. C . Polanyi J. Chern. Phys. 1959 30 317; M. J. Y . Clement and D. A. Ramsay Cunad. J . Phys. 1961 39 205; M. A. A. Clyne and B. A. Thrush Trans. Faraday SOC. 1961 57 1305 Discuss. Faruday SOC. 1962 33 139; R. Simonaitis J. Phys. Chern. 1963 67 2227; 0. P. Strausz and H. E. Gunning Trans. Faraday SOC. 1964,60 347. A. N. Wright J. W. S. Jamieson and C. A. Winkler J. Phys. Chent. 1958 62 657. R. Berisford and D. J. LeRoy Cunud. J . Chern. 1958 36 983. TABLE B.-continued Reactions§ 9 H + C3H6 -+ n -GH,* H + C4Hs-2 --t CqHg* -+ iso:GH7* 10 + C4H,-l -+ C4H9* 11 0 + so-+ SO,? -H# Evidence for chemical Reactions of activated Refs.(kcal./mole) activation molecule (other than deactivation or reverse reaction) 98 d a d . - ZCH3 7 8 7 93 d.u.d. -CHQCHZ + CH 7a 8 38 cis-trans-Isomerisation of 8 207 t2H2] ethylene. Hydrogen- 82 9 deuterium exchange of ole- fin. Product dependence on pressure 35 d.u.d. Prod. depend. on -CH3 + C2H4 85 9b press 10 37-38 d.u.d. cis-trans-Isomerisation -+ CH,. + C,H 19,94 115 Emission (continuum in the I 1 39 of but-Zene. Prod. depend. -t CH3CH + C,H4 on press. visible) ' (a) E. Gorin W. Kauzman J. Walter and H. Eyring J . Chem. Phys. 1939 7 633; (b) B. deB. Darwent and E. W. R. Steacie ibid. 1948 16 381. O (a) S. Toby and H. I. Schiff Cunad. J. Chem. 1956,34 1061; (b) P. J. Boddy and J. C. Robb Proc. RoySoc. 1959 249 A 518; A. H. Turner and R. J. CvetanoviC,J.Chern. Phys. 1959,37 1075; K. R. Jennings and R. J. CvetanoviC ibid. 1961,35 1233; J. H. Current and B. S. Rabinovitch ibid. 1963 38 783 1967. lo W. J. Moore,J. Chern. Phys. 1948,16,916; M . D. Scheer and R. Klein J. Php. Chern. 1961,65,375; W. E. Falconer B. S. Rabinovitch and R. J. CvetanoviC J . Chem. Phys. 1963,39,40. l1 R. G. W. Norrish and A. P. Zeelenberg Proc. Roy. Soc. 1957 240 A 293. C. A. Heller and A. S. Gordon J. Phys. Chem. 1960 64 390. i 4 2 in .. Reactions5 CL TABLE B.-continued ch a? - AH# Evidence for chemical Reactions of activated Refs. (kcal./mole) activation molecule (other than 12 0 + NO -+ N02t (C state) 71 13 0 + NO + NO3* 14 0 + 0 2 + 0 3 * 15 0 3- CO 3 CO,t(triplet) -+ CO,* (singlet) 50 24 127 16 O(1D) + C02 - C03* pent-2-ene -+intermediatet 17 0 + ethylene pr opylene butenes butadiene other olefins 80-90 i -1 30 18 O(lD) + C4H -+ epoxide* deactivation or reverse reaction) Emission (“air afterglow”) NO,t(C) - NO,t(B) NO27 + NO2 + hv d.u.d.Isotopic exchange d.u.d. d.u.d. Emission in visible. CO,t(triplet) -+ CO,t(singlet) CO,t(singlet) -f C02 + hv d.u.d. Isotopic exchange d.u.d. Prod. depend. on -+ epoxide press + carbonyl compound -+ other rearrangement or fragmentation 37a 12 13 37,46a 12b ,Q 73 4 14 4 E s 3 53 54 2 F 57a 15 m Prod. depend on press. -+ carbonyl compound 576 -+ fragmentation products l2 (a) H. P. Broida H. 1. Schiff and T. M. Sugden Trans. Faraday Soc. 1961 57 259; (6) D. Garvin P. P. Gwyn and J. W. Moskowitz F. E. Blacet T. C. Hall and P. A. Leighton J. Amer. Chem. Soc. 1962,84,4011; M. A. A. Clyne and B.A. Thrush Trans. Farahy Soc. l4 R. J. CvetanoviC Chem. in Canada 1963 15 No. 4 56. l6 R. J. CvetanoviC J. Chem. Phys. 1956,25,376; 1960,33,1063; Canad. J. Chem. 1955,33,1684; 1958,36,623; S . Sat0 and R. J. Cveta- Canad. J. Chem. 1960,38,1795. 1962,58,511; J. Chem. Phys. 1963,38 1252; S . W. Benson ibid. 1963,38 1251. novie ibid. 1958,36,279,970; 1959,37,953; R. J. CvetanoviC and L. C. Doyle ibid. 1960,38,2187. TABLE B.-continued - AH# Evidence for chemical Reactions of activated (kcal. /mole) activation molecule (other than deactivation or reverse reaction) Prod. depend. on press. - decomposition products Reactions§ 19 N + ethylene 1 } -+ intermediate* prop ylene butenes J 20 X + oleh + Xolefin* X = F C1 Br I olefin = dihalogeno-olefin or but-2-ene 21 c1 + 0 2 + c102* 22 RS- + but-Zene - RSC,H,* -1 5( ?) 23 RR’P.+ but-2-ene- RR’PC,H8* Olefin isomerised or exchanged d.u.d. C1 catalysed isotopic scrambling in 0,. 0 cata- lysed recomb. of C1 atoms Olefin isomerised Olefin isomerised Refs. r 16 s z s g c3 0 X 14 15 u 17 crj m 3 .. 0 18 g E5 19 20 (3 F l6 (a) H. G. V. Evans G. R. Freeman and C. A. Winkler Canad. J. Chem. 1956,34 1271; (b) W. G. Zinman J. Phys. Chem. 1960,64 b ’’ (a) R. G. Dickenson R. F. Wallis and R. E. Wood J. Amer. Chem. Suc. 1949 71 1238 (references to earlier work cited here); P. B. 2 Ayscough A. J. Cocker and F. S. Dainton Trans. Furaduy SOC. 1962,58,284; N. C. Craig and E. A. Entemann J. Amer. Chem. SOC. 1961 $ 83 3047; (b) H . Komazawa A. P. Stefani and M . Szwarc ibid. 1963 85 2043; (c) N. Colebourne and R. Wolfgang J. Chem. Phys.1963 3 38 2782. z lD R. H. Pallen and C. Sivertz Canad. J. Chem. 1957 35 723; C. Walling and W . Helmreich J. Amer. Chem. SOC. 1959 81 1144; C. Sivertz J. Phys. Chem. 1959 63 34. 2o J. Pellon,J. Amer. Chem. Suc. 1961 83 1915. 1343; J. T. Herron J. Chem. Phys. 1960,33,1273. 4 R. A. Ogg jun. J. Chem. Phys. 1953,21,2078; S. W. Benson and J. H. Buss ibid. 1957,27 1382. c VI 4 Reactions5 24 Re + olefin --f radical* R. = 'CH3 'CD3 *CF3 24a R. + acetylene -+ radical* R. = i-propyl or t-butyl 26 S(l0) + C2H4 -+ CH2CH2* 25 NO + C2H2D2 -+ NO.C2H,D2* \/ S S(l0) + C3HG -+ CH&H-CH2* \ / S S(l0) + alkanes -+ mercaptans* 27 :CH2 + CH2:CH2 --f cyclopropane* 27a :CH2 + C2H,F4- -+ fluorocyclopropane* c-r v1 00 TABLE B.-continued - AH# Evidence for chemical Reactions of activated Refs.(kcal./mole) activation molecule (other than deactivation or reverse reaction) 25 Proposed intermediate in -f olefin' + R' displacement or olefin iso- merisation reaction CH3C0 C1 R' = CD3 CH3 30 Proposed intermediate -f possible rearrangement - 27 Olefin isomerised 73 Kinetic characteristics d.u.d. Prod. depend. on press. -85 d.u.d. Prod. depend. on press. - CH3CH =CH2 -+ geometrical isomer d.u.d. -+ fluoropropylenes 26 17b 21a 21 b 21 0 54 s 22 z 23a 32 lOla 23b 21 (a) J. N. Pitts R. S. Tolberg andT. W. Martin J. Amer. Chem. SOC. 1954,76,2843; R. E. Varnerin ibid. 1955 77 1426; J. R. McNesby and A. S. Gordon ibid. 1957,79,5902; P. Kebarle and W. A. Bryce Canad. J. Chem. 1957,35,576; J. N. Pitts jun. D. D. Thompson and R. W. Woolfolk J. Amer. Chem. SOC. 1958 80 66; J. H. Binks J.Gresser and M. Szwarc J. 1960 3944; (b) J. A. Garcia Dominguez and A. F. Trotman-Dickenson J. 1962,940. z3 (a) H. M. Frey J. Amer. Chem. SOC. 1957 79 1259; B. S. Rabinovitch E. Tschiukow-Roux and E. W. Schlag ibid. 1959 81 1081 ; (b) B. A. Grzybowska J. H. Knox and A. F. Trotman-Dickenson J. 1963,746. 0. P. Strausz and H. E. Gunning J. Amer. Chem. SOC. 1962,84,4080; Chem. in Canada 1963 15 No. 4 58. TABLE B .-con t inued - AH# Evidence for chemical Reactions of activated (kcal./mole) activation molecule (other than Reactions5 28 :CHz + CH,CH=CH -+ CHSCH-CH2* -85 \ / CH2 29 :CH2 + but-2-ene -+ dimethylv * -8 5 + isobutene2 30 :CH2 + CHECH -+ CH=CH* \ / CH2 31 :CH2 f CH2=C=CH2 - CHZ=C-CH,* \/ CH2 -+ CH,CH=C=CH2* 32 :CH2 + CH2=CHCH=CH2 -+ deactivation or reverse reaction) d.u.d.Prod. depend. on press. --f all isomeric butenes d.u.d. Prod. depend. on press. -f structural isomers - geometrical isomer By analogy -+ CH,=C=CH (no CH=CH isolated) - CH,C_CH \ / CH2 Prod. depend. on press. CHECH + C2H structural isomers structural isomers - pentadienes Prod. depend. on press. cyclopentene* 24 J. H. Knox and A. F. Trotman-Dickenson Chem. and Ind. 1957 1039. 25 H. M. Frey J. Amer. Chem. SOC. 1958 80 5005; Proc. Roy. SOC. 1959 A 250 409; J. H. Knox and A. F. Trotman-Dickenson J. 26 H. M. Frey Chem. and Ind. 1960 1266. 1958 2897. 27 H. M. Frey Trans. Faraday Sac. 1961 57 951. c 111 \o H. M. Frey Trans. Furaday SOC. 1962 58 516; B. Grzybowska J. H. Knox and A. F. Trotman-Dickenson J. 1962 3826. Reactions5 33 34 35 36 37 38 39 40 :CH2 + CM2CO -+ CHs-CO* \ / CH2 :CH2 + H --t CH4* :CH2 + CH*-+CzHG* :CH2 + CH2-CH2 4 CH3CH-CH2* \ / CH2 \ / CH2 I / :CH2 + CHZ-CH + CH,CH-CHZ* CHZ-CHZ I I CHZ-CH :CH2 + C02 -+ CHz-CO* :CH2 + CO -+ CH2CO* \ / 0 CH3.CH + CH3*CH =CH2 -+ CH3CH-CHCH3* (cis and trans) \/ CH2 c-r TABLE B.-continued - AH# Evidence for chemical Reactions of activated Refs.(kcal. /mole) activation molecule (other than 103 87 91 -9 1 72 Proposed intermediate Proposed mechanism Proposed mechanism Prod. depend. on press. Prod depend. on press. Proposed mechanism Proposed mechanism Assumed by analogy deactivation or reverse reaction) - C2H4 + CO 29 40 30 - -CH3 + H - 2CH3 40 31 + all isomeric butenes 100 lOla,b ~ s 24 w 5 CA ? 33 -CO + HCHO 34 35 29 D. A. Semenow E. F. Cox and J. D. Roberts J. Amer. Chem. SOC. 1956 78 3221; G. B. Kistiakowsky and K.Sauer ibid. 1956 78 5699; G. B. Kistiakowsky and P. H. Kydd ibid. 1957 79,4825; W. B. DeMore H. 0. Pritchard and N. Davidson ibid. 1959,81 5874. 30 J. Chanmugan and M. Burton J. Amer. Chem. SOC. 1956 78 509. s1 F. Grard and M. Vanpet Bull. SOC. chim. belges 1951,60,208. 32 H. M. Frey Trans. Furaduy SOC. 1960 56 1201. ss G. B. Kistiakowsky and W. L. Marshall J . Amer. Chem. Soc. 1952 74 88; T. B. Wilson and G. B. Kistiakowsky ibid. 1958 80 2934. 34 G. B. Kistiakowsky and K. Sauer J. Amer. Chem. SOC. 1958 80 1066. s5 H. M. Frey Chem. and Ind. 1962 218; J. 1962,2293. TABLE B.-continued React ions§ 41 :CCO( ?) + - AH# Evidence for chemical Reactions of activated (kcal. /mole) activation molecule (other than deactivation or reverse reaction) C2H4 - CH2CH2* Proposed mechanism + CH,CH,* \ / 'JI C \ / cco CH2=C =CH2 or CH,C=CH 42 43 44 CH3 + CH3 3 C2H6* CH3 + NO - CH3*NO* CH3 + O2 -+ CH302* 83 Observed pressure dependence -51 d.u.d.-5 d.u.d. - HCHO + OH 45 CH3 + (CF,),CO -+ (CF3)2CO*CH3* Proposed intermediate in + CF&OCH + .CF displacement reaction 46 Re + NO2 --t RN02* -57 d.u.d. -+ RONO* -57 d.u.d. -RO+NO R* =(CI-C,) 36 K. Baves. J. Arner. Chern. SOC.. 1962.83. 3712 1962.84.4077 R. T. Mullen and A. P. Wolf. ibid.. 1962. 84. 3214. Refs. N 2 8 2 5 36 0 x v la 37 S la 38 $ la 38a .. 39 42 3 : 2 U * 4 37 D. M." Miller and E. W. R. Steacie,'J. Chern.'Phys.; 1951,19,73; R. Gomer and G. B. Kistiakowsky,*ibid. 1951 19 85; K. U. Ingold and F. P. Losing ibid. 1953,21,368,1135; G. B. Kistiakowsky and E. K. Roberts ibid. 1953,21,1637; R. E. Dodd and E. W.R. Steacie Proc. Roy. SOC. 1954,223 A 283. 38 (a) W. C. Sleppy and J. G. Calvert J. Amer. Chem. SOC. 1959 81,769; (b) D. E. Hoare Canad. J. Chem. 1962 40,2012; B. W. Wojcie- chowski and K. J. Laidler Trans. Faraday SOC. 1963,59 369. 30 (a) N. R. Subbaratnam and J. G. Calvert J. Amer. Chem. Soc. 1962 84 1113; (b) M. Barber J. Farren and J. W. Linnett Proc. Roy. So& 1963,274 A 306 317 (a) R. A. Sieger and J. G. Calvert J. Arner. Chem. Soc. 1954 76 5197; (b) G. 0. Pritchard and E. W. R. Steacie Canad. J. Chem. E - 1957,354 1216. s Reactions3 47 48 49 50 51 52 53 1 41 R\ R. + CH,CN -+ CN* (R- = H CN CH, CF,) F + NO -+ NOF* R3N + BF - R3NBF3* NO2 + NO - N205* CH3/ ' NH3 + BF3 -+ NH3BF3* NO2 + NO 4 N204* *NF2 -+ .NF2 -+ N2F4* x + x 3 x2* X = atom e x . Br(2P3/2> + W2P,/,) - ~ r ~ ( ~ n ~ + ~ ) w o\ h TABLE B.-con timed - AH# Evidence for chemical (kcal. /mole) activation Proposed intermediate - 55 Emission in visible 28 d.u.d. -28 d.u.d. 13 d.u.d. 21 d.u.d. 19 d.u.d. d.u.d. Continuous emission Electronic excitation occurs as well as vibrational excitation. Reactions of activated Refs. molecule (other than deactivation or reverse reaction) + CH + RCN 40b 41 79,44 2 R 2 3 45 17 81 1 Vibrational excitation only. D. E. McEcheran M. H. J. Wijnen and E. W. R. Steacie Cunad. J . Chem. 1958 36 321. 42 D. Rapp and H. S. Johnston J. Chem. Phys. 1960 33 695. 43 G. B. Kistiakowsky and R. Williams J. Chem. Phys. 1955 23 334; S. H. Bauer in the press. 44 F. H. Verhoek and F. Daniels J. Amer. Chem. Soc. 1931 53 1250; W. F. Giauque and J. D. Ketnp J. Chem. Phys. 1938 6,40; T.45 R. A. Ogg jun. J. Chem. Phys. 1947 15 337 613; J. H. Smith and F. Daniels J. Amer. Chem. Soc. 1947 69 1735; R. L. Mills and Carrington and N. Davidson J. Phys. Chem. 1953 57 418. H. S. Johnston ibid. 1951 73 938. Reaction Cj 1 H + o~-+oH* + 0 2 H + O,+OH* + 02t 3 H+HOz-+OH*+OH 4 H+N02+OH*+N0 5 H+N,H+NHf+NH 6 H + Cl 3 HCl* + C1 D + Cl --+ DCl* + C1 7 H + Br + HBr* + Br 8 H + NOCl 4 HC1* + NO 9 H + F - t H F * + F TABLE C. -AH (kcal. /mole) 80 42 38 31 45 47 41 65 98 Bimolecular exchange reactions. Electronic state Level of vibra- of excited tional product excitation # G (ground state) v < 9 (75 kcal.) (35.5 kcal.) 1E; v = 4 G v < 3 G v < l In (27.6 kcal.) G G G G G v < 5 (36.8 kcal.) v < 5 v < 6 v < ( l o ? ) (63.6 kcal.) v < 9 (86 kcal.) Detection of Activation 1.r.emission Emission from lz; Higher proportion of OH* formation in v = 4 state 1.r. emission Emission lI7 3 lA 1.r. emission 1.r. emission 1.r. emission 1.r. emission 1.r. emission Refs. w z 87 3 5 5 87c e 46 44 33 13 88 0 7 2 iE E 886 R 48 3 0 .. > r 47 88b 91 cl 5 $ Reactant atoms are in ground electronic state unless otherwise indicated. 2 # Values in parentheses refer to the highest value of v cited. l6 J. K. Cashion and J. C. Polanyi J. Chem Phys. 1959,30 316; P. E. Charters and J. C. Polanyi Cunud. J. Chem. 1960 38 1742. 47 J. K. Cashion and J. C. Polanyi Proc. Roy. Soc. 1960 258 A 570. 48 B. A. Thrush 7th International Symposium on Combustion 1959 p. 243. c m w Reactions 10 0 + NO2 -+ 02* + NO 11 0 + c10 - 0 2 * + c10 12 0 + 0 -+ 0 2 * + 0 2 14 O('0) + H2 3 OH* + H O('0) + H,O 3 OH* + OH O('0) + HC1- OH* + C1 O('0) + NH3 - OH* + .NH2 O(l0) + CH 3 OH* + CH3 C2H + 0 - CHt + CO C2H + O(3P) + CHt + CO 15 or 16 CH* + O(3P) -+ Cot + H - AH (kcal./mole) 46 61 93 138 75 67 or TABLE C.-continued Electronic state Level of vibra- of excited tional product excitation # G v < 8 (34 kcal.) v < 12 (49 kcal.) G v < 8 (34 kcal.) G v < 17 (19) (66-5 kcal.) G v < 18,19 G v < 2 A ~ A B2C A l n (19.8 kcal). Detection of Activation U.V. absorption; non- Boltmann distribution 'o = 1 mainly U.V. ab- sorption U.V. absorption U.V. absorption 1.r. emission 02* can promote de- composition of O8 1.r. emission Possible reactions for production of CH* from which emission occurs in acetylene flames hv (vacuum U.V. emission) CO(AlI7) - CO(X'C) + c m P Refs.9Oa 90b 43a 90 lo 49 43p 46b 2 43a 46 49a T1 E 4 43b m 3 50 50 '@ (a) W. D. McGrath and R. G. W. Norrish Proc. Roy. Soc. 1957,242 A 265; (b) W. M. Jones and N. Davidson J. Amer. Chem. SOC. 1962 84 2868. C. W. Hand and G. B. Kistiakowsky J. Chem. Phys. 1962 37 1239. TABLE C.-continued Reaction3 17 N+NO-N,* + O 19 Br + O3 --t BrO* + 0 18 N + NC2H4 -t C2H4* + N2 20 c1 + 0 3 --t c10* + 0 21 22 X + Na - NaX* + Na s + S,Cl - s,* + SCI 23 Na + CIX --f NaX* + C1 24 Na + HgX -+ NaX* + H g - AH (kcal. /mole) 75 Br 39? I 53? c1 75 Br 71? Electronic state of excited product G G G G G G Level of vibra- tional excitation # >24 kcal. (8 kcal.) 2; < 5 (?) (11 kcal.) v = 5 - 1 2 (25 kcal.) At least 48.5 kcal. Detection of Activation Refs. r N2* + 03-+N2 + 0 + 0 45 Suggested reaction 16a 51 Absorption spectra 52 ?2 ..n Absorption spectra 53 ki "i --t 3c; L5 c? F 9 Sodium D line emission 9 70 2 71 2 b1 B. Dunford H. G. V. Evans and C. A. Winkler Canad. J. Chern. 1956,34,1074; R. Kelly and C. A. Winkler ibid. 1960,38 2514. 69 W. D. McGrath Z. phys. Chem. 1958 15 245. 63 W. D. McGrath J. Chem. Phys. 1960 33 297. L o\ wl CL Q\ TABLE C.-continued Reaction4 - AH Electronic state Level of vibra- Detection of Activation Refs. 25 26 27 28 29 (kcal. /mole) RX + Na-R + NaX* (R = olefin or cyclic compound) NO + O3 -+ NO2* + O2 49 C(3P) + CH(,n) 61 c2t + W2S) C(3P) + CCl(W) -+ c,t + c1 CH3CH + CH3CHN2 --+ tr~n.~-C,Hs* + N2 CH3 + 0 2 -+ CH,O + OH 43 of excited product G High vibrational electronic ground state or metast- able electronic state A3.ZIg G tional excitation# At least Sodium D line emission 72 48.5 kcal.Emission 54 (5900-10,850 A) lo w 2 3 Swan emission band 55 R E 2 5 v < 2 v < 8 ZI = 7,8 preferentially excited cc Isomerisation to 35 cis-C4H8* unless deactiv- ated Proposed reaction cf. 39b Table B. reaction 44 64 J. C. Greaves and D. Garvin J. Chem. Phys. 1959,30,348; M. A. A. Clyne B A. Thrush and R. P. Wayne Trans. Faraday SOC. 1964 66 H. B. Palmer and W. J. Miller J. G e m . Phys. 1963 38 278. 60,359. TABLE D. Terniolecular reactions. Reaction Evidence for chemical activation Refs. 1 H + H $ - M - + H + M Third body requirement 56 2 H + H + N a + H + N a t Na D-line emission 2b 57 3 5 N + N + M - + N + M Third body requirement 75 76 3 5 7 8 O + O + M - + O f M Third body requirement 52d 61 9 Br + Br + M -Br2 + M Third body requirement 51,526,62 u 10 I + I + M - + I + M Third body requirement 51,52a-c 1 1 H + C1 + M + HCI* $- M Emission from HCl* 886 12 w H + H + Tl(or Pb) - H + Tlt (or Pbt) Emission from TI (2S1/2) [or Pb(3P,) J 53 P 4 H + H 4 S24H2 + SZt Emission from S2t 58 z 6 N + N + 0 4 N 2 + O(%) Emission from O(lS) 59 !? cl N + N + TlX --t N i- X + Tlt Emission from Tl? 60 ( X = F C1 Br I) ?! Emission from Tlt (or Pbt) 55 3 E 13 H + Br + M 3 HBr* + M Emission from KBr* 47 ..14 H -I- Br + T1+ HBr + Tlt Emission from Tlt 55 Emission from NOT 63 ti 16 N+Br+M-NBrl. + M Emission from NBrt 64 5 M. M. Sutton and A. Williams Discuss. Faraday SOC. 1962,33,205. H + C1 + T1 (orPb) - HCl + TIT (orPbT) c 15 N + O + M + N O t + M 0 $ (Reactions given are not intended to represent the mechanism necessarily,) 56 E.M. Bulewicz and T. M. Sugden Trans. Furaday Soc. 1958 54 1855; J. P. Rink J. Chem. Phvs. 1962 36 262; G. Dixon-Lewis 58 T. M. Sugden and A. Demerdache Nature 1962 195 596. 'l S. R. Byron J. Chem. Phys. 1959 30 1380; J. E. Morgan L. Elias and H. I. Schiff ibid. 1960 33 930; C. B. Kretschmer and H. L. Peterson ibid. 1960,33,948; M. Camac and A. Vaughan ibid. 1961,34,460; J. P. Rink H. T. Knight and R. E. Duff ibid. 1961 34 1942. 62 E. Rabinowitch and W. C. Wood Trans. Faraday SOC. 1936 32 907; W. G. Givens and J. E. Willard J. Amer. Chem. SOC. 1959 81 4773; G. Burns and D. F. Horning Canad. J. Chem. 1960,68,1702. 63 Y . Tanaka J. Chem. Phys. 1954,22,2045; F. Kaufman and J. R. Kelso ibid. 1957,27,1209; C. A. Barth W. J. Schade and J. Kaplan ibid.1959,30 347. P c 2 4 P 5 2 57 K. F. Bonhoeffer Z. phys. Chem. 1925,116 391; J. C. Polanyi and C. M. Sadowski J. Chem. Phys. 1962,36 2239. 59 J. Kaplan W. J. Schade C. A. Barth and A. F. Hildebrandt Canad. J. Chem. 1960 38 1688. " L. F. Phillips Canad. J. Chem. 1963 41 732. w rn - 64 E. R. V. Milton and K. B. Dunford J. Chem. Phys. 1961,34,51.

ISSN:0009-2681    

DOI:10.1039/QR9641800122

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

PHOSPHONITRILIC DERIVATIVES AND RELATED COM- POUNDS By N. L. PADDOCK (UNIVERSITY of MANCHESTER) THE essential structural characteristic of phosphonitrilic derivatives is the formally unsaturated repeating unit (I) which occurs in a large number 4 .. (I) of cyclic and linear molecules. The best-known phosphonitrilic compound is the trimeric chloride (11) which was discovered by Liebigl in an attempt to prepare the amides of phosphoric acid though the correct empirical and molecular formulae were not established till later. Stokes3 showed that the series (NPCI,) extended at least as far as the heptamer and suggested cyclic formulae which have been confirmed in many ways. The method of preparation of the chlorides has since been improved and related pro- cedures have been used for alkyl- aryl- and bromo-phosphonitriles.Substitution reactions especially of the trimeric chloride have been ex- plored in detail and have raised interesting questions of positional geometrical and configurational isomerism. In the fluoride series cyclic structures persist up to at least (NPF,),, rivalling the organic macrocyclic compounds in size though not yet in detailed chemistry. The polymerisa- tion of the phosphonitrilic halides has also been studied rubber-like solids of high thermal stability being obtained; both thermal and chemical stability are sensitive to substitution. The physical and chemical properties of phosphonitrilic derivatives depend on their electronic structures. Theo- retical studies show that although such structures as (11) are formally the same as the KekulC structures of benzene there is an essential difference because d-orbitals are necessarily involved in the formation of double bonds between quinquevalent phosphorus and nitrogen.The valency problem is more complicated than in carbon compounds because of the wider variety of d-orbitals and the different symmetries of p - and d-orbitals. The theo- retical work is however sufficiently advanced for useful comparisons to be made with experiment for both phosphonitrilics and for other series in which pr-dr bonds make some contribution to the structure notably siloxanes polyphosphates and thiazyl compounds. Among a number of reviews,* that by Haber*“ is valuable for its infor- Liebig Annalen 1834 11 139. 2 Gerhardt Cumpt. rend. 1846,22,858; Gladstone and Holmes J. 1864,17,225. a Stokes Amer.Chem. J. 1897 19 782. 4 (a) Haber “Inorganic Polymers” Chem. SOC. Special Publ. No. 15 London 1961 p. 115; (6) Shaw Fitzsimmons and Smith Chem. Rev. 1962,62,247; (c) Schmulbach “Progress in Inorganic Chemistry,” Interscience New York 1962 Vol. 4 p. 275; (d) Audrieth Steinman and Toy Chem. Rev. 1943 32 109; (e) Paddock and Searle “Advances in Inorganic Chemistry and Radiochemistry,” Academic Press New York 1959 Vol. 1 p. 347;(f)Gribova and Ban-Yuari Russ. Chem. Rev. 1961,30,1. 168 PADDOCK PHOSPHONITRILIC DERIVATIVES 169 mation on recently-developed preparative methods and the most recent by Shaw Fitzsimmons and B. C. Smith,4b and by Schm~lbach,~~ for their extensive coverage of the literature and their detailed numerical information. All three contain references to other reviews.The chemistry of phosphonitrilic derivatives is extensive and perhaps for that reason is often considered in isolation though many of the relevant ideas are applicable to the chemistry of other systems. Our object here therefore is to outline the main preparative methods of phosphonitri- lic chemistry to discuss the theoretical concepts and their experimental basis and to illustrate the application of the ideas to other properties of phosphonitrilic derivatives and to groups of related compounds of silicon phosphorus and sulphur. Preparative Methods Formation of the P=N Grouping-The simplest compounds containing the phosphonitrilic grouping (I) are the phosphinimines R3P :NR' in which R R' may be alkyl aryl or halogen. They can be synthesised by the reaction of organic azides with tertiary phosphines ;5 R3P + R'N -+ R3P-N3R' -+ R3P:NR' + N2 (1) The rate of reaction of a series of aromatic azides R'N with triphenyl- phosphine6 increases in the order R'=p-Me,N-C6H, C6H5 p-N02.C& suggesting that the reaction involves nucleophilic attack of the phosphine on nitrogen.Another method depends on the decomposition of the conjugate acids of phosphinimines the aminophosphonium ions [R,P-NHR' I+. The salts can be prepared (a) by the reaction of chloramine,' methyl~hloramine~ or hydroxylamine-o-sulphonic acid7"p8 with triphenylphosphine Ph3P + ClNH2 + [Ph3P*NH2]+C1- (2) or (b) from the reaction of triphenylphosphine dihalides with ammonia or primary aliphatic or aromatic amines. Ph3PBr + 2RNH2 -+ [Ph3P-NHR]+ + RNH$ + 2Br- (3) They can also be obtained by the interaction of triphenylphosphine with hydrazoic acid5 Ph3P + 2HN3 -+ [Ph3P-NH2]+N3- + N,.(4) (a) Staudinger and Meyer Helv. Chim. Acta 1919,2,635; (6) Staudinger and Hauser ibid. 1921 4 861. Horner and Gross Annalen 1955 591 117. (a) Appel Angew. Chem. 1959 71 374; (6) Sisler Sarkis Ahuja Drago and N. L. Smith J. Amer. Chem. SOC. 1959,81,2982; (c) Appel and Hauss Chem. Ber. 1960,93 405; (d) Sisler Ahuja and N. L. Smith J. Org. Chem. 1961,26 1819. * Appel Buchner and Guth Annulen 1958,53 618. Horner and Oediger Annalen 1959 627 142. 170 QUARTERLY REVIEWS Aminophosphonium salts are comparatively stable in water and the anions can be exchanged readil~.~~t* The N-aryl salts can be deprotonated with t riet hylainine ( 5 ) but the analogous formation of triphenylphosphinimine itself requires such stronger bases as sodamide in liquid a m m ~ n i a ~ ~ l ~ or magnesium h ~ d r i d e .~ ~ The P-N bond in phosphinimines is polar in the sense P+-N-. Those formed from tertiary aryl phosphines and Chloramine-T take up water if the aryl group carries electron-withdrawing substituents ;I1 the product can be formulated either as an aminophosphonium hydroxide or a derivative of quinquecovalent phosphorus. [R,P.NHAr]+ + Et,N -+ R,P:NAr + [Et,NH]+ Ph3P.N.S02CgH4-CH3 + H20 -+ Ph3P(OH)*NH.S02.C6H4.CH (6) Triphenylphosphinimine itself re-forms aminophosphonium salts with both hydrogen halides5 b ~ 1 2 and alkyl halides,12 and like the iso-electronic phosphine oxides R,PO and methylene-phosphoranes R3P :CH, it forms complexes with ions of the transition rnetals,l3 e.g. (PH,P-NH),CoCl 2.Triphenylphosphinimine reacts with halogens to give N-halogeno-phos- phinimine~,~~,~* (7) and N-bromotriphenylphosphinimine analogously to chloramine reactsI2 with triphenylphosphine (8) 2Ph,P:NH + Br -+ Ph,P:NBr + [Ph3P.NH2]+Br- Ph,P:NBr + Ph3P -+ [Ph,P:N:PPh,]+Br- The same product is obtained by the reaction of triphenylphosphine dibromide with triphenylphosphinimine Ph,PBr2 + 2Ph,P:NH -+ [Ph,P:N:PPh,]+Br- + [Ph,P-NH ]+Br- (9) In the last reaction the formation of a linear P-N-P system is limited to a triatoinic species by the phenyl groups. If however the phosphorus atom carries e.g. a chlorine atom which is stable as an anion the condensa- tion process can continue with the formation of polymeric phosphinimines better known as phosphonitrilic derivatives.* The general methods for the preparation of phosphinimines are therefore also appropriate for phos- phonitriles.Linear phosphonitriles have also been prepared by the action of e.g. phosphorus trichloride on tetrasulphur tetranitride a reaction * Another system of nomenclature which recognises the unsaturation of the P-N grouping has been pr~posed.~b Linear and cyclic phosphonitriles are consistently named phosphazenes with appropriate prefixes. l o Appel and Hauss Angew. Chew. 1959 71 626. l1 Mann and Chaplin J. 1937 527. l a Appel and Hauss 2. anorg. Chem. 1961 311 290. l3 Appel and Schaaf 2. Naturforsch. 1961 16b 405. 14 Appel Hauss and Buchler 2. Naturforsch. 1961 16b 405. PADDOCK PHOSPHONITRILIC DERIVATIVES 171 without a parallel in the phosphinimine series (see Ref. 4a for details).The use of azides in the preparation of phosphonitriles is comparatively recent.4a The general reaction is R2PX + MN3 -+ l/n (NPRJ + N + MX (10) in which X is a halogen and M is an alkali metal preferably lithium. In published examples R = Br Ph Clf5 or CF3;16 the substituents on the phosphorus atom need not be identical and the method is probably widely applicable. The product in each case is predominantly highly polymeric though the first related compound of arsenic (NAsPh,) is prepared in this way,17 and is probably cyclic. Phosphonitrilic derivatives are more commonly prepared by the ammo- nolysis of the pentahalides of phosphorus or their substituted derivatives the most familiar of such reactions being that between phosphorus penta- chloride and ammonium ~hloride,~ usually carried out in an inert solvent such as tetrachloroethane.18 The simple equation PCl + NH4C1 -+ l/n (NPC12)n + 4HC1 (1 1) conceals some complexity ; according to conditions either cyclic or linear products can p r e d ~ m i n a t e .~ ~ ~ ~ The covalent form of phosphorus pentachloride in solution is in equilibrium21 with the ions PCl,+ + PC1,- and derived ionic intermediates are important in its reaction with ammonium chloride. In tetrachloro- ethane the electrical conductivity of the solution increases by a factor of about 5000 to a maximum near half-reaction,22 when the average composi- tion of the dissolved reaction mixture is NPCl,-PCI,. At this stage only traces of cyclic polymers are present in the solution. During the completion of the reaction the conductivity falls continuously with the conversion of the ionic intermediates to the cyclic chlorides.The first half of the reaction (only) is accelerated by the use of highly polar solvents such as nitro- benzene which facilitate charge separation. The first reaction scheme to be suggestedlg involved the formation of ammonium hexachlorophosphate and its dehydrochlorination to P-tri- chlorophosphinimine - 2HC1 - HCI NH4CI + PCI -+ NHdPC16 ----+ NHZ'PCI4 -+ NH:PCI3 (12) followed by successive reactions with phosphorus pentachloride and l5 Herring Chem. and Ind. 1960 717. l6 Tesi Haber and Douglas Proc. Chem. Sac. 1960 219. l7 Reichle Tetrahedron Letters 1962 5 1 . l8 Schenck and Romer Chem. Ber. 1924,574 1343. 2o Lund Paddock Proctor and Searle J. 1960 2542. 21 Payne J . 1953 1052. 22 Unpublished work from laboratories of Messrs.Albright and Wilson ; Paddock Proctor Thesis London 1958. 135th Nat. Meeting A.C.S. Boston 1959. 172 QUARTERLY REVIEWS ammonium chloride. The characteristic products of the reaction are then expected to be the linear derivatives PCl,(NPCl,),Cl and H(NPCl,),Cl the cyclic chlorides being formed from the latter compounds by intra- molecular elimination of hydrogen chloride. Both types of linear derivative have been i ~ o l a t e d . ~ ~ ~ ~ ~ Recent work on the intermediates has shown that they are mainly ionic their chemistry following naturally from that of the phosphinimines discussed above. The (indirect) evidence for the participation of ammonium hexachlorophosphate has been summarised el~ewhere.~~ While amino- tetrachlorophosphorane NH,-PCl, the immediate precursor of trichloro- phosphinimine is no doubt too reactive to be isolated the analogous compound NH,.P(CF,),Cl is formed by chlorination of NH,.P(CF,) in chloroform at -3O"c and decomposes on warming to room temperature to give a mixture of the previously unknown trimeric and tetrameric derivatives [NP(CF,),],,, together with some high polymer.24 Hexa- (perfluoropropy1)triphosphonitrile has been obtained in high yield by a similar rea~tion.,~ Another amino-phosphorane NH2.PPh2Cl, is formed by the reaction of chloramine with chlorodipheiiylphosphine and can be dehydr~chlorinated~~ to the tetrameric diphenylphosphonitrile [NPPh,],.It is therefore likely that the formation of cyclic phosphonitriles in ammono- lysis reactions depends on the decomposition of compounds NH,.PR,Cl (R = C1 Br CF, alkyl aryl) perhaps predominantly as amino-phos- phonium salts [NH,.PR,Cl]+Cl- as suggested for the reaction of the phenyl deri~ative.,~ The evidence for the phosphinimine NH:PCl is again indirect but strong.Phosphorus pentachloride reacts with both amides26 and primary a ~ n i n e s ~ ~ s ~ ~ to give compounds of general formula RN :PCl, which are often dimeric. In polar solvents at low temperatures phosphorus penta- chloride and ammonium chloride react29 to give a compound P3NClI2 proved to have the structure [PCl,:N:PCl,]+PCl,- (cf. eqns. 8 9) by its nuclear magnetic resonance spectrum (N.M.R.) the suggested reaction path30 involving nucleophilic attack of NH :PCl on PC14+ (13) PC14+PCl,- + NH:PCl + [PCl,:N:PCl,]+PCl,- + HC1 At 150°c this salt reacts with more NH:PCl to give31 another cation [PCl,(NPCl,)Cl]+ + NH:PCl -+ [PCl,(NPCl&,Cl]+ + HCI (14) 23 Becke-Goehring and Koch Chem.Ber. 1959,92 1188. 24 Tesi and Douglas J. Amer. Chem. SOC. 1962 84 549. 25 Sisler Ahuja and N. L. Smith Znorg. Chem. 1962 1 84. 26 Kirsanov Khim. i Prim. Fos. Akad. Nauk S.S.S.R. Trudy l-oi Konferents 1955 Zhmurova and Kirsanov J. Appl. Chem. (U.S.S.R.) 1960 30 3044; Chem. Abs. Chapman Holmes Paddock and Searle J. 1961 1825. 29 Becke-Goehring and Lehr Chem. Ber. 1961,94 1591. 30 Becke-Goehrjng and Fluck Angew. Chem. 1962,74,382. 31 Becke-Goehring Fluck and Lehr Z. Naturforsch. 1962 17b 126. 99 (Publ. 1957); Chem. Abs. 1958,52,251. 19:/ 55 17551. PADDOCK PHOSPHONITRILIC DERIVATIVES 173 which is found in solution as its hexachlorophosphate. The latter com- pound had been recognised earlier32 by its empirical formula P2NCl, and thought to have the structure PCl,+NPCl,-; the structure given above is required by its electrical conductivity its apparent molecular weight and especially by its nuclear magnetic resonance spectrum.Both these new cations can be regarded as generalised chlorophosphonium ions. Their hexachlorophosphates are more completely dissociated than the analogous phosphorus pentachloride mainly because of the large size of the ions and possibly because of some resonance stabilisation. A similar mode of ionisation has been proposed to account for the reactions of triaryl phosphite dihalides., Further condensation with NH:PCl can take place and the linear polymers which have been formulated20 as (NPCI,),PC15 should perhaps be regarded as [PC13(NPC12)2nCl]+PC16- consistent with their high dielectric constants and insolubility in non-polar media.Ring-closure on the other hand now becomes a competitive process (111) and depends on the reaction of the polynuclear cations with either ammonia or ammo- nium chloride with a simultaneous decrease in conductivity. A similar scheme can be written for the higher cyclic polymers which are normally formed in decreasing amount with increasing ring size. If the chains are long enough chain growth predominates over cyclisation ammonolysis of a typical mixture of higher linear polymers (NPCl,);PCl, (n-17) giving a rubber-like high polymer.2o All the processes considered involve the elimination of hydrogen chloride and it is likely that removal of the chloride ion is aided by partial transfer to phosphorus of lone-pair electrons from neighbouring nitrogen atoms.The cyclisation of phosphonitriles can be compared with the forma- tion of B-chloroborazoles from amines and boron trichloride. In the latter reaction part of the driving-force is the delocalisation energy of the trimeric product the release of a chloride ion from boron being aided by the partial charge-transfer in the intermediates RNH,.BCl and RNH-BCl,. It is also catalysed by some transition metals and their chlorides.34 Similarly the ammonolysis of phosphorus pentachloride is catalysed by metal chlorides with acceptor proper tie^.,^ Some (e.g. MgCl, MnCl,) increase the rate of cyclisation; others like AlCl, accelerate the earlier stages of 32 (a) Groeneveld Visser and Seuter J.Inorg. Nucleur Chem. 1958 8 245; (b) Glemser and Wyszomirski Nuturwiss. 1961 48 25. 33 Rydon “Phosphoric Esters and Related Compounds,” Chem. SOC. Special Publ. No. 8 London 1957 p. 61. 34 Emeleus and Videla Proc. Chem. Soc. 1957 288. 36 Paddock and Searle B.P. 905,315/1962. 174 QUARTERLY REVIEWS the reaction but remain attached to the chain,* probably as end-groups in such structures as AlCl,(NPCI,),Cl. Phosphonitrilic chloride chains can be terminated with the elements of other acid chlorides by direct a d d i t i ~ n ~ ~ " ~ ~ by the reaction of hydroxyl- amine hydrochloride with phosphorus penta~hloride,~~ and in other ways.38 The compound PCl :N.POCl has been i ~ o l a t e d ~ ~ ~ ~ and its structure proved from its nuclear magnetic resonance Cyclic phosphonitrilic bromides40 (NPBr,), and chloridebromide~~l N3P3CI,Br,- have been prepared similarly but less is known about the intermediate stages.An orange crystalline NPBr,*PBr gives the cyclic trimeric and tetrameric bromides on further treatment with ammonium bromide; its structure is unknown. Alkyl- and aryl-phospho- nitriles have been prepared by the ammonolysis of Me,PC1,,42 Et 2PC13,43944 Ph,PC1,,44y45 PhPC144s and PhPBr4.*' Removal of the elements of the hydrogen halide from the intermediates in these reactions is more difficult than in the preparation of the chlorides the base strength being increased by the organic groups. Cyclisation of alkyl-phosphonitriles requires either heat43 or like N-trialkyl-B-trichloroborazoles,48 the use of a tertiary base.4z It is possible4b that (NPEt,),,HCI reported as linear,44 is in fact the hydrochloride of the cyclic molecule.A crystalline omp pound^^,^^ N3P2H4Ph,Cl (IV) prepared by the reaction of Ph2PCI with liquid ammonia gives mainly the cyclic tetramer [NPPh,] on pyrolysis. The structural formulae that have been suggested for (IV) all involve covalently bound chlorine; it is also possible that it may be a salt [NH2.PPh :N-PPh,.NH,]+CI- like many other phosphonitrilic interme- diates. In most of the above compounds each phosphorus atom is symmetric- ally and similarly substituted. If each phosphorus atom is similarly but * For a fuller account of the interaction of metallic halides with phosphonitrilic chlorides which includes information from publications with a restricted circulation see Ref. 4c. 36 Paddock Can. P. 575,069/1959.3' Barth-Wehrenalp and Kowalski Abstracts 135th Nat. Meeting of Amer. Chem. SOC. Boston 1959; Kahler U.S.P. 2,925,320/1960. 38 Becke-Goehring Mann and Euler Chem. Ber. 1961 94 193 ; Becke-Goehring Debo Fluck and Goetze Chem. Ber. 1961,94 1383. 3B Fluck Chem. Ber. 1961 94 1388. 40 (a) Besson Compt. rend. 1892 114 1479; (b) Bode 2. anurg. Chem. 1944 252 113; (c) John and Moeller J . Amer. Chem. Soc. 1960,82 2647; ( d ) Bean and Shaw Chem. and Ind, 1960 1189; (e) John and Moeller J. Inorg. Nuclear Chem. 1961,22,199. 41 Rice Daasch Holden and Kohn J. Inorg. Nuclear Chern. 1958 5 190. 42 Searle Proc. Chem. SOC. 1959 7 . 43 Bilbo 2. Naturforsch. 1960 15b 330. 44 Korshak Gribova Artamanova and Bushmarina Vysokomol Soedeninya 1960 46 (a) Haber Herring and Lawton J . Amer. Chem. SOC.1958 80 21 16; (6) Bezman 46 (a) Shaw and Stratton Chem. and Ind. 1959 5 2 ; (6) J. 1962 5004; (c) Humiec and 47 Moeller and Nannelli Inorg. Chem. 1962 1 721. 48 Turner and Warne Chem. and Ind. 1958 526. 2 377. and Smalley Chem. and Ind. 1960 839. Bezman J. Amer. Chern. SOC. 1961 83 2210. PADDOCK PHOSPHONITRILIC DERIVATIVES 175 unsymmetrically substituted and the rings are (statistically) planar two geometrically isomeric molecules (NPXY)3 and four (NPXY) are possible as for cyclopropane and cyclobutane derivatives. Both isomers are known for the trimeric phenyl-~hloro-~~ and phenyl-bromo-phos- ph~nitriles,~~ and three of the possible four tetrameric phenyl-chloro- phosphonitriles (V) have been obtained,46b and many derivatives of them prepared. Their configurations have not been determined.The possibility of configurational isomerism arising from non-planarity of the ring will be discussed later. yh yh Th Th Th $1 "d$:"/ I / ,'h 5)N '+lN/tL 51/N C1~h/N/Ph Cl CL T-N-P ,y-N-P ,P-N-y T Yh P-N-P 7-N-7 I/ 7-N-T 7-N-7 P-N-? Ph CL ph Ph Ph C l (v) Substitution Reactions.-Among phosphonitrilic compounds obtained by direct synthesis the chlorides are the most readily obtained and most other derivatives have been made from them and especially from the trimeric chloride. The majority of the reactions appear to be nucleophilic displacements. The orientation pattern depends on a number of factors but especially on whether the entering group decreases or increases the electron density at phosphorus. The first type of reaction is exemplified by the fluorination of the trimeric chloride with potassium fluoro~ulphite~~ (VI).(For clarity only the substituting groups are shown in VI IX X XI). The orientation of the substituents has been established by infrared and NMR spectroscopy ;49 geminal substitution takes place almost exclusively. One of two trimeric fluoride-chlorides described earlierso does not fall into this pattern redistribution reactions evidently occurring at high temperatures. Sodium mercaptides react readily with trimeric phos- phonitrilic ~ h l o r i d e ~ ~ especially in polar solvents and here too geminal substitution has been proved by NMR spectros~opy.~~ By contrast substitution by amines increases the electron density at phosphorus important contributions being made53 by such resonance structures as (VII). As will be seen later the ability of phosphorus to accept 4 9 (a) Chapman Paine Searle D.R. Smith and White J. 1961 1768; (b) Allen Barnard J. Emsley Paddock and White Chem. and Znd. 1963 952; (c) Heffernan and White J. 1961 1382. 50 Schmitz-Dumont and Braschos 2. anorg. Chem. 1940 243 1 1 3. 51 A. P. Carroll and Shaw Chem. and Znd. 1962 1908. 52 Boden J. W. Emsley Feeney and Sutcliffe Chern. and Znd. 1962 1909. 53 Becke-Goehring John and Fluck 2. anorg. Chem. 1959 302 103. 176 QUARTERLY REVIEWS electrons has important consequences; its effect here is that succes- sive amination tends to take place on different atoms (VIII; R represents ci ,CL Cl an amine residue) the increasing difficulty of substitution being attributed partly to electronic53 and partly (especially for bulky amines) to steric (VI I I) The orientation of the substituents has been established both spectros~opically~~~~~ and by the preparation of isomers.53 It has recently been that in the bis-dimethylamido-derivative (IX) prepared from the trimeric chloride and dimethylamine both substituents are on the same side of the ring.Geminal substitution by amines also occurs. It has been suggested for aromatic a m i n e ~ ~ ~ ~ ~ and established for partial substitution by ethylene- imine.57 The fully substituted derivative had been described earlier.58 For other amines information is less detailed. Pi~eridine,~~ morpholine,60 and pyrrolidine61 each give a series of compounds N3P3ClnR6_ (n = 0-5). Two isomers have been obtained for each of the bis- tris- and tetrakis- pyrrolidides but it is not known whether these compounds are cis- and trans-isomers or whether geminal substitution takes place to some extent.Similar behaviour is found in the tetrameric series. Successive substitu- tion of chlorine by fluorine takes place geminally followed by attack at a neighbouring phosphorus atom49 (X). Other tetrameric fluoride-chlorides N,P4F,C14 and N4P,F,C1 are known,g2 in which the orientation-pattern i\ / \ i\ 54 Ray and Shaw J. 1961 872. 66 Bezman and Ford Chem. and Ind. 1963 163. 57 Kobayashi Chasin and Clapp Inorg. Chem. 1963 2 212. 58 Ratz and Grundmann U.S.P. 2,858,306/1958. 6 8 Kropacheva Mukhina Kashnikova and Parshina Zhur. obschei. Khim. 1961 31 Kropacheva and Kashnikova Zhur. obschei Khim. 1962 32 521. 61 Kropacheva and Kashnikova Zhur. obschei Khim. 1962,32 652. Schmitz-Dumont and Kulkens 2.anorg. Chem. 1938 238 198. Bode Butow and Lienau Chem. Ber. 1948,81 547. 1036. PADDOCK PHOSPHONITRILIC DERIVATIVES 177 must be different ; since they were produced by the reaction of lead fluoride and the trimeric chloride at a high temperature rearrangement of the primary products is likely to have occurred. Successive substitution by amines takes place preferentially at remote phosphorus atoms (XI) b k p 3 and tetrakiQ4 amido-derivatives having been isolated. \ t R' In both series the orientations have again been proved by the structure of the NMR spectra involving the isotope phosphorus-31; e.g. the bis- amido derivatives show a pair of triplets of equal intensity (excluding successive substitution at the 1 3 positions) and in the tetra-substituted products the environments of all the phosphorus atoms are i d e n t i ~ a l .~ ~ ~ Other tetrameric amido-chloro-compounds have been described.65 Partially-substituted derivatives of known orientation have also been made by the Friedel-Crafts reaction and by reaction of the halides with organometallic compounds. The trimeric chloride with benzene and aluminium chloride gives66 the geminally-substituted diphenyl-tetra- chloride N3P3Ph2C14 which reacts further with amine~.~' As would be expected successive phenylation of a partially aminated trimer occurs pre- ferentially at the aminated p o s i t i o n ~ ~ ~ (XIT). C( ,CL cL\ ,Cl c\ ,c1 Cl,P N4P< c1 Me2N 7 P &Pc N Me2 ~~Q$J-,P,NYp<NMe Mc,NH fP*y C,H,+ AICI yNPy ___c - yP*s.' CL C1 Cl CL Ph ph (XI 0 The main product of the reaction of the trimeric chloride with phenyl- magnesium bromide66 was found to be a halogen-containing derivative which gave a linear compound Ph(PPh,N),H.HClO on treatment with alcoholic silver perchlorate.Small yields of the cyclic hexaphenyl tri- phosphonitrile have also been reported.68 Trimethyltrichlorotriphospho- nitrile has been indirectly by the following series of reactions (XIII) which establishes the orientation. The trimeric fluoride reacts with organolithium derivatives,'* and a series of phenyl-fluoro compounds 63 John Moeller and Audrieth J . Amer. Chem. Soc. 1960 82 5616. 64 John Moeller and Audrieth J. Amer. Chem. Soc. 1961 83 2608. 65 de Ficquelmont Ann. Chim. (France) 1939 (1 l) 12 214. 66 Bode and Bach Chem. Ber. 1942 75 215. 67 Becke-Goehring and John 2. anorg. Chem.1960 304 126. 68 Rosset Bull. Soc. chim. France 1925 37 518; Ramain Runavot and Schneebeli .I. Chimphys. 1959 56 659. Tesi and Slota Proc. Chem. Soc. 1960 404. Moeller and Tsang Chern. and Ind. 1962 361. 3 178 QUARTERLY REVIEWS N,P,F,Ph,- (n = 1-5) has been obtained; their orientations are unknown. Me,N ,CL Me,N /Me (Xlll) Tetrameric phosphonitrilic chloride reacts with phenylmagnesium bromide to give two phenylated compounds N,P,Ph,Cl, which from a study of their decomposition are formulated as position isomers (XIV). Two octaphenyl-derivatives N4P4Ph (possibly configurational isomers) were obtained in the same work. Only the higher-melting compound was found in later investigations of the ammonolysis of di- phenyltrichlor~phosphorane,~~ and a repetition of the work and a study of the structures of the two compounds should give interesting results.An isomeric phenyl-chloro-compound (NPPhCl) has been converted to the octaphenyl-derivative by phenylmagnesium bromide.46 Fully-substituted trimeric and tetrameric amido-phosphonitriles pre- pared by the action of ammonia or amines on the chlorides are very numerous. A list of over 150 of them which includes hydrazides and phenylhydrazides is given in Ref. 4b. They will be referred to here only briefly ; their properties will be considered later. The pho~phonitrilamides~~ [NP(NH,),],, are stable in water and give adducts of the formula [NP(NH2)2HAc],, with acetic acid. On being heated amorphous phosphams of empirical composition PN :NH are eventually f ~ r m e d ~ ~ # their infrared spectra74a$ b * showing them to contain -P=N-P= and -P-NH-P- groups.Derivatives of primary amines decompose with elimination of part of the amine substituted phosphams of empirical formula NP:NPh having been so obtained from the aniiide [NP(NHPh),],. The reaction is reversible suggesting that the trimeric ring structure is retained in the p h o ~ p h a m . ~ ~ 71 Bode and Thamer Chem. Ber. 1943 76B 121. 72 Audrieth and Sowerby Chem. and Znd. 1959 748; Sowerby and Audrieth Chem. Ber. 1961 94 2670. 73 de Ficquelmont Compt. rend. 1935 200 1045. 74 (a) Steger Angew. Chem. 1957 69 145; (6) Chem. Ber. 1961 94 266; (c) Steger and Lunkwitz Naturwiss. 1961,48 522; ( d ) Pustinger Cave and Nielsen Spectrochim. Acta 1959 11 909. 75 Bode and Clausen 2. anorg. Chem. 1949 258 99. PADDOCK PHOSPHONITRILIC DERIVATIVES 179 The phosphonitrilic halides are hydrolysed to the metaphosphimic acids the reaction involving a tautomeric change (XV).74d The basicities of the trimeric and tetrameric acids are therefore three and four on normal neutrali~ation,~~ though all the hydrogen atoms are replaceable by silver.76 N-Chloro-derivatives are formed by the reaction of the salts of the tri- meric and tetrameric acids with sodium hyp~chlorite.~~ Further hydrolysis of trimetaphosphimate takes place,78 imino-groups being replaced by oxygen atoms apparently without opening of the ring (XVI).The cyclic (XVI) trimetaphosphate is however often formed from linear phosphates e.g. from long-chain polyphosphates even in the absence of water.79 The marked preference for six-membered rings appears to be common to both series.The tetrametaphosphimate ion is degraded on hydrolysis first into an equimolar mixture of orthophosphate and di-imido-trimetaphos- phate,so (XVII) the imino-groups then being replaced by oxygen atoms. Trimeric ring imidophosphates are also formeds1 on hydrolysis of the pentametaphosphimate ion [NH.P02- J5. Phosphonitrilic esters of the type [NP(OR)2]3,4 (R = Me Et Prn Bun) have been prepared from the corresponding chloride and either the 76 Stokes Amer. Chem. J. 1896 18 629. 77 Taylor U.S.P. 2,796,321 ; 2,796,322 (1957). i 8 Narath Lohman and Quimby J . Amer. Chem SOC. 1956 78,4493. Thilo “Advances in Inorganic Chemistry and Radiochemistry” Academic Press Pollard Nickless and Warrender J . Chromatog. 1962 9 506. 81 Pollard Nickless and Warrender J . Chromatog. 1962 9 513.New York 1962 Vol. 4 p. 1. 180 QUARTERLY REVIEWS alcohol (pyridine being used as an acceptor for hydrogen ~ h l o r i d e ) ~ ~ ~ ~ or the alkali metal a l k o ~ i d e . ~ ~ ~ ~ ~ ~ ~ The former method has been used in the preparation of nitroxyalkyl-esters,8e and the latter for a series of fluoroalkyl phosphonitrilates of extreme thermal and chemical ~tability.~' Aryl esters have been prepared by both methods;* a summary of earlier work not yet published in the open literature is given in Ref. 4c. Pseudo-halogen groups can also be introduced; the trimeric azideaa [NP(N3)2]3 and an amide-azideS9 N,P,(NH,),(N,) have also been described and a particu- larly rapid reaction takes place between potassium thiocyanate and the trimeric and tetrameric chlorides in acetone,90 with formation of the cor- responding isothiocyanates N,P,(NCS) and N4P4(NCS),.The foregoing reactions have concerned the trimeric and tetrameric phosphonitrilic chlorides almost exclusively. The only series of larger ring compounds so far investigated is that of the fluorides. The trimeric and tetrameric fluorides were first prepared by the action of KS0,F on the chlorides,g1 and have also been obtained by the fluorination of a phosphorus nitride.92 Subsequent work has shown that a mixture of potassium fluoride and sulphur dioxide is also e f f e c t i ~ e ~ ~ ~ ~ and the larger-ring compounds in the range (NPF,),-, have been similarly prepared from the chlorides.94 Acetonitrileg5 and nitrobenzeneg6 are also suitable media for the reaction with simple fluorides which is catalysed by water and hydrogen fluoride.96 The properties of the fluorides and other phosphonitrilic derivatives are considered below in relation to their electronic structures.Electronic and Molecular Structure By using the preparative methods described above a broad range of substituent groups can now be attached to the 6- and 8-membered phos- phonitrilic rings. The properties of the derivatives so obtained are diverse. The chlorides (except the heptamer) are crystalline solids easily soluble in organic media and in sulphuric acid. They undergo hydrolysis and other 82 Yokoyama J. Chem. SOC. Japan 1959,80 1192. s3 Fitzsimmons and Shaw Chem. and Ind. 1961 109. s4 Dishon J. Amer. Chem. SOC. 1949 71 2251. 85 Ratz and Hess Chem. Ber. 1951 84 889. 86 Chang and Matuszko Chem. and Ind. 1962,410. *' (a) Ratz 2nd Grundmann U.S.P.2,876,247; 2,876,248/1959; (6) Ratz Schroeder Ulrich Kober and Grundmann J . Amer. Chem. Soc. 1962,84,551; (c) Mao Dresdner and Young J. Inorg. Nuclear Chem. 1962 24 53. s8 Grundmann and Ratz 2. Naturforsch. 1955 lob 116. 89 Chang and Matuszko J . Amer. Chem. SOC. 1960 82 5756. Otto and Audrieth J . Amer. Chem. SOC. 1958 80 5894; Tesi Otto Sherif and 91 (a) See1 and Langer Angew. Chem. 1956 68,461 ; (6) 2. anorg. Chem. 1958 295 92 Mao Dresdner and Young J . Amer. Chem. SOC. 1959 81 1020. 93 Haber and Uenishi Chem. and Eng. Data Ser. 1958 3 323. y4 Chapman Paddock Paine Searle and D. R. Smith .I. 1960 3608. 95 Tullock and Coffman J . Org. Chem. 1960 25 2016. g6 Moeller John and Tsang Chem. and hid. 1961 347. Audrieth J . Amer. Chem. SOC. 1960 82 528. 316.PADDOCK PHOSPHONITRILIC DERIVATIVES 181 substitution reactions comparatively slowly. The fluorides are all volatile and except for the trimer and tetramer are mobile liquids which resemble fluorocarbons in their behaviour as solvents. The alkyl derivatives are soluble in water as are many amido-phosphonitriles which are fairly strong bases. Aryl- aryloxy- and especially fluoro-alkyl- and fluoro- alkoxy-phosphonitriles are stable both to high temperatures and to chemical attack. The halides and isothiocyanates have been polymerised thermally the rubber-like polymers being easily hydrolysed in damp air. Because of its variety within a uniform structural scheme phosphonitrilic chemistry forms a good basis for assessing theories of bonding which are applicable to second-row elements generally.The formulae have so far been written as if single and double bonds alternated in the ring though in fact in all the structures determined so far the ring bonds are equal in length within experimental error. To a first approximation a system of r-electrons which is at least partly delocalised is formed by overlap of alternate 3dr-orbitals (on phosphorus) and 2pr- orbitals (on nitrogen). Some of the properties of this system follow from the nature of the localisedpr-dr bond. We shall consider them first by com- paring the properties of phosphoryl and carbonyl bonds and shall after- wards discuss phosphorus-nitrogen compounds in which the bonding shows additional features due to delocalisation. Comparison of Phosphoryl and Carbonyl Groups.-The important differ- ences between the phosphoryl and carbonyl groups arise from the d-orbitals in the phosphorus atom.The characteristic properties of carbonyl com- pounds as compared with saturated compounds depend on the increased electronegativity of the carbon atom by reason of the increased s-com- ponent of the a-bonds and the transmission of the inductive effect of the more electronegative oxygen atom by the polarisable r-system. On balance the carbon acquires acceptor properties and the other substituents interact with the carbonyl group to an extent dependent on their electronegativities and their abilities to conjugate with it either directly or by the release of electrons to carbon.97 Carbonyl compounds can be regarded formally as polar compounds (XVIII) the three groups donating electrons competitively into the 2pr-orbital of carbon conjugation with the ligandsg8 being expressed by such structures as (XIX).The effect of the electranegativity of the ligand can be seen by comparing carbonyl fluoride and acetone (Table 1); substitution of fluorine for methyl shortens and strengthens the carbonyl bond. Tn methyl acetate conjugation (as above) ensures that the total 97 Ingold. “Structure and Mechanism in Organic Chemistry” Cornell University Press Ithaca New York 1953. 98 Hartwell Richards and Thompson J . 1948 1436. 182 QUARTERLY REVIEWS TABLE 1. Properties qf carhonyl compounds Compound L(C = 0) (A)& V(C -0) (crn.-l)b E(C=O) (kcal.)c Me,CO 1 .22d 1 742h 1 79k MeC0,Me 1 *22f 1 774h 197* MeC(O)NH 1.219 17101 1 9gn F2C0 1*17e 1 92gi 189 a L(X-Y) stands for the length of the bond X - Y .Determined in vapour phase. Obtained by subtracting average bond energy terms [Cottrell “The Strengths of Chemical Bonds,” Butterworths London 1958 (2nd edn.)] from the total atomisation energy. The references give the heats of formation. Allen Bowen Sutton and Bas- tiansen Trans. Farada-v SOC. 1952,48,991. e Brown and Livingston J . Amer. Chem. Soc. 1952 74 6084. O’Gorman Shand and Schomaker J . Amer. Chem. Suc. 1950 72 4222. Kimura and Aoki Bull. Chem. Sac. Japan 1953 26 429. Hartwell Richards and Thompson J. 1948 1436. Nielsen Burke Woltz and Jones J . Chem. fhys. 1952 20,596.3 Estimated from the value obtained in dilute solution (Richards and Thompson J . 1947 1248). Miles and Hunt J . fhys. Chem. 1941 45 1346. Patrick “Advances in Fluorine Chemistry” Butterworths London 1961 Vol.2 p. 1 based on heat of hydrolysis (von Wartenburg and Riteris Z . anot-g. Chem. 1949 258 356). Green Quart. Rev. 1961 15 125; Landolt-Bornstein 4 Teil Kalorische Zustandgrossen Springer-Verlag 1961. Selected Values of Chemical Thermodynamic Properties Circ. 500 Nat. Bur. Stand. Washington 1952. bond energy remains high partly at the expense of the carbonyl bond itself. In acetamide the contrast between v(C=O) and the increase in total atomisation energy here artificially concentrated in E(C= 0) is still greater nitrogen being the better donor atom. L(C-0) and L(C-N) are also reduced by 0-071$ and 0.1 1 A r e s p e c t i ~ e l y ~ ~ ~ * ~ ~ from the accepted values for single bonds.lol Such electron-releasing groups also make the carbonyl carbon atom less electropliilic; the increase in activation energy (9 kcal./mole) for the hydrolysis of ethyl chloroformate,102 as compared with acetyl chloride,lo3 is attributed to conjugation of the ester oxygen atom with the carbonyl group.lo4 The differences between phosphoryl and carbonyl compounds arise because of the increased size of the phosphorus atom and the different sizes symmetries and numbers of available 2p- and 3d-orbitals.Overlap schemes for pn-prr and pn-di-r bonds are shown in Fig. 1. Because of the large size of the phosphorus atom overlap takes place in a compara- tively weak nuclear field so that pn-dn bonds are weaker than their p n - p ~ counterparts.105 Their polarity is accentuated by the shape of the &orbital which tends to concentrate charge close to the oxygen atom.If the three other groups attached to phosphorus are identical the molecule has 3-fold symmetry and the n-system is doubly degenerate. It now includes another pair of orbitals like that in Fig. 16 but in a plane rotated by 90” gg O’Gorman Shand and Schomaker J . Amer. Chem. SOC. 1950,72,4222. loo Kimura and Aoki Bull. Chem. SOC. Japan 1953,26,429. lol “Interatomic Distances” Chem. SOC. Special Publ. No. 1 1 1958 pp. 516,517. lo2 Bohme and Schurhoff Chem. Ber. 1951 84 28. ln3 Zimmerman and Ching Yuan J . Amer. Chem. Suc. 1955 77 332. lo* Hudson and Keay. J . 1960 1859. lo5 Craig MaccoII Nyholm Orgel and Sutton J . 1954 332. PADDOCK PHOSPHONITRILIC DERIVATIVES 183 FIG. 1. Overlnp schernes ( a ) for pn-pn horid i/i > C- 0; ( b ) px-dr bond in 'P=O; ( c ) and (d) x orid n' borids in PO,".about the bond axis. If the ligands can release electrons to phosphorus they can participate in the double n-system; in the limit of regular tetrahedral symmetry as in the Pod3- ion the two n-systems sometimes distinguished as n and r' become equal and equally shared among the four ligands. In the orientation of axes shown in Fig. l(c,d) they are formed from the dy pair the dx2-y2 and dzl ~ r b i t a l s . ~ ~ ~ ~ ~ ~ ~ (The dc triplet interacts less strongly.) The availability of this strongly r-bonding pair of orbitals leads to two differences from y7-r-p7-r systems the central atom becomes more electrophilic and conjugation does not depend on ~ 1 a n a r i t y . l ~ ~ The extent of conjugation in phosphoryl compounds is still controversial. The ultraviolet spectra of phenylphosphonates suggest only slight inter- action of the phenyl groups with the phosphoryl group,loB though this is very much increased if the ligands are strong donors such as N-alkyl- pyrryl (conjugation is even then not as strong as in carbonyl compounds).1oY Conjugation is however indicated by nuclear quadrupole resonance measurements 110 and ll1 by the reduction in v(P=O) in the series POCl (1 305 cm.-l) POCl,NMe ( 1 268 cm.-l) and POCI(NMe,) (1 241 cm.-l).Quantitative information is given in Table 2 for a series of compounds R,PO. If we take v(P=O) as a measure of the strength of the phosphoryl lo6 Cruickshank J . 1961 5486. lo' Jaffe J . Phys. Chem. 1954 58 185. lo* Jaffe and Freedman J . Amer. Chem. SOC. 1952 74 1069; Jaffe J . Chem. Phys. log Griffin and Polsky J. Org.Chem. 1961 26 4772. l l o Lucken and Whitehead J . 1961 2459. 1954 22 1430. Harvey and Mayhood Cattad. J . Chem. 1955 33 1552. 184 QUARTERLY REVIEWS TABLE 2. Properties of phosphoryl compounds R Br C1 F OEt NEt Ph Me v(P=O)" (c1n-l) 1261' 1290" 1395d 1272c 1212f 119Y 1170h D(P=O) (kcal./mole) 119i 1222 130' 151k 156' 128'" 139T2 a Dilute solution. Gerding and van Driel Rec. Trav. chim. 1942,61,419. Cabannes and Rousset Ann. Physique 1933 (lo) 19 229. dDelwaulle and Franqois Compt. rend. 1948 226 894. eAverage value (Bellamy and Beecher J. 1952 475). f In PO(NMe,) (Paddock unpublished work). Cotton Barnes and Bannister J. 1960 2199. hDaasch and Smith J. Chem. Phys. 1959 19 22. lCharnley and Skinner J. 1953 450. Ebel and Bretscher Hefv. Chim. Acta 1929 12 450. li Chernick and Skin- ner J.1956 1401. Kearney quoted by Claydon and Mortimer J. 1962 3212. nz Bed- ford and Mortimer J. 1960 1622. Claydon Fowell and Mortimer J. 1960 3284. bond it can be seen that among the halides the phosphoryl bond strength and the total additional bond energy (over PR,) increase together with the electronegativity of the halogen. D(P=O) in fact contains both 0- and r-components in all the bonds but an approximation to the total n-energy can be obtained by subtracting E(P-0) here taken as 92 kcal./rno1e.ll2 It is noticeable that even in phosphoryl fluoride the (approximate) n-energy (38 kcal./mole) is very much less than in carbonyl compounds where it is 80-90 kcal. In triethyl phosphate (58 kcal.) and in trisdiethyl- amidophosphine oxide (64 kcal.) the total n-energies are very much higher even though the phosphoryl bond itself is somewhat weaker and are con- sistent with strong interaction of the oxygen and nitrogen lone pairs with the phosphorus centre.The detailed allocation of energy in such com- pounds has been considered by McCoubrey and his co-workers.l12 Some conjugation of the phenyl groups in triphenylphosphine oxide is also suggested and the combination of an especially low value of v(P=O) with a high rr-energy (47 kcal.) in trimethylphosphine oxide is noteworthy. Similar values of D(P=O) are found for other trialkyl phosphine 0 ~ i d e s . l ~ ~ The importance of pn-dn interaction between phosphorus and carbon at least in the transition state is confirmed by the rapid exchange of the protons in the tetramethylphosphoriium ion with heavy water 114 and by the easy elimination of hydrogen bromide from /3-bromoethylphos- ~ h 0 n a t e s .l ~ ~ (Eqn. 15). Stabilisation of carbanions by pr-dn bonding is ,OEt ,OEt OEt OEt @.r.CH;CH;P\=O --- CH,:CH.PqO + HBr well known,116 and perhaps occurs because it does not involve a large shift of electron density towards the d-centre. Conjugation of the phosphoryl group with electron-releasing substitu- ents also lengthens the phosphoryl bond and increases its polarity and 112 S. B. Hartley W. S. Holmes Jacques Mole and McCoubrey Quart. Rev. 1963 113 Chernick and Skinner J. 1956 1401. 114 Doering and Hoffmann J . Amer. Chem. SOC. 1955 77 521. 116 Ford-Moore and Williams J. 1947 1465. 116 Henbest Ann. Reports 1956 53 137. 17 204. PADDOCK PHOSPHONITRILIC DERIVATIVES 185 hence its donor properties.Phosphate esters unlike most carbonate esters are readily soluble in water,l17 and dialkylphosphinic acids R,P(O)OH have basic properties,l18 forming hydrochlorides which are presumably dialkylphosphonium salts [R2P(OH)2]+C1-. The phosphorus centre also is affected. Displacement reactions of phosphinyl chlorides R,P(O)Cl obey the same (second-order) rate law as those of acyl halides,llg and for both types of compound the increase in activation energy accompanying the introduction of electron-releasing substituents is attributed to stabilisation of the ground state by coi~jugation.~~~ As a result of the enhanced acceptor properties however the phosphorus compounds are more sensitive to substitutional change.l17 Phosphoryl compounds therefore resemble their carbonyl analogues qualitatively but effects due to polarity and polaris- ability are more strongly marked with them.The combination of electro- philic character with competitive donation from the ligands into a double pr-dr system is important for other series of compounds and especially for phosphorus-nitrogen compounds. The P=N Bond in Phosphinimines.-The simplest compounds containing the P=N bond are the phosphinimines R,P:NR’ which may be expected to resemble phosphoryl compounds. The limited thermochemical evidence suggests that the total n-energy is strongly dependent on the nature of the attached groups. In the singly-bonded compound trisdiethylamidophos- phine P(NEt,), E(P-N) = 66.8 kcal.,120 and in the two phosphinimines Ph,P:NEt and Me,P:NEt E(P=N) is 98.4 kcal. and 69.7 kcal.respec- tively.121*122 The large difference between these two figures is in the opposite sense to that between corresponding phosphoryl compounds. The P=N bond in phosphinimines is highly polar a typical reaction (16) being that with ketones to give ketimine~.~~ Similarly phosphinimines R’N :PR are dimeric if the strength of the parent amine is great enough.27 Dimerisation (1 6) R2C0 + R’N:PR -+ R’N:CR2 + R3P0 also requires the substituents at phosphorus to be electron-withdrawing groups; Ph,P:NEt is monomeric whereas C13P:NMe and a number of other compounds in which the phosphorus atom carries such groups as P= 0 or P= S are d i m e r i ~ . l ~ ~ Dimeric N-methyltrichlorophosphinimine has been investigated more closely than the other molecules its vibrational spectra showing the presence of a four-membered planar P2N2 ring.28 The conditions for dimerisation are precisely those for development of such structures as (XX) and there is spectroscopic evidence for the partial delocalisation of the nitrogen lone pairs.In addition the average value of 117 G. S. Hartley Ref. 33 p. 33. 118 Crofts and Kosolapoff J . Amer. Chem. Soc. 1953 75 3379. 119 Dostrovsky and Halmann J . 1953 502 508 51 I 516; 1956 1004 lZo Fowell and Mortimer J . 1959 2913. lZ1 Mortimer Pure and Appl. Chem. 1961 2 71. lZ2 Claydon Fowell and Mortimer J . 1960 3284. lZ3 Trippett J. 1962 4731. 186 QUARTERLY REVIEWS E(P-N) (74.3 kca1.)12* is greater than that of a single bond. It is surprising that trimeric molecules are not formed preferentially since increased M&=PCL C1,P =!$Me stability should result both from the attainment of the natural valency angles and from the accompanying increased n-electron energy.A four- membered silazane ring presents a similar problem;125 it is possible that in both compounds steric interactions would be greater in the larger rings. Bonding in Cyclic Phosphonitri1es.-The structural effect of variations in n-bonding is better understood in the cyclic phosphonitrilic derivatives (NPX2),. It is possible to regard these compounds as constructed from linked tetrahedra and the n-bonds in them as formed by linking the orbitals of the double n-system of the individual units. This model has been developed especially by Cruickshank ;loS the sharing of r-orbitals gives an acceptable picture of the bond-lengths and angles in many com- pounds of tetra-co-ordinated silicon phosphorus and sulphur and the double n-system is expected to make an important contribution to the bonding in phosphonitrilic compounds too.The more detailed knowledge of these compounds however makes a deeper analysis necessary. Formally the new features arise because of the lower molecular site symmetry of the phosphorus atom which is now at most CLV rather than Td so that the two strongly n-bonding orbitals are no longer degenerate with each other.lL6 It is now convenient to use a co-ordinate system in which the z-axis is perpendicular to the local NPN plane (which for symmetry Dnh is the plane of the molecule) so recognising the importance of overlap with the 2pn(p,) orbital on nitrogen. The orientation of the five d-orbitals in the co-ordinate system employed is shown in Fig.2. The n-system in the ring is now composed of the two phosphorus orbitals dzz and d,, and the n'-system of d, and dx2--ye; each system in a non-planar molecule over- laps both 2 p and an s-p hybrid at nitrogen. Exocyclic n-bonding also has two components using respectively dz2 and the pair d,, d,, but it is at present less profitable to distinguish between them.* The reality of the difference even here can however be seen in structure of the tetrameric dimethylaniide (see below). Conjugation between the ring and the exocyclic mystems will still occur especially with electron-releasing ligands but because of the greater deviation from regular tetrahedral geometry it is likely to be less important than in phosphoryl compounds. * Notation:-In a planar molecule the n'-orbitals lie in the plane of the molecule and might well be labelled u'.In general however the relevant molecules are non-planar so that the reference plane is not defined. The notation used recognises that both n- and *'-orbitals change sign on rotation by 180" about the local bond axis. I I (XX) Fowell and Mortimer Chem. arid ]/Id. 1960 444. 125 Wheatley. J. 1962. 1721. 126 Craig and Paddock J . 1962 4118. PADDOCK PHOSPHONITRILIC DERIVATIVES 187 Z Z (d FIG. 2. Overlap schemes for 7r-bonding of (u) d3, ( 6 ) d, with p z ; ( c ) r’-bonding of dxt-yg and ( d ) dxy with an s-pv hybrid; ( e ) 7r-bonding of d,i with ligandp-orbital. Of the two possible types of interaction within the ring106p126 (T and n’) most attention has been devoted to the 7~-systern.l~~,l~* The two dr- components are not required to be equal by symmetry and it is likely that dxz is more strongly involved than dyz.If it is a delocalised n-system is formed of a different type from that in benzene because the interactions of the d,,-orbital with the 2pn orbitals on either side are of opposite sign (Fig. 2a b). The essential results of a simple molecular-orbital treatmentla’ are that (1) for unequal electronegativities of the n-orbitals any even number of electrons forms a closed shell and (2) for a given difference of electronegativity n-electron energies (per electron) increase steadily with ring size. Fig. 3 shows n-electron energies as a function of ring size for pn-pn and for pr-dn systems. On this basis it is not to be expected that trimeric and tetrameric phosphonitrilic derivatives will differ as much as do benzene and cyclo-octatetraene and indeed the separate series of phosphonitrilic halides (NPC1Jn and (NPF,) have comparable proper- ties among themselves.The effect of increased mixing-in of the d,,-orbital is to reduce the expected differences between successive members of these series. In the limit of equal contributions,128 the n-system is resolved into a series of 3-centre P-N-P “islands” (Fig. 4). The secondary n’-bonds in the ring are formed in planar molecules by 12’ (a) Craig and Paddock Nature 1958 181 1052; (b) Craig Chern. Soc. Special Publ. No. 12 1958 p. 343; (c) J. 1959 997; (d) “Kekule Symposium on Theoretical Organic Chemistry” Butterworths London 1959 p. 20. lPB Dewar Luckeii and Whitehead J. 1960 2423. 188 QUARTERLY REVIEWS 20.2 4 8 4 6 8 10 12 14 Number of Tr-electrons FIG.3. Delocalisatinn energies in planar (AB) systems. Upper diagram; Coulomb integrals diflering by 8 lower diagram equal Coulomb parameters. Full lines refer to p r - p r and broken lines to pr-d&,. (Reproduced with permission from Craig J. 1959 997 * ) FIG. 4. Orbital overlap scheme for 3-centre P-N-P phosphorus here seen in plan are formed by taking the (Fig. 14) and make angles of 45" with them. bonds. The two d-orbitals at sum and difference of dxz d, overlap of the dd-orbitals with s-p hybrids on nitrogen and as the shape departs from planarity increasingly with the p ,-orbital. Release of elec- trons to phosphorus is important competitively from the ligands and from the lone pairs on the ring nitrogen atoms,129 and in the latter case contri- butes predominantly to the d-system.The two dd orbitals however differ in their interactions with p (Fig. 2 4 ) ; dxe-y2 which according to overlap calculations should be the more strongly involved interacts (like a p-orbital) with the same sign on its two sides and to the extent that it is used should impose a minor alteration on the pn-dr energies shown in Fig. 3. However the electrons are distributed between the r- and d-systems the multiple bond system is highly polarisable and the different distributions of electrons in differently-substituted molecules have important effects on their physical and chemical properties. In the next sections we shall discuss the structural spectroscopic and thermochemical evidence on electron distri- lpg Shustorovich Zhiir.strukt. Khim. 1962 3 218. PADDOCK PHOSPHONITRILIC DERIVATIVES 189 bution and shall later show the applications to other properties of phos- phonitrilic derivatives and to other types of compound. Structural Investigations.-The cyclic structures suggested by Stokes3 for the phosphonitrilic chlorides typically the trimer (11) have been amply confirmed by later work. Perhaps the clearest demonstration is provided by their NMR spectra using phosphorus-31 which show that the phos- phorus atoms in any particular member of the series have identical environ- ments.20 A similar conclusion applies to the fluorides up to at least (PNF&. The chemical shifts due to phosphorus-3 1 of some phosphonitri- lic derivatives are shown in Fig. 5 . X= F X= CL d .- .I= 5 X=Me u -20 2%3 4 5 6 7 8 PL in (PNX,) FIG.5. 31P N.M. R. spectra of chloro- Jluoro- and methyl-phosphonitriles relative to 85% H3P0,. (R. F. M. White quoted in Ref. 20 and unpublished work.) For (NPBI-~)~,~ chemical shifts are + 49.5 + 71.8 p.p.m. (John and Moeller J. Inorg. Nuclera Chem. 1961 22 199). The molecular structures of seven phosphonitrilic derivatives are known. Details are given in Tables 3 and 4 which also contain information on corresponding phosphines and phosphine oxides. The lengths of the exocyclic bonds in the phosphonitrilic compounds [with the possible excep- tion of (NPBr,),] are very similar to those in the phosphoryl compounds as expected. The ring angles at phosphorus are all close to 120' and can be compared with 117'19' for O=P-Cl in PocI,.130 They are slightly greater in the tetrameric molecules especially in the halides and are associated in these molecules with slightly decreased angles between the ligands.In the trimeric molecules the ring angle at nitrogen hardly deviates from 120' but in the 8-membered rings with their greater geo- metrical freedom it is appreciably increased consistent with some de- localisation of the nitrogen lone pairs. Remarkably the angle is the same in three of the molecules; in the fluoride inductive effects are so great that the bond angle at nitrogen is increased to 147" and the ring becomes planar. The ring bond lengths in all the molecules are similar and are appreciably shorter than the 1.78A found131 for L(P-N) in the phosphoramidate ion [P03NH3]-. Again the fluorides stand out; not only are their ring bond lS0 Badgley and Livingston J.Amer. Chem. SOC. 1954 76 261. 131 Hobbs Corbridge and Raistrick Acta. CryJt. 1953 6 621. 190 QUARTERLY REVIEWS lengths shorter than those of the other compounds but L(P-N) in the tetramer is significantly shorter than in the trimer. All these features are generally consistent with the existence of a T- (and d-)system in the ring the strength of which is controlled partly by the electronegativity of the ligands and partly by ring size. Stronger bonds are possible in the larger rings because of the improved orbital overlap greater cyclic delocalisa- tion and because of the possibility of increased delocalisation of lone pairs on the nitrogen atom.126 TABLE 3. Structural information on -trimeric phosphonitrilic derivatives (NPR2)3 and related compoundsa R Br c1 F L(P-R) (A) in R3P 2.23 2.04 1-55 R3P0 2.06 1 -99 1 -52 (NPR2)3 2.18 1 *99 1-52 R3P0 108" 103.5" 102.5 " <RPRin R3P 100" 100" 104" (NPR2)3 103" 102" 99.5 O L(P = 0) in (A) R3P0 1-41 1.45 1 -45 LNPN 9 117" 119.5" 119.5" LPNP 9 9 123" 120.0" 120.5 O L(P=N)in (A)(NPR,) 1.53 1.59 1-56 a References for the structures of individual compounds are PBr, Gregg Hampson Jones and Sutton Trans.Faraday Soc. 1937 33 852 as revised by Lister and Sutton ibid. 1941 37 393; PCl, Kisliuk and Townes J . Chem. Phys. 1950 18 1109; PF3 Gilliam Edwards and Gordy Phys. Rev. 1949,75,1014; POBr, Secrist and Brockway J . Amer. Chem. Soc. 1944 66 1941; P0Cl3 Badgley and Livingston J. Amer. Chem. Soc. 1954 76 261 ; POF,# Williams Sheridan and Gordy J . Chem. Phys. 1953 20 164; (NPBr,), de Santis Giglio and Ripamonti J.Inorg. Nuclear Chem. 1962 24,469 (preliminary determination) ; (NPCI,), Pompa and Ripamonti Ricerca sci. 1959 29 1516; Wilson and D. F. Carroll J . 1960 2548; (an apparent inequality in L(P-N) in (NPCI,) (Giglio Ricerca sci. 1960,30,721) may be due to the use of isotropic tempera- ture factors); (NPF,), Dougill J. 1963 3211. Equivalent bond lengths and angles are averaged in the phosphonitrilic molecules. TABLE 4. Structural information on tetrameric phosphonitrilic derivatives (NPR,) R NMeZa Meb CIC Fa LRPR 104" 104" 103" 100" W-R) (A) 1.68 1.80 1-99 1.51 L(P=N) (A) 1-58 1.60 1.58 1.51 LNPN 120" 120" 121 " 122.5 " LPNP 133" 132" 132" 147" a Bullen J. 1962 3193; Dougill J . 1961 5471 ; in Me,P L(P-C) = 1.85 A LCPC = 98.6" (Bartell and Brockway J . Chem.Phys. 1960 32 512); in Me,PO .L(P-C) = 1.81 A LCPC == 106" L(P=O) = 1.48 8 [Wang Forsvarets-Forsknmgsinstitutt (Norway) Intern. Rapport IR-K-225 (1960)l; Hazekamp Migchelsen and Vos Acta Cryst. 1962,15 539; McGeachin and Tromans J . 1961,4777. PADDOCK PHOSPHONITRILIC DERIVATIVES 191 The bond system is modified by electron release from the ligands in competition with release from the ring nitrogen atoms; ring vibration frequencies reaction rates the orientation of substituents and molecular structures are all affected. The importance of electron release from exocyclic groups is shown in the structure of the tetrameric dimethylamide. The average exocyclic P-N bond length is 1-68A shorter than expected for a single bond as corrected for hybridisation changes (1*74A) and com- parable with L(P-N) in (NH2)3P-BH3 (1~65A).l~~ The average angle CNC angle (1 16") is also greater than that133 in trimethylamine (108O) the exocy- clic groups being almost flat.Interaction of the dimethylamido-groups with the ring although large is limited by the mutual steric requirements of a pair attached to the same phosphorus atom. The one with its N-p orbital orientated most suitably for overlap with the strongly n-bonding P-d, orbital has the slightly smaller exocyclic L(P-N) (1 -67 1A) and more significantly is the more nearly planar the sum of the angles at nitrogen being 358.5 '. The other dimethylamido-group is necessarily orientated so that its main pn-dr overlap takes place with the d, and orbitals which interact less strongly the former because it is already involved in the ring n-bonds.L(P-N) is therefore slightly greater (1-686A) and the sum of the angles at nitrogen is smaller (349-5") indicating decreased delocal- isation of the lone pair on the ligand. The structures are further differentiated by their configurations which also show qualitatively the relative importance of T- and d-bonding in different molecules. The trimeric halides are all close to planarity the chloride and bromide being slightly distorted by lattice forces. As already discussed the tetrameric fluoride also is planar because the strong induc- tive influence of the fluorine atoms permits extensive delocalisation of the lone pairs on the nitrogen atoms with consequent increase of the PNP bond angle. The other three molecules all have the same minimum sym- metry S4 but in the series N4P4C1, N4P4Me, and N4P4(NMe2) tend increasingly towards the higher symmetry D2d the configuration of the phosphorus atoms becoming more nearly planar.It seems probable that the differences are due to differences in the compromise made between steric interactions and the inequality of 7 ~ - and d-bonding.126 The two &-orbitals d, and d,, intersect in the z-axis and the relative orientations of local z-axes on successive ring atoms gives a measure of the overlap of the N-pn orbitals with the two dr-orbitals or linear combinations of them. Figs. 6-8 show stereographic projections of the three possible configur'a- tions of non-planar 8-membered rings (NPX,) which exhibit four-fold symmetry the relative orientation of n-orbitals in them and the steric relations between non-bonded groups.The first configuration (Fig. 6) is similar to that of cyclo-octatetraene if all the displacements from the 132 Nordman Actn Cryst. 1960 13 535. 133 Brockway and Jenkins J . Amer. Chem. Soc. 1936 58 2036. 192 QUARTERLY REVIEWS plane of projection are assumed to be equal. The relative orientation of ocal z-axes (Fig. 6a) shows that n-interaction is strong between adjacent FIG. 6. S (tub) configuration; (a) orientution of z-axes (b) stereographic projection (c) steric interactions. atoms on the same side of the plane and weak otherwise as found experimentally for cy~Io-octatetraene,~~~ In the second (saddle) configura- tion (Fig. 7) the four phosphorus atoms have been brought into a plane so introducing two-fold axes and planes of symmetry and increasing the I n I U (4 FIG.7. D z d (saddle) configuration; (a) orientation of z-axes (b) stereographic projec- tion (c) steric interactions. symmetry to DZd necessarily equalising the interactions between successive n-orbitals. In the third (C4v crown) configuration (Fig. 8) the interactions are again all equal but are almost zero the z-axes on adjacent atoms being nearly mutually perpendicular. 134 Bastiansen L. Hedberg and K. Hedberg J . Chem. Phys. 1957 27 1311. PADDOCK PHOSPHONITRILIC DERIVATIVES 193 FIG. 8. C, (crown) configuration; (a) orientation of z-axes (b) stereographic projec- tion (c) steric interactions. There are two other important factors. First we have to include 7 ~ ’ - bonding which uses d-orbitals with their density lobes concentrated in a plane perpendicular to that of the dn-orbitals and is therefore strong where n-bonding is weak and conversely; if n- and n’-contributions were equal the bonds would have cylindrical symmetry.Secondly the configur- ations differ in the importance of steric interactions. In the tub form (Fig. 6c) neighbouring PX groups are staggered whereas in the saddle and crown forms (Figs. 7c 8c) the planes of symmetry bring them into the eclipsed position. The crown form combines the weakest v-bonds with the strongest repulsive interactions and is not found in phosphonitrilic structures though it is the preferred configuration when steric interactions on adjacent atoms need to be minimised as in S8 and in the sulphur i m i d e ~ l ~ ~ S4(NH)4 and S,(NH),. In the phosphonitrilic compounds strengthening of the n-system by equalisation of the dihedral angles is offset by the repulsion between ligands on successive phosphorus atoms.For similar steric interactions therefore real configurations will approach the saddle form as n-bonding predominates over n‘-bonding. The deter- mined structures allow some assessment of the two types of contribution. Table 5 gives the displacements of the ring atoms from the mean molecular plane for the three non-planar tetrameric phosphonitrilic molecules ; the ratio decreases as the symmetry Ozd is approached and becomes zero in the limit. 135 Abrahams Acta Cryst. 1955 8 661. 136 Sass and Donohue Acta Cryst. 1958 11,497; Weiss Z . anorg. Chem. 1960,305 190. 194 QUARTERLY REVIEWS TABLE 5. Displacements of ring atoms from mean molecular plane (NPCl,) (NPMe,) “P(NMe,),I Displacements of Phosphorus (A) 0.35 0.21 0.18 Displacements of Nitrogen (A) 0.47 0.54 0.52 Ratio of Displacements P/N 0.75 0.39 0.35 From work on phosphoryl compounds described above the ability to release electrons to phosphorus decreases in the order Me,N > CH3 > C1.In the tetrameric dimethylamide such electronic feed-back tends to prevent delocalisation of the lone pairs on the ring nitrogen atoms so that T I - bonding is weak and the mismatch of dihedral angles small; in this mole- cule we have the closest approach to pure n-bonding. Electron release is less from methyl and least from chlorine so that n’-bonding increases and the configuration controlled more by steric interactions departs increasingly from D2d symmetry towards the S4 structure in which the phosphorus and nitrogen atoms are equally displaced from the plane of projection (Fig.6b). In the limit of equality the total n-system exerts no control over mole- cular shape and the existence of another configuration chair-shaped and of symmetry CZh (Fig. 9) in which steric repulsions are as small as in the FIG. 9. C2 (chair) configuration; (a) orieiitatioii of z-axes (6) stereographic projcc- tion (c) steric interactions. tub configuration opens the possibility of configurational isomerism. The first known example is the tetrameric phosphonitrilic chloride (NPC1,J4 in which the large mismatch of ring dihedral angles (Table 5) indicates a large relative 7i-I component and therefore the highest probability (among known structures) of configurational inversion. This compound exists in two polymorphic forms one of which ( K ) (for Ketelaar13’) has already 13’ Ketelaar and de Vries Rec.Trav. chinz. 1939 58 1081. PADDOCK PHOSPHONITRILIC DERIVATIVES 195 been described. The other (T; a label originally due to a mis-reading of 1 by the Reviewer) is required by space-group symmetry and packing con- siderations to be ceiitrosymmetrical and therefore ~hair-shaped.'~~ The detailed structure of T-(NPCI,) is at present being determined ;139 bond lengths and angles are unlikely to differ greatly from those in the K-form. While the nuclear quadrupole resonance spectrum of the trimeric140 chloride is consistent with its crystal structure having two pairs of lines of intensity 2 1 that of the tetrameric chloride140 sometimes shows four lines of equal intensity characteristic of the centrosymmetric T-form historic- ally the first indication of configurational isomerism.141 At ordinary temperatures the T-form is the more stable.Configurational isomerism is also likely if the ligands are bulky enough irrespective of the inequality of 7- and n'-bonding. If this is large however the ring bonds would be expected to alternate in the S4 configuration and to be unequal in pairs in the C,,-form. No example is yet known in the phosphonitrilic series but as will be seen later thiazyl compounds and metaphosphates sometimes show inequalities in bond lengths which can be attributed to sterically forced variations in orbital overlap. Possible bond-length variations in simple delocalised pn-dn-systems have been considered the~retically,~~~ with interesting results.In cyclic aromatic hydrocarbons with 4n + 2 7-electrons the n-electron energies decrease steadily to a limit with increasing ring size (Fig. 3). The energy of compression of the o-bonds is therefore offset decreasingly by the 7-energy as ring size is increased and for a sufficiently big ring localised pn-pn bonds are more stable and the skeletal bonds alternate in length.143 In pn-dn-systems on the other hand 7-electron energies increase to the same limit (Fig. 3) (for equal electronegativities) so that inequalities in bond lengths are to be expected for small rings only and even then only if the electronegativity difference is small enough. Such conditions have not yet been found among phosphonitrilic compounds but these investigations lend fresh point to a search for compounds (PX,=CH), in which the ring bonds might alternate.Vibrational Spectra.-The complementary information derived from the Raman and infrared spectra of phosphonitrilic derivatives is less detailed than that given by direct structural determination and is sometimes ambiguous. There are for instance some disagreements about individual 13* A. Wilson unpublished work. lP0 Negita J. Sci. Hiroshima Univ. 1958 A21 261 (Chem. Abs. 1958 52 19468); Negita and Satou J. Chem. Phys. 1956 24 621 ; Bull Chern. SOC. Japan 1956 29,426 (Chem. Abs. 1957,51,12646). See also Torizuka J . Phys. SOC. Japan 1956,11,84 (Chem. Abs. 1956,50,9149). 141 A. C. Chapman and A. Wilson unpublished work. 142 Davies Nature 1962 194 82; Haigh and Salem Nature 1962 196 1307; Davies Nature 1962 196 1309. A. J. Wagner personal communication.Longuet-Higgins and Salem Proc. Roy. SOC. 1959 A 251 172. 196 QUARTERLY REVIEWS assignments for the trimeric ~hloridel~~-l~7 and f l ~ o r i d e ~ 4 ~ 3 ~ ~ ~ though their vibrational spectra are both consistent with D, symmetry which requires the ring bonds to be equal in length within each molecule. In solution and in the vapour p h a ~ e ~ ~ - ~ ~ ~ * ~ ~ ~ the tetrameric fluoride is slightly distorted from the planar structure found in the crystal into a chair form of symmetry C2h the deviation from planarity depending on the solvent. The tetrameric chloride is less straightforward. Although its spectra in solution a i d in the vapour are very similar to those in the crystal (K-form) in which the molecule has the symmetry S, they are more nearly compatible with D4h,14544Q D2d145 or other The symmetry Dlh has also been suggested149 for the tetrameric bromide (NPBr,),.Although no detailed crystal structure is available for comparison the identity of space-groups of the tetrameric chloride and bromide and the close similarity of their c e l l - ~ i z e s ~ ~ ~ ~ ~ suggests that the molecular symmetry of (NPBr,) is really S4 a similar contradiction existing for both the chloride and the bromide. As a further difficulty the vibrational spectra of T-(NPCl,) appear to be inconsistent with the presence of a centre of The higher chlorides and fluorides have been studied in less detai1,146,150 though the structure of the series of PF symmetrical stretching bands in the fluorides (NPF2) provides a further proof of their cyclic nature.Q4 The most obvious feature of the infrared spectra of phosphonitrilic derivatives is a strong band at approximately 1200-1400 cm.-l usually known as the “P-N stretching frequency” and covering about the same range as v(P=O) in phosphoryl compounds. It can be regarded as v,,(P-N-P) but is strictly a degenerate ring-stretching vibration with (if it is the first of the degenerate bands) an amplitude phase difference of 8 = 2w/N between the vibrational motions of adjacent atoms N being the number of atoms in the ring.” A simplified treatment analogous to that of a zig-zag hydrocarbon chain shows that if interaction with deformation is ignored v(P-N) should decrease with increase in ring size as found for the chloride20 and fluorideg4 series above (NPX2)5. The initial increase in frequency between trimer and pentamer,4p20,153 is probably due neither to the change in phase angle nor appreciably to strengthening of the ring * For the use of 6 in interpreting the spectra of polymeric chains especially hydro- carbons see Ref.151. The application to cyclic molecules generally appears to have been made first by Parodi,162 and to phosphonitrilic molecules by Chapman.94 344 de Ficquelmont Magat and Ochs Compt. rend. 1939 208 1960. 145 Daasch J. Amer. Chem. SOC. 1954 76 3403. 146 Chapman and Paddock J. 1962 635. 147 Califano J . Inorg. Nuclear Chem. 1962 24 483; Califano and Ripamonti ibid. 148 Becher and Seel 2. anorg. Chem. 1960 305 148. 14s Steger and Stahlberg 2. Naturfursch. 1962 17b 780. 150 Steger and Mildner 2. Naturforsch. 1961 16b 836. l5l Brown Sheppard and Simpson Discuss.Faraday Soc. 1950 9 261; Deeds 162 Parodi Mem. des Sci. physiques 1944 47 1 (73). 153 Shaw Chem. and Ind. 1959 54. p. 491. Thesis Ohio State University 1951. PADDOCK PHOSPHONITRILIC DERIVATIVES 197 bonds; the same type of variation of ring-stretching frequency with ring size is found in the series of cyclic dimethylsil~xanes.~~~ For a particular ring size v(P-N) depends primarily on the electro- negativity of the ligands. Some typical values are shown in Table 6. The TABLE 6. v(P=N) in trimeric and tetrameric phosphonitrilic derivatives R Mea Phb BrC NMe,d Cle OMed Ff (NPR,),(cm.-') 1180 1190 1175 1195 1218 1275 1297-1305 (NPR,),(cm.-l) 1180 1213 1272-1280 1265 1315 1337 1435-1438 a Searle Proc. Chem. SOC. 1959,7. Bilbo 2. Naturforsch. 1960,15b 330. John and Moeller J .Inorg. Nuclear Chem. 1961 22 199; Steger and Stahlberg 2. Naturforsch. 1962 17b 780. Shaw Chem. and Ind. 1959 54. Daasch J . Amer. Chem. SOC. 1954 76,3403.f See1 and Langer 2. anorg. Chem. 1958,295,316; Chapman Paddock Paine Searle and D. R. Smith J. 1960,3608. value of v (P-N) is also affected to an important extent by the donation of electrons to phosphorus by the exocyclic groups. In the dimethylamides especially electron release from the ligands limits the delocalisatioii of the lone pairs on the ring nitrogen atoms and analogously to the behaviour of phosphoramides,lll depresses v(P-N) even though the electro- negativities of N and C1 are usually taken to be the same. The importance of steric effects on v(P-N) has also been investigated. In both the t r i m e r i ~ l ~ ~ and the tetramerics4 series the same trends are observed (Table 7).The relatively high frequencies exhibited by the TABLE 7. v(P=N) in trimeric and tetrameric alkylamino-phosphonitrilesa R NH MeNH EtNH PPNH BunNH n-C5HllNH n-C,H,,NH (NPR2) 3 (cm.-l) 1170 1175 - 1183 1195 1190 1192 (cm.-l) 1240 1215 1262 1266 1260 1265 1265 a References are for R=NH2 Audrieth and Sowerby Chem. and Ind. 1959 748; Sowerby and Audrieth Chem. Ber. 1961 94 2670; for other trimers Kokalis John Moeller and Audrieth J . Inorg. Nuclear Chem. 1961 19 191; other tetramers John Moeller and Audrieth J. Amer. Chem. Soc. 1961 83,2608. compounds carrying the larger groups is attributed155 to their greater difficulty in adopting the planar configuration of exocyclic nitrogen characteristic of appreciable electron release to phosphorus.In agreement with this interpretation v(P-N) is also high in the tetrameric dimethyl- amide (1265 cm.-l) and as we have seen the complete development of exocyclic n-bonding in this molecule is prevented by steric interactions of the two dimethylamido-groups. v(P-N) would probably be lower if both these groups were planar. It is also possible that v~P-N) in the amino- and monomethylamino-derivatives is depressed by hydrogen-bonding of 15* Wright and Hunter J. Amer. Chem. SOC. 1947,69 803; Richards and Thompson 155 Kokalis John Moeller and Audrieth J. Inorg. Nuclear Chem. 1961 19 191. (NPR,) J. 1949 124. 198 QUARTERLY REVIEWS the ligands to the ring nitrogen atoms of other molecules. Such bonding would also be subject to steric interference and its occurrence would be consistent with the insolubility of these compounds in organic media.The observed frequency shifts are rather smaller than those found for the pro- tonation of the phosphoryl Thermochemistry.-The heats of formation of several phosphonitrilic derivatives have been determined (Table 8) and allow some estimate to be made of the total n-bond energies. The total bond energy exceeds the sum of nominal single bond energies by the amounts shown in the last TABLE 8. Thermochemistry of phosphonitrilic derivativesQ (NPC12)3 (NPC12)4 (NPMe2)3 (NPPh,) “P(OC6H11)213 -AH (Cryst.) (kcal./mole) 194.1 40.7 259.2fl.2 125.1 f2.7 -40.7f11.5 582.0f4.0 E(P-N) +E(P-X) (kcal.)b 152.3 1 5 2 ~ 7 ~ 140-8 153.2 174.9’ dE(P-N) + AE(P-X) ( kcal.)E 9.3 9.7d 12.4 18.9 16*le a Data for the chlorides are from S.B. Hartley Paddock and Searle J. 1961 430; for the other compounds from Bedford and Mortimer J. 1960 4649. X refers to the first exocyclic atom. Excess of measured E(P-N) + E(P-X) over sum of single-bond energy terms taken to be E(P-N) = 66.8 E(P-0) = 92.0 E(P-C1) = 76.2 E(P-C) = 61-6 (Me,P) and 67.5 (Ph,P)kcal. See Ref. 112 for detailed sources. Calculated from dHr(NPCI2) and measurements of the heats of polymerisation of (NPCI,),, (Table 12). The use of dHr(NPCI,) gives values 0.2 kcal. lower. Arithmetical error in source corrected. row of the Table the individual entries strictly including both u- and T- components. They are useful quantities only to the extent that the single- bond energy terms are appropriate. E(P-N) is particularly suspect because even tervalent phosphorus has acceptor properties (the dissocia- tion energy of PCl,.NMe is 6.4 kcal./m01e)~~’ and the apparent value of E(P-N) in P(NEt2) may well include a contribution €rom partial lone- pair delocalisation the P-N bonds having partial double-bond character.Although this does not affect a comparison between phosphonitrilic compounds it is still impossible on thermochemical grounds alone to allocate the extra energy to individual bonds or to distinguish 0- and 7 ~ - contributions. From the values given three tentative conclusions seem possible. (1) r-Electron energies are smaller in phosphonitrilic than in phosphoryl compounds partly because nitrogen is less electronegative than oxygen and partly because the overlap of the individual orbitals is less effective. In phosphonitrilic chlorides it is of the order of 6-10 kcal./bond.(2) The difference between the trimeric and tetrameric chlorides is real and dis- 166 Zingaro and White J . Inorg. Nuclear Chem. 1960 12 315. 16’ R. R. Holmes J . Phys. Chem. 1960 64 1295. PADDOCK PHOSPHONITRILIC DERIVATIVES 199 tinguishes pr-dn from p7r-p~ systems. Comparison of their heats of combustion shows that the average ring-bond energy in cyclo-octatetra- ene158 is 5.5 kcal. less than in benzene.159 In the phosphonitrilic series the ring bonds are stronger in the 8-membered ring as expected theoretica1ly,l2’ though there is no doubt that increased lone-pair delocalisation in the larger molecule accounts for part of the difference. (3) It is noteworthy that the additional bond energy is appreciably greater in the methyl- and phenyl- phosphonitriles than in the chlorides in spite of the lower electronegativity of carbon suggesting that here (surprisingly) as in phosphoryl com- pounds there is some electron-release to phosphorus.It is clear from the foregoing paragraphs that the mutual interaction of the groups attached to phosphorus is too great for the concept of “resonance energy” as a distinguishable contribution to the total heat of formation to be of much use in phosphonitrilic chemistry and other methods of organising the information have been developed.l12 There is another important difference between doubly-bonded phosphorus and carbon. In organic compounds the change from saturation to a double-bonded system is accompanied by a change in a-hybridisation from sp3 to sp2 whereas in the change from (e.g.) PCl to POCl the change in a-hybridisa- tion is likely to be smaller so that the r-bond energy is offset by a greater a-compression energy than in carbon compounds.112 In phosphorus com- pounds therefore a - ~ interaction may be very important and the high compression-energy of an sp3 a-bond may explain why a pn-dr bond may be both short and apparently weak.160 Chemical Properties of Phosphonitrilic Derivatives Delocalisation.- As we have seen the bond system in phosphonitrilic derivatives is composed of two parts (r and T’) in both the ring and the ligands.The structural investigations allow a qualitative relative estimate of those two major contributions but do not distinguish for instance between the d, and d, components of 7r-bonding which are in general unequal.The relative heats of formation of (NPC12)3,4 are compatible with the results of the simple theory in which d is dominant but a part of the difference may arise from the greater contribution from delocalisa- tion of the lone pairs on the nitrogen atom in the eight-membered ring. Thermochemical evidence alone does not therefore fix the relative im- portance of d, and d,, or the extent of n-delocalisation which depends on it. Nor is other evidence decisive. The ultraviolet spectrum of a p7r-dr delocalised system is not expected to resemble those of benzene derivatives because the top occupied and the bottom occupied r-levels in phospho- nitrilic compounds are expected to be non-degenerate (and a transition 16* Springall and White Trans. Faraday SOC. 1954 50 815. 15’ Prosen Gilmont and Rossini J .Res. Nat. Bur. Stand. 1945 34 65. J. C . McCoubrey personal communication. 200 QUARTERLY REVIEWS between them is in any case forbidden by symmetry). The spectra of the chlorides20Js1 have absorption maxima below 1 850A (but see also Krausela2) and those of the fluorides occur at 1494 & 5 A for (NPF2) and at 1475 & 5 A for (NPF,), log, Emax. > 4 in both cases. For (NPBr,), Amax = 2015 A log, E = 4.44 and in sulphuric acid Amax. = 1930 A log, E = 4 ~ 2 4 . l ~ ~ For (NPCI,), the interpolated value of Amax is 1755 A. It is likely that the bands in all three halides are due to the same type of transition the insensitivity to protonation especially suggesting excitation of halogen lone p a i r ~ . ~ ~ ~ ~ ~ ~ J ~ ~ A ring n-v* transition is not ruled out,161 though the chemical effects of ultraviolet radiation are to promote re- placement of the chlorine atoms by organic groups without breaking the ring Two phosphonitrilic derivatives are magnetically anisotropic in the same sense as b e n ~ e n e .l ~ ~ l ~ ~ The a-anisotropy has to be estimated and the residual ring current in (NPCI,), small because of the large electro- negativity difference between consecutive atoms is paramagnetic as expected for apw-dn system. The result however is not a conclusive demon- stration of cyclic delocalisation the evidence for which consists of numer- ous suggestive indications from molecular structure and other physical properties rather than of assignable quantitative data. In the following sections we shall illustrate the consequences of the concepts developed above in relation to typical chemical properties of phosphonitrilic deriva- tives avoiding as far as possible invoking cyclic delocalisation.It is often sufficient to regard the primary T-system as supplemented by competitive donation to phosphorus by the ring nitrogen and the ligands the balance being influenced by inductive conjugative and in some cases steric effects. Base Strength.-The phosphonitrilic chlorides are very weak bases. The trimer forms a complex with nitrogen dioxide,ls7 and adds three molecules of sulphur trioxidels8 to give (NPCI2),,3SO3. The trimeric chloride also takes up two molecules of aluminium chloride,66 but does not react with a number of transition-metal halides.lsg All the chlorides take up protons in sulphuric acid (Table 9). Electron release to phosphorus especially by amino-groups increases the base strength considerably.In nitrobenzene the amidophosphonitriles are stronger bases than the parent amine.170 The relative strengths are reversed in water because of the greater stabilisa- Foster Mayor Warsop and Walsh Chem. and Ind. 1960 1445. 16a Krause 2. Elektrochem. 1955 59 1004. 163 Lakatos Hess Holly and Horvath Naturwiss. 1962 21 493. Dishon and Hirshberg J . Polymer Sci. 1949 4 75. 165 Craig. Heffernan. Mason and Paddock J . 1961 1376. 166 Bull& J . 1962 3193. 16' Besson and Rosset Compt. rend. 1906 143 37. 16* Goehring Hohenschutz and Appel 2. Naturfursch. 1954 9b 678. 169 Lakatos Bohus and Hess Magyar Kem. Fulydirat 1961 67 374. 170 Ray and Shaw Chem. and Ind. 1961 1173. ' PADDOCK PHOSPHONITRILIC DERIVATIVES 20 1 TABLE 9.Number of protons taken up by the phosphonitrilic chlorides (NPC12)n in 100% sulphuric acidu I1 3 4 5 6 7 8 No. of protons (cryoscopic) 1.2 2.0 2.0 2.0 2.3 2.8 No. of protons (conductimetric) 1.14 1 *72 2.20 2.63 2-20 2.60 hexameric chloride is unknown. D. R. Smith unpublished work. The reason for the discrepant results for the tion of the aminium ion by hydrogen-bonding,171 and relative base strengths in nitroben~enel~~ are therefore a better measure of proton-accepting capacity. The conclusion170 that protonation takes place on the ring rather than at exocyclic positions is by measurements of pKa for the addition of a second proton to trimeric amido-derivatives.* The general interpretation is that charge is drained from exocyclic to ring nitrogen atoms via 7r-bonding between the nitrogen lone pairs and the phosphorus d-0rbita1s.l~~~ There are other points of interest.The trimeric and tetra- meric ethylphosphonitriles are also quite strongly basic (Table lo) like the trialkylphosphine oxides. It is also possible that the induction coefficients for correlating the strengths of substituted fatty acids are also applic- able here but with a greatly increased transmission coefficient as aresult of TABLE 10. Base strengths of trimeric and tetrameric phosphonitrilic derivat ivesa C1 NH NHMe NMe Et Ph OEt pK,(NPR,) -8*2b 7.65 8.8 7.6 6.4 1.5 0.20 pK,(NPR,) -8.6b 7.55 8.2 8.3 7.6 2.2 0.60 a Feakins Last and Shaw Chem. and Ind. 1962 510 Feakins Last Neemuchwala and Shaw Chem. and Ind. 1963 164 except for chlorides. Determined in nitrobenzene except for R=NH2 determined in water.D. R. Smith unpublished work. the polarisability of the 7r-system. The relative strengths of the trimeric and tetrameric compounds are also interesting. The electron density at nitrogen should be smaller in the latter compounds because of increased lone-pair delocalisation and this is reflected in the Table in weaker basici- ties for the tetrameric molecules in which R = C1 NH, or NHMe. The reversal in the other cases quoted may be a result of differential steric hindrance of solvation reducing especially the base strength of the more * For later work on base properties and the formation of molecular addition com- pounds see Moeller and Kokalis J. Inorg. Nuclear Chem. 1963 25 875; Das Shaw B. C . Smith Last and Wells Chem. andZnd. 1963 866.I i 1 Trotman-Dickenson J. 1949 1293. l i 2 (a) Feakins Last and Shaw Chem. and Znd. 1962 510; (6) Feakins Last Nee- muchwala. and Shaw. Chem. and Ind.. 1963.164. l i 3 Branch and Calvin “The Theory of Organic Chemistry” Prentice-Hall New York 1941. 202 QUARTERLY REVIEWS hindered trimer molecules as proposed for the protonation of 2,6-di-t- b~ty1pyridine.l~~ Molecular Rearrangements.-Electron release from exocyclic groups leaves them relatively electrophilic. Polymeric phosphonitrilic chloride with sodium t-butoxide gives i ~ o b u t e n e ~ ~ presumably by nucleophilic attack of a ring nitrogen atom on a methyl group (XXI) and the partial rearrangement of a methyl ester (XXIT) has been attributed to the electro- philic behaviour of the methyl groups. Complete rearrangements in the same sense comparable to others in the triazine have been Me Me \ / 0 NH I CH* I 0% ,OEt MeO ,OMe MeO 40 Et / p NEt (XXII) /‘\N /‘\?Me EtO onP,N,P\ ‘ 40 (XXI’’) I OEt Et achieved175 for both triineric and tetrameric alkyl esters.N-Ethyl esters of the same type have been prepared by the action of ethyl iodide 011 the silver salts of the acids;176 they have singly-bonded structures (XXIII) and form stable hydrochlorides. The reaction of trimeric phosphonitrilic chloride with sodium benzoate leads to a more deep-seated rearrangement,177 the first-formed normal benzoate apparently undergoing a tautomeric change to an N-benzoyl compound (XXIV). Benzoic anhydride is eliminated in a reaction with a third molecule of sodium benzoate the final products being benzonitrile and a metaphosphate.Salts of other organic acids are also effecti~e.l~O>~~ Nitriles are also formed in a Wittig-like reaction by the decomposition26 of N-acyl-trichlorophosphinimines. Cl,P:N.C(O)R -+ C1,PO + RCN (17) Rates of Substitution Reactions.-The kinetics of the hydrolysis of phosphoryl halides are uniformly b i m o l e c ~ l a r ~ ~ ~ ~ ~ ~ ~ and optical inversion characteristics of rear-face attack at a tetrahedral centre has been demon- 174 Gold in “Progress in Stereochemistry,” Butterworths London 1962 Vol. 3 175 Fitzsimmons and Shaw Proc. Chem. SOC. 1961 258. 176 Ratz and Hess Chern. Ber. 1951 84 889. li7 Bezman and Reed J . Amev. Chem. Soc. 1960 82 2167. p. 169. PADDOCK PHOSPHONITRILIC DERIVATIVES 203 ~tratedl7~ for the analogous reaction (18). The phosphorus atoms in (18) phosphonitrilic compounds would be expected to be similarly electro- philic and the orientational effects referred to earlier do not suggest any important difference in mechanism.Kinetic information is given in Table 11. RIR2P(0)OMe* + MeO- -j R,R,P(O)OMe + Me*O- TABLE 11. Kinetics of substitution reactions of phosphoryl and phos- yhonitrilic derivatives Compound Reagent Solvent Temp. Kinetic parameters Ref. E t P( 0)CI H,O Acetone various 7.3 5.5 a (MeO)EtP(O)Cl , 7 9 9 9 8.4 5.3 9 9 7 10.6 5.3 9 Ace t oni tri le 0-35" 18.3 12.1 b ( MeO),(P(O)Cl ( N PClJ NPCl,) 7 9 7 7 16.3 12.0 , (NPCI,) 5 7 7 9 9 9 17.0 11.9 , (NPCI,) 1 7 9 7 16.3 11.2 , 1.mole-lsec-l C l.mole.-lsec.-l 9 9 1. mole .-lsec .-l d k = 1.7 1.2mole-2sec.-1 , E(kcal./mole) logl,A ei- (NPCI,) Aniline Ethanol-benzene 34.5' k = 1.48 x 9 Ethanol 7 9 7 , h- = 0.01 x 10-3 3 Piperidine Toluene 00 k = 2.2 x 10-3 9 , Catalysis by tri-n- k = 1.4 x 10-21.2 butylamine mole-2sec.-1 a Hudson and Keay J.1960 1859. b Sowerby personal communication. and Parker Chenz. and Itid. 1962 1823. 1962 390. Bailey Capon Hills and Shaw Proc. Chem. SOC. The rate of the bimolecular reaction of trimeric phosphonitrilic chloride with bases increases in the order ethanol <aniline<piperidine<chloride ion. The smallness of the discrimination between ethanol and aniline,l'Q as compared with the similar reaction of picryl chloride shows that bond- breaking is more important in the transition state in the phosphonitrile. This finding is by the unreactivity of the trimeric fluoride to iiucleophilic attack.g1b On the other hand the importance of a pentaco- ordinated complex in the transition state is suggested by the reaction of the trimeric chloride with trimethylamine in which methyl chloride is elirninated,laO and especially by the strong catalysis of its reaction with piperidine by tri-n-butylamine and by piperidine itself.lsl The two results are not incompatible.Hudson and Keaylo4 have pointed out that the transition-state structures in acylation and phosphylation are different IiR Green and Hudson Pmc. Chem. SOC. 1962 307. l i 9 Bailey and Parker Chem. atid Id. 1062 1823. lR'' Burg and Caron J. Amer. Client. Soc. 1959 81 836. Capon Hills and Shaw Pmc. Chenz. Soc. 1962 390. 204 QUARTERLY REVIEWS being based respectively on sp3 and sp3d hybrids bond-breaking being more important in the latter case and a similar argument would apply to com- parative substitution in the aromatic and phosphonitrilic series.The participating d-orbital may come from either the 7~' or the 7~ set. Bailey and Parker suggest the formei (dXJ reaction involving the flank attack established for the hydrolysis of a silyl hydride.lS2 Alternatively if d, is used the reaction should proceed by inversion. In this connection Sowerby's results (Table 11) on the exchange of radioactive chloride ion with a series of phosphonitrilic chlorides are interesting. Electron density at phosphorus is expected to increase with increase in ring size (a) because of the increased n-delocalisation energy and (b) because of increased delocalisation of electrons of nitrogen. The NMR evidence (Fig. 7) is in agreement with this.Further on the basis of the crystal structure d e t e r m i n a t i ~ n ~ ~ ~ steric hindrance (by a chlorine atom on an adjacent phosphorus atom) to rear-face attack is greater in the tetrameric than in the trimeric chloride. On all counts the reactivity of the tetramer should be the less and yet general experience now re- inforced by a preliminary estimate of relative rate+ and the determination of activation energies (Table 11) shows that it is usually greater. Similar difficulties exist for the hypothesis of flank attack. Although steric effects no longer favour the trimer a decrease in activation energy from trimer to tetramer is difficult to understand. The explanation is probably that the reactions do proceed via inversion the sp3d transition state involving deformation of the ring reactivity being dependent on ring flexibility.We have already seen that the near-equality of V- and n'-bonding in (NPCI,) allows an easy conversion between tub (S,) and chair (C2h) configurations access of a reagent to the phosphorus centre being particularly easy in the latter case. This concept adds a dyna- mic steric effect to the static effects already to account for the slowness of successive substitution by bulky amines.* Molecular flexibility induced especially by electronegative ligands arises from delocalisation of electrons on nitrogen and is of two types. Re- sistance to angular deformation is decreased so that increase of angle at nitrogen is expected to be accompanied by decrease of deformation con- stant. Also insofar as the local ring configuration approximates to planar- ity the delocalised electrons tend to occupy the &-orbitals of phosphorus so reducing the inequality of T- and n'-bonding and with it the torsional rigidity of the P-N bond.In the trimeric molecules in which delocalisa- tion of nitrogen electrons and n'-bonding are restricted geometrically deformability is least and the activation energy for displacement reactions greatest. * Steric effects of the type discussed above have been recognised by Moeller and Kokalis ( J . Inorg. Nuclear Chen?. 1963 25 1397) in the relative rates of aminolysis of trimeric and tetrameric phosphonitrilic fluoride$ chlorides and bromides. la2 Sommer Bennett Campbell and Weyenberg,J. Arncr. Chern. Soc. 1957,79 3295. 183 Hazekamp Migchelsen and Vos Acta Cryst. 1962 15 539.PADDOCK PHOSPHONITRILIC DERIVATIVES 205 There are no direct measurements of deformation or torsional constants. The small dipole moments of cyclic phosphonitrilic h a l i d e ~ l ~ ~ ~ ~ ~ prevent a measurement of molecular relaxation time from dielectric absorption though to take the extreme case the infrared spectra of the fluorides and the absence of configurational i ~ o m e r i s m ~ ~ ~ ~ ~ both suggest that the life-time of any particular configuration is short. Activation volumes for viscous flow are small and even less dependent on ring sizeg4 than for the cyclic dimethylsilo~anes.~~~ Polymerisation.-The extent of lone-pair delocalisation also has an important effect on polymerisability. It is well known3 that the cyclic chlorides polymerise on heating to about 300" to a solid with the mechani- cal properties of natural rubber,lss though if the material is pure enough polymerisation is extremely slow.ls7 The fluoridesgla and bromides40 c y d also polymerise as do the isothiocyanate~,~~ at a much lower temperature (1 50").The kinetics of the polymerisation of the trimeric chloride have been investigated by several authors.lg8 The trimeric chloride polymerises more rapidly than the tetramer,lS8 the rate being proportional to the concentra- tion of trimer. The reaction is catalysed by metals and by oxygen-containing organic compounds benzoic acid being especially effective. It seems likely that the mechanism is the same in each case and it has been suggested188e that a chloride ion is first detached propagation occurring by the attack of a linear species P,N,C15+ on the cyclic trimer.A similar ionisation has been proposedlS to account for the semiconductivity of the trimeric chloride. Radicals appear to play only a small part in polymerisation which results neither from high-energy electrons1 nor from y-radiation,lgl though 50kv X-rays are more effective especially just below the melting Some molecular chlorine is released from the trimeric chloride and a little nitrogen from larger-ring compounds.1go Irradiation of trimeric phospho- nitrilic chloride in solution in butanol by high-energy electrons causes esterification without appreciable increase in degree of p01ymerisation.l~~ Graft copolymers have been prepared by irradiation of the phospho- nitrilic chloride polymer together with styrene,lg4 the product having a reduced susceptibility to hydrolysis.ls4 Corfield J. 1962 4258. lE5 Gee Proc. Chem. SOC. 1957 1 1 1 . I E 6 Meyer Lotmar and Pankow Helv. Chim. Acta 1936 19 930. l a i Colclough personal communication. lSa (a) Patat and Kollinsky Makromol. Cllem. 1951 6,292; ( h ) Patat and Frombling Monatsh. 1955,86,718; ( c ) Patat and Derst Angew. Chem. 1959,71 105; ( d ) Konecny and Douglas J . Polymer Sci. 1959 36 195; (e) Konecny Douglas and Gray J . Polynier Sci. 1960,42 383. ls9 Eley and Willis J . 1963 1534. lS0 Spindler and Vale Makrornol. Chem. 1961 43 232. l g l Manley Nature 1959 184 899. l D 2 Caglioti Cordischi and Mele Nafure 1962 195 491. lg3 Spitzyn Afanasieva Rikaev Kolli and Glazunov Dokladv Akad. Narik S.S.S. R . lg4 Spindler and Vale Makromol. Chem. 1961 43 237. 1960 131 1106. 206 QUARTERLY REVIEWS The difference (1.7 kcal./mole) between the energies of activation for polymeri~ation~~~ (24.3 kcal./mole) and depolymerisation188c (26.0 kcal./ mole) agrees with a direct determination of the heat of polymerisation of the trimeric chloride by differential thermal analysis196 (Table 12).De- polymerisation of polyphosphonitrilic chloride begins at about 350"c TABLE 12. Heats of polymerisation of phosphonitrilic chlorides (kcal./mole) 0.46 0.22 0.16 0.04 and an equilibrium between the cyclic molecules is believed to be set up at 600"c though no quantitative information is a~ai1able.l~' The molecular weights of different samples of polyphosphonitrilic chloride have been estimated in many ways. Typical values are 3.7 x lo4 (from elastic con- s t a n t ~ ) ~ ~ ~ and 3 1.7 x lo4 (from light-scattering measurement^).^^^ The polymer has a helical structure,186 which a recent structure determina- tion200 shows to be nearly flat (Fig.10). Intramolecular steric interactions between PCI groups are much reduced as compared with the tetrameric I FIG. 10 chloride so that the small excess of 7- over 7'-bonding is sufficient to bring the chain close to planarity the torsion angles about N-P and P-N bonds being alternatively 14" and 156" a mismatch of 10". The high polymer is much more reactive to water and other nucleophiles than the cyclic polymers presumably because of its open structure and ease of deformation. The initial rate of hydrolysis in aqueous acetone is of the first-order with respect to the polymer,201 the rate constant at 25"c being (5.48 0.28) x min.-l It is striking that apart from the isothiocyanates which may polymer- ise through the ligands the only phosphonitrilic derivatives to polymerise lg6 Gimblett Polymer 1960 1 418.lP* Jacques unpublished work. lU7 Schmitz-Dumont Angew. Chem. 1939 52 498; 2. Elektrochem. 1939 45 651. 19* Specker 2. aiiorg. Chenr. 1950 263 133; Angew. Chern. 1953 65 299. 199 Knoesel Parrod and Benoit Compt. rend. 1960,251 2944. * O 0 Giglio Pompa and Ripamonti J. Polymer Sci. 1962 59 293. 201 Gimblett Trans. Faraday SOC. 1960 56 528. PADDOCK PHOSPHONITRILIC DERIVATIVES 207 on heating are the halides. This may result from the strong inductive influence of the halogens an appreciable part of the heat of polymerisa- tion of (especially) the trimeric molecules coming from the opening of the ring angle at nitrogen and delocalisation of its lone pair.Less electronegative ligands and especially electron-releasing groups tend to prevent delocalisation of electrons from nitrogen and reduce the difference between units in the trimer and the high polymer so that the heat of poly- merisation may be inadequate to compensate for the loss of translational entropy. High polymers prepared at low temperatures (e.g. by the azide method) may therefore be thermodynamically unstable even with high bond energies and may depend on kinetic factors to ensure stability at moderate temperatures. Similar views have been expressed by Burg.202 Applications to Other Molecules The importance of p.rr-d?r bonding in other compounds of silicon phosphorus and sulphur has often been pointed out and the structural evidence for a double v-system in many of them both linear and cyclic has been reviewed by Cruickshank.lo6 Compounds of sulphur and nitrogen include many with formally alternating double and single bonds which are in this respect analogous to phosphonitrilic compounds.The results of three recent structure deter- minations are shown in Fig. 11. In both trithiazyl chloride and a-sulpha- nuric chloride all the ring bonds are equal in length consistent with at Cl C'l070 0 2 . " / 2.00&$.40 113.4' Cl-s N F / 0 F FIG. 1 1 . Structures of (a) trithiazyl chloride (b) a-sulphanuric chloride,aJJ and ( c ) tetrathiazyl ,fluoride.e a Wiegers and Vos Proc. Chern. SOC. 1962 387; b Bannister and Hazell Proc. Chew. SOC. 1962 282; Wiegers and Vos Acta Cryst. 1961 14 562.*02 Burg J. Chent. Educ. 1960 37 482. 208 QUARTERLY REVIEWS least partial cyclic delocalisation and are shorter in the latter molecule ; an oxygen atom doubly-bonded to sulphur or phosphorus is especially effective in polarising d - ~ r b i t a l s . ~ ~ ~ The ring angle at sulphur is kept small in the thiazyl compound by the lone pair on sulphur which also prevents delocalisation of the nitrogen lone pair and in a-sulphanuric chloride by the concentration of electrons in the exocyclic double bonds. Here too the angle at nitrogen is small perhaps because the environment of sulphur is nearly regular tetrahedral and the dc orbitals which would be used by the electrons delocalised from nitrogen are only weakly T-bonding.lo6 The ring is therefore non-planar with symmetry close to C3v.In tetra- thiazyl fluoride the local geometry at both sulphur and nitrogen is very similar to that in the first two compounds but the symmetry of the con- figuration (S4) while minimising repulsive interactions also ensures that T-interactions and consequently bond lengths alternate round the ring ; the difference from (e.g.) tetraphosphonitrilic chloride may arise because the lone pair on sulphur prevents delocalisation of the lone pairs on nitrogen.126 In other molecules notably polyphosphates and siloxanes development of n-character in the skeletal bonds requires delocalisation of lone pairs is therefore weaker and is more sensitive to extramolecular influences. The trimetaphosphate anion is chair-shaped its symmetry being close to CSv (Fig. 12). The tetrametaphosphate ion in the monoclinic form of Na4P4012 4H20 is chair-shaped nearly C2h a symmetry required by the space- I FIG.12. Structures of the anions in LiK2P309,H20a and in monoclinic Na4P40,,,4H20.b a Eanes and Ondik Acta Cryst. 1962 15 1280; bOndik Block and MacGillavry Acta Cryst. 1961 14 555. group for ammonium tetrametaphosphate.204 Like the tub the chair con- figuration is well adapted to minimising exocyclic repulsive interactions here of PO2- groups but its symmetry is too low for equalisation of T- interactions round the ring (Fig. 9) so that the bond lengths are unequal in pairs126 (Fig. 12). In a second (triclinic) modification of Na4P4012,4H20 in which the anion is similarly chair-shaped the ring bonds are more nearly While the reason for the difference is still unexplained it is known that the 203 K.A. R. Mitchell Thesis Univ. of London 1963. 204 Romers Ketelaar and MacGillavry Acta Cryst. 1951 4 114. 206 H. M. Ondik personal communication. PADDOCK PHOSPHONITRILIC DERIVATIVES 209 more highly charged cupric and magnesium ions cause an inversion to the Ded symmetry characteristic of stronger. rr-bonding,206 and it is possible that a change of cation arrangement in the sodium tetrametaphosphates may alter the T/T' ratio without change of configuration. A description of the bonds in metaphosphates in these terms is still incomplete however because it does not account for the instability of a polyyhosphate chain as compared with trimetaphosphate at ordinary temperature^.^^ The high polymer is stable only above the melting point of trimetaphosphate and must therefore be formed from it endothermically contrary to expectation for simpleprr-drr bonding.The ring angle at phos- phorus in the trimetaphosphate ion however is smaller than in phosphoni- trilic structures so permitting some delocalisation of the lone pairs on the ring oxygen atoms and more effective use of the dxp-y2 orbital with a pattern of n-electron energies like prr-prr interaction (Fig. 3). Such in- teraction would stabilise especially the six-membered ring. There is extensive stereochemical evidence for pn-dn contributions to the bonding in simple compounds in which silicon is bound to a more electronegative element carrying lone pairs ; the planarity and low basicity of t r i ~ i l y l a m i n e ~ ~ ~ and the linearity of silyl isothiocyanate208 are well known each being consistent with some contribution from forms like (XXV).Similarly LSiNC is large (130 + 5") in Me,SiNHMe.209 Com- + I I >N Si- (XXV) parative values of bond angles are given in Table 13. The possible tendency towards an expansion of the central valency angle from propane to dimethyl ether as a result of increased bond polarity is counteracted by the increased repulsion from the unshared pairs on the central atorn.2lO TABLE 13. Bond angles und force constants in some compounds of carbon and silicon MeCH,Me MeNHMe MeOMe Me,SiCH,SiMe Me,SiNHSiMe Me,SiOSiMe Lcxca 11 1.5" 111" 111" LSiXSib 120" 131" 150" Deformation const. SiXSi 0-05 1 0.03 1 0.021 Stretching const. Si-X a From Ref. 101. b Bond angles and force constants estimated from vibrational spectra (Kriegsmann Z.Elektrochem. 1957 61 1088). Owing to the simplicity of the assumed force field the results for the silicon compounds are approximate only. 206 Steger and Simon 2. anorg. Chem. 1958 294 1 ; Steger ibid. p. 146. 207 Hedberg J. Amer. Chem. SOC. 1955 77 6491. 208 Jenkins Kewley and Sugden Proc. Chem. SOC. 1960 220. 2 0 9 Roper and Wilkins Trans. Faraday Soc. 1962 58 1686. 210 Dickens and Linnett Quart. Rev. 1957 11 291 ; Gillespie and Nyholm ibid. p. 339; Gillespie J. Amer. Chem. Soc. 1960,82,5978. 4 210 QUARTERLY REVIEWS Partial delocalisation of the lone pairs into the acceptor orbitals of silicon decreases the deformation constant and increases the bond repulsion especially because the asymmetry of the pn-dn bond (Fig. 1 b) concentrates electron density near the central atom.The effect of relative electro- negativity on bond angles in ring and chain molecules211 in which Si P S alternate with C N or 0 is therefore indirect and it is possible to under- stand on this basis why E(Si-0) in hexamethylcyclotrisiloxane (Me2SiO) is hardly larger (108 kca1.)212 than would be expected for a single bond even though 213iSiOSi = 136" indicating appreciable lone-pair delocalisation. It is also understandable that this angle increases214 in the centrosymmetrical octamethylcyclotetrasiloxane to an average of 142.5" without the attachment of electronegative ligands and that the increase is accompanied by an increased bond strength [dH,o~.(Me2SiO)3 == -3.5 k~al./mole;~l~ dHpo1.(Me2SiO) - 0216] and a decreased base Because of the tendency of the oxygen atom to localise the n-bonding pairs close to itself these effects which are similar to those found in a comparison of trimeric and tetrameric phosphonitrilic derivatives are probably better regarded as due to lower steric strain in the larger molecules; certainly the comparative thermochemistry2ls and base strengths217 of cyclic and linear siloxanes are consistent with this view the weakness of v-bonding in (Me,SiO) being confirmed by the small average ring angle at silicon (109°).214 Although they have the same origin and their qualita- tive effects are similar n-bonding and steric strain are distinct the balance between them depending on relative electronegativity.In silicates as in phosphates the nature of the cation has important effects on the distribu- tion of n - e l e c t r o n ~ . ~ ~ ~ These examples show that the concepts of d-orbital interaction developed especially in connection with phosphonitrilic chemistry are relevant to other compounds of silicon phosphorus and sulphur and often show up their chemistry in a new light.There is a useful gain in coherence that leads to an indication of areas of new experiment especially in those cases where the relation between structure and reactions is still imperfectly understood. The author is very grateful to Professor D. P. Craig Mr. H. T. Searle and Mr. S. M. Todd for their helpful comments and to Dr. D. B. Sowerby and to Dr. R. F. M. White for permission to quote unpublished results. 211 Liebau Acta Cryst. 1961 14 1103. 212 Tanaka Takahashi Okawara and Watase Bull. Chem. Soc. Japan 1955,28 15. 213 Peyronel Afti. Accad.naz. Lincei Rend. CIasse Sci. 8s. maf. nut. 1954 16,23 1. 214 Steinfink Post and Fankuchen Acta Cryst. 1955 8 420. 215 Piccoli Haberland and Merker J. Amer. Chem. Soc. 1960 82 1883. 216 A. J. Barry personal communication. 217 West Whatley and Lake J . Amer. Chem. SOC. 1961 83 761. 218 Tanaka Bull. Chem. Soc. Japan 1960 33,282. 218 Noll Angew. Chem. I n f . Edn. 1963 2 73.

ISSN:0009-2681    

DOI:10.1039/QR9641800168

出版商:RSC    

年代:1964

数据来源: RSC

摘要:

THE CYCLISATION OF OLEFINIC ACIDS TO KETONES AND LACTONES By M. F. ANSELL (DEPARTMENT OF CHEMISTRY QUEEN MARY COLLEGE UNIVERSITY OF LONDON) and M. H. PALMER (DEPARTMENT OF CHEMISTRY UNIVERSITY OF EDINBURGH) STRONG acids such as polyphosphoric acid or sulphuric acid wil1,l under suitable conditions convert an olefinic acid into the isomeric y- and/or &lactone together with a mixture of a 2-substituted cyclopent-2-enone and a 2-substituted cyclohex-Zenone as illustrated schematically below. R-CH;CH=CH[CH& C02H -+ + n+l 30 Such reactions occur irrespective of the relative positions of the double bond and carboxyl group provided that the double bond is free to migrate (even if in some cases with concommitant skeletal change) to the corres- ponding alk-4-enoic acid. Related to these reactions are the lactonisation of non-olefinic acids via carbonium-ion intermediates and of alka-di- and tri-enoic acids which are considered later.The closely related cyclisa- tions of arylal kanoic acids to benzocycloalkanones are not included since such reactions have been reviewed elsewhere.2 The relative proportions of lactones and ketones formed on cyclisation of an olefinic acid are dependent on both the cyclising agent and the reac- tion temperature. Strong protonating agents such as sulphuric acid,3 toluene-p-sulphonic acid,4 formic acid,5 oxalic acid,6 trifluoroacetic acid,ld hydrogen fluoride,' and hydrogen halides in acetic a ~ i d ~ ~ yield mainly (a) Ansell and Brown J. 1958 2955; (b) Ansell and Coombs unpublished results; (c) Ansell and Emmett unpublished results; (d) Ansell and Palmer J.1963 2640. Johnson Org. Reactions 1944,11 114. (a) Linstead J. 1932 115; (b) Linstead and Rydon J. 1933 580; (c) 1934 1995. Plattner and St. Pfau Helv. Chim. Acta 1937 20 1474. (u) Collin-Asselineau Compt. rend. 1952 235 634; 1953 237 1535; (b) Dietrich and Lederer Heh. Chim. Acta 1952 35 1148; (c) Buchi Saar and Eschenmoser Experientiu 1956 12 136; (d) Davy Halsall and Jones J. 1951 2696; (e) Robertson Soliman and Owen J. 1939,2696; Schenk Gutman Jeger and Ruzicka Helv. Chim. Acta 1952 35 817. (a) Elliot and Linstead J. 1938,660; (b) Burnop Elliott and Linstead J. 1940,727; (c) Mathieson J. 1951 177. ' (a) Johnson Johnson and Peterson J. Amer. Chem. SOC. 1945 67 1360 1366; (b) Johnson Stromberg and Peterson ibid. 1949,71 1384. El-Abbady J. Amer. Chem. SOC 1957 79 1757; Johnson Davis Hunt and Stork ibid.1948 70 3021 ; Reigel Siegel and Kritchevsky ibid. 1948 70 2950. 21 1 4* 212 QUARTERLY REVIEWS lactones whereas hot polyphosphoric acid,lpg phosphorus pentoxide,6bp10 zinc chloride in acetic acid acetic anh~dride,~ c 9 7 9 8 stannic chloride’ll and trifluoroacetic anhydridef2 yield mainly ketones. It seems probable that the first group of reagents preferentially protonate the ethylene linkage whereas the latter group preferentially attack the carbonyl group produc- ing either acylium ions or highly polarised donor-acceptor co-ordination these mechanistic differences leading to different products. Thus little double-bond migration occurs in reactions with phosphorus pentoxideSb~l0 or trifluoroacetic anhydridelb9 and cyclisation occurs only with d4 d5 or d6 alkenoic acids.Lactonisation of suitably substituted alk-3 and 4-enoic acids may be induced by electrophiles other than protons. Thus halogens14 or cyanogen iodide15 yield halogeno-lactones peracidP yield hydroxy-lactones tellurium halides17 and aryltellurium halides1’ yield trichloro tellurium and aryldichloro tellurium lactones sulphenyl halides18 yield alkylthio- lactones and in suitable cases mercuric acetate and sodium chloride yield chloromercuri-lactones.l The formation of halogeno-lactones can occur under mild conditions and has been used to determine the positions of ethylene linkages relative to carboxyl groups particularly among the triter~en0ids.l~~ Lactonisation of Olefinic Acids.-Mechanism and stereochemistry . The stereospecific nature of many lactonisations of olefinic acids both in proton and general acid (electrophile) catalysed reactions has led to a modification of Robinson’s original mechanism,2o which postulated addition of the electrophile to the double bond followed by attack of the resulting carbo- nium ion on the carboxyl group.Furthermore some reagents (e.g. ICN) show little tendency15 to add to an ethylene linkage in the absence of a neighbouring carboxyl group or one of its derivatives. In general lactonisa- tion of cis- and trans-isomers of an alkenoic acid leads to diastereoisomeric lactones irrespective of the lactonising agent.14a It has been that trans-addition to the double bond is usual in these reactions but that the product isolated may (especially if a strong acid catalyst is used) be ( a ) Jacobs and Dev Chem.and Ind. 1956 576; J. Indian Chem. SOC. 1959 36 429; (6) Rai and Dev ibid. 1957 34 178; Experientia 1955 11 114; (c) Riobe Compt. rend. 1958,247 1016. lo Frank and Pierle J. Amer. Chem. SOC. 1951 73 724 and references there cited. l1 Mathieson J. 1953 3251. l2 Ferrier and Tedder J. 1957 1439. l3 Palmer Chem. and Ind. 1963 589. l4 (a)Berti Tetrahedron 1958,4 393; (b) Simonsen and Ross “The Terpenes” Vol. 5 l5 Arnold and Lindsay J. Amer. Chem. SOC. 1953 75 1048. l6 Berti Bottari and Macchia Gazetta 1960 90 1763. l7 de Moura Campos Tetrahedron 1962 18 521. l* de Moura Campos J. Amer. Chem. SOC. 1954 76 4480. l9 Malaiyandi and Wright Canad. J. Chem. 1963,41 1493; Arakelyn Dangyan and 2o cf. Linstead and May. J. 1927 2565. 21 Klein J . Arner. Chem. SOC. 1959 81 3611.Cambridge Univ. Press 1957. Avetisyan Izvestea Akad. Nauk Arm. S.S.R.; Khim. Nauk 1962 15,438. ANSELL AND PALMER CYCLISATION OF OLEFINIC ACIDS 213 that expected from a cis-addition. This is explicable on the basis of an acid-catalysed epimerisation (see p. 214). The steric course of the lactonisa- tions is consistent with a concerted or near-concerted mechanism,22 which is also consistent with the fact that lactonisation of esters of alkenoic acids leads23 to an inversion of the configuration of the expelled alkyl group. Thus the following mechanism exemplified by the formation of a y- lactone seems probable. 1' R = H or alkyl X = Cl- H,O etc. Whether a y- or 6-lactone is formed in a particular reaction depends mainly on the stability of the lactone under the reaction conditions (thermo- dynamic control).In cases where kinetic control is dominant e.g. in many halogeno-lactonisations the carbonyl group (nucleophile) attacks the more stable carbonium ion. That is in reactions where steric factors are relatively unimportant the order of preference is tertiary > secondary > primary carbonium Steric factors are important by virtue of the fact that the two reactive centres must approach sufficiently closely for bond formation to occur. Lactonisation of alk-4-enoic acids by trifluoroacetic acid,ld that is under conditions where both y- and 6-lactones are stable yield mainly the y-lactone. Molecular models of the suggested transition state (see above) show a greater overlap between the 2p orbitals of the carbonyl oxygen and the vacant orbital of the y-carbonium ion than with that of the 6-carbonium ion.Other examples of the influence of steric factors are the lact~nisation~~ of the substituted stilbenecarboxylic acids (1) which yield more of the y-lactone as the size of the group R increases. R 0"" CO H ( 1 ) The lactonisation of d4-octalin- 1 ,Zdicarboxylic acids26 and related has been studied; the syn-cis (1-CO,H,ax; 2-C02H,eq) and 22 Stork and Burgstahler J. Amer. Chem. Soc. 1955 79 5068. 23 Arnold de Moura Campos and Lindsay J. Amer. Chem. Soc. 1953 75 1044. 24 Linstead J. 1932 115. 25 Dr. G. Berti personal communication. 26 Nazarov Kucherov and Andreev Izvest. Akad. Nauk S.S.S.R. Otdel khirn. Nauk. 1955,89 289; ibid. 1956 951. 27 Nazarov Kucherov and Segal ibid. 1956 559; Kucherov Andreev and Grigo- reva Bull. SOC. chim. France.1960 1406 and later papers. 214 QUARTERLY REVIEWS syn-trans (1-CO,H,eq ; 2-C02H,eq) isomers readily from the y-lactones (2) in the latter case presumably via a syn-trans (eq,eq) + syn-trans (ax,ax) + lactone (2) equilibrium. Hydrogen halide addition (3) via a migration competes with formation of &lactone (4) from the anti-cis-isomer (1-CO,H,eq; 2-C02H,ax) and the anti-trans-isomer ( 5 ) ( l-C02H,eq; 2-C02H,eq) is unreactive the change to anti-trans ( 5 ) (1-CO,H,ax; 2-C02H,ax) being inhibited by Csa-H C,-CO,H interaction. Stability of lactones in acid solution. The nature of the lactonic product from reactions catalysed by strong acids depends on the stability of the lactone in the reaction media. For simple lactones such as hexano- heptano- and octano-lactone the a-lactone ( 6 ; R=H R’=n-alkyl) is converted into the isomeric y-lactone by hot 50% sulphuric acid.ld However the &lactone (7; R=R’=Me) which is stable to cold concen- trated sulphuric acid undergoes a skeletal rearrangement (see below) in hot 50% sulphuric acid but y-lactones of the type (7; R=Me; R’=Et) are not isomerised by hot 50% sulphuric acid.lC In cyclic systems examples are known of the conversion of y- into &lactones e.g.(8)+(9)26 and (1 O)+( 1 l)? Acid catalysis will sometimes as often noted in the diterpene field effect epimerisation of lactones. Thus norambreinolide (12) is according to the reaction conditions converted into either the lactone (13) or the lactone (14).29 j-ro EtW-HI3 z* Noyce Weingarten and Dolby J. Org. Chem. 1961,26,2101. 1B (a) Lucius Chem.Ber. 1960,93 2663; (b) Corey and Sauers J. Amer. Chem. Suc. 1959 81 1739. ANSELL AND PALMER CYCLISATION OF OLEFINIC ACIDS 215 Analogous reactions have been observed in the santonin series.3o Such epimerisations presumably lead to the release of steric strain in the lactone ring formed by methods other than lactonisation of the related un- saturated acid and appear to occur via alkyl-oxygen bond fission of the lactone ring followed by re-lactonisation either directly or via the related unsaturated acid. The latter route may occur particularly in examples involving &lactones which are known3 to exhibit ring-chain tautomerism. Recently it has been shown31 that some yy-dialkyl-y-lactones (e.g. y- methyl-y-caprolactone) are hydrolysed in aqueous ethanolic acid via acyl-oxygen bond fission.Should this be a general mode of cleavage of lactones and applicable to reactions in stronger-acid media then the role of the tertiary alcohol in epimerisation reactions must be considered. The configuration of the lactonederived from an olefinic acidmay depend on the nature of the acid catalyst used as is shown by the conversion32 of oleanolic acid (1 5) into either oleanolic lactone (1 6) or 18-iso-oleanolic lactone (17) which is obtainable directly from iso-oleanolic acid (18). H8r -AcOH t Examples of structural changes occurring when lactones are treated with strong acid have been observed. For example,33 the conversion of the lactone (19) into dihydro-/3-camphenolactone (20) and the conversion1 of y-isopropyl-y-butyrolactone (21) into y-ethyl-y-methylbutyrolactone (22).30 Barton J. Org. Chem. 1950 15 466; Huang-Minlon J. Amer. Chem. Sac. 1948 41 Sandberg Acta Chem. Scand. 1962,16 1124. 32 Barton and Holness J. 1952 86. 33 Sauers J. Amer. Chem. Sac. 1959 81 925. 70 611. 216 QUARTERLY REVIEWS The rearrangement34 of lactone (23) into lactone (24) on silica-gel chroma- tography shows the danger of even attempting to separate some com- pounds by this technique. Lactonisation of Diolefinic Acids.-Alka-2,4-dienoic acids (e.g. 25) lactoni~e~~ in strong acids to the isomeric a/3-unsaturated Nactones (e.g. 26); conversion of the usually all-trans-acid to the all-cis-acid is apparently a n.ecessary prerequisite since cis,trans-hexa-2,4-dienoic is only slowly lactonised. Analogous lactonisations have been car- ried out with 3-hydroxyalk-4-enoic acids (27) and thionyl chloride36u and potassium b i s ~ l p h a t e .~ ~ ~ Halogeno-lactonisation of such acids under the usual conditions is unsuccessfu1,37 probably because of failure of the re- agent to isomerise the all-trans- to the all-cis-acid. Lactonisation of piperi- tylideneacetic acid (28) yields38 the lactone (29) and not an a/3-unsaturated lactone as formation of the latter is not sterically possible from the inter- mediate transoid diene. An interesting photochemical lactonisation of the trans trans-dienoic acid (30) has been reported39 and the following mechanism suggested 34 Buchi and Goldman J. Amer. Chem. SOC. 1957 79 4741. 35 ( a ) Korte and Machleidt Chenz. Ber. 1955 88 136; (6) Eisner Elvidge and Lin- 36 ( a ) Korte and Scharf Chem. Ber. 1962 95 443; (b) Mousseron and Neyrolres 37 Bougault Compt.rend. 1906 143 398. 38 Tribolet Gamboni and Schinz Helv. Chim. Acta 1958 41 1587. 39 Mousseron-Canet Mousseron and Legendre Compt. rend. 1961 252 3928. stead J. 1953 1372. Rull. SOC. chim. France 1960 598. ANSELL AND PALMER CYCLISATION OF OLEFINIC ACIDS 217 The acid-catalysed conversions of conjugated alkenynoic acids into unsaturated lactones has been reported recently,40 and are illustrated below. 0 R.C=-C.CH=CH.CO,H R,/QAO Or R (R=H ref. 4Oa) (R= Ph ref. 40a,b) 8 CC$= CC L *C=C* CO,H - [CCl,=CC1*C(OH)=CHCO2H] - (ref. 40c) The lactonisation of many monocyclic and acyclic nonconjugated diolefinic acids has been recorded,41 in which lactonisation is accompanied by intramolecular alkylation. The order in which the cyclisation steps occur can often be inferred from the stereochemistry and structure of the product.Thus the acid (31) could yield the lactone (32) by either a concerted or non-concerted alkylation-lactonisation process whereas the actual product (33) cannot be formed by a concerted process but must arise via the carbonium ion (34) formed from the ion (35) either by a 1,2-hydride ion shift or via the d1,8a or dsa9ga olefin. ___c CO H CO,H Cycli~ation~~ of geranylacetic acid (36) with phosphoric acid at 25" gives a separable mixture of the epimeric lactones (37 and 38) with the cis-isomer (37) predominating indicating that the reaction is essentially non-concerted but that steric factors are important. However it is possible that the reaction is concerted and that the trans-product is then mainly (37) (38) (39) 40 (a) Schutte and Basanowsky Pharm.Zentralhalle 1959,98,403 ; (6) Jacobs Danker and Danker J. Amer. Chem. Soc. 1958,80,864; (c) Roedig Kleppe and Mark] Chem. Ber. 1962 95 1245. 41 Mousseron-Canet and Mousseron Compt. rend. congr. internat. chim. et. ind. 31e Liege (published as Ind. chim. belge (suppl. 1952 2 625) cf. Chem. Abs. 1960,54 10966. 42 Mondon and Teege Chem. Ber. 1958 91 1020; Mondon and Erdman Angew. Chem. 1958,70 399. 21 8 QUARTERLY REVIEWS epimerised to the cis-product. The same acid (36) with sulphuric acid at 60” yields the isomeric y-lactones with epimer (39) predominating. Lactonisation of Triolefinic Acids.-Many of the early studies of the cyclisation of triolefinic acids were made under conditions where either kinetic or thermodynamic control of the reaction may have been operating and interpretation of the origin of the mixed isomers is not always possible.Furthermore many of the acids were obtained by dehydration of p-hy- d r ~ x y - e s t e r s ~ ~ ~ ~ ~ thus leading to uncertainty as to the position of the ethylene linkage. Cyclisation of farnesylacetic acid (40 ; n = 2)22944 and homofarnesylic acid (40; n= l)29a in the presence of formic and sulphuric acid leads mainly to the products (41; n=2) and (41; n=l) respectively derived from a nearly concerted series of additions to the ethylene linkages although in the latter other stereoisomers were also isolated when sulphuric acid was present. The failure29b of the aj3-conjugated isomers of “a)’- and “/l”-rnono- cyclohomofarnesylic acid (42) to cyclise under the conditions where the /3-non-conjugated isomer and farnesic acid (43) react is due to steric and electronic factors being unfavourable for a concerted or near-concerted Lewis-acid catalysis has been little used in lactonisations but is important since in some cases22 it is possible by choice of reaction conditions to arrest the cyclisations at different stages and thus identify possible intermediates.(40;~=2) SnCL4. (41 ; n = 2) Skeletal Rearrangements during Lactonisation of Olefinic Acids.- Although the occurrence of skeletal rearrangement during lactonisation in 43 Trebolet Gamboni and Schinz Helv. Chim. Acta 1958 41 1589; 1962 45 1036. 44 Dietrich and Lederer Helv. Chim. Acta 1952 35 1148. ANSELL AND PALMER CYCLISATION OF OLEFINIC ACIDS 219 complex systems has been known for some time (see below) it has recently been observed in simple systems.Thus although 5-methylhex-4-enoic acid (45) with cold concentrated sulphuric acid y i e l d ~ l ~ ? ~ ~ the expected 8-methyl-8-hexanolactone (44) treatment with boiling 50 % sulphuric acid yields1 y-methyl-y-hexanolactone (46) as the main lactonic product. (44) (45) (46) This rearrangement can be rationalised by the following reaction sequence which involves the rearrangement of a tertiary carbonium ion to a second- ary carbonium ion and the subsequent migration of a methyl group. A similar mechanism has been suggested45 for the aluminium chloride cata- lysed rearrangement of 2-chlor0-2-methy1[2-C~~] butane to 2-chloro-2- methyl[3-C14] butane. (CH3)2C=CH. [CH,] ,C02H + (CH3),C*CH2*[ CH2]2C02H + H+ + HQ CH3 + C H 3 0 + I H a (CH3)2CH.CH*[CH2]2*C02H + CH3-CH*CH*[CH2],CO2H + CH3 I CH3*CH2*C-[CH2],CO2H + (46).+ Examples of rearrangements in more complex systems are observed in the lactonisation of dihydroabietic and of dihydro- and iso-pimaric acid.47 In these cases the initial protonation is followed by a methyl shift (12-+13) as is illustrated below for dihydropimaric acid (47+48 + 49). 1 45 Roberts McMahon and Hine J. Amer. Chem. SOC. 1950 72 4237. 46 Subluskey and Sanderson J. Amer. Chem. SOC. 1954 76 3512. 47 Wenkert Chem. and Znd. 1955 282; J. Amer. Chem. Soc, 1958 80 2912. 220 QUARTERLY REVIEWS More extensive rearrangements have been observed in the triterpene series; thusg8 quinovic acid (50) on treatment with either concentrated sulphuric acid or zinc chloride in acetic acid yields novic acid (51) probably via the route indicated.Another example is the action of acid on betulic acid (52) in which4@ lactonisation is accompanied by expansion of ring E resulting in the forma- tion of lactone (53). 4 @ 2H H+ [" C02H - &o (~~~ HO H 62) Owing to the possibility of rearrangement occurring lactonisation is not always applicable to the determination of the exo- or endo-nature of substituents in various bridged ring compounds. Thus the Diels-Alder adducts from cyclopentadiene and alk-Zenoic acids might be expected to lactonise only if the carboxyl group occupies the endo (axial) position. However both the endo- (54) and the exo- (55) isomer yield the same lactone (56) because of rearrangement both in acid-catalysed reactions0 & f - + Lbg2 CO,H 48 Barton and de Mayo J.1953 31 11. 49 Davy Halsall and Jones (a) Chem. andInd. 1951 233; (b) J. 1951 2696. 5 0 Beckmann Schaber and Bamberger Chem. Ber. 1954,87 997; McBee Hsu and Roberts J. Amer. Chem. SOC. 1956 78 3389. ANSELL AND PALMER CYCLISATION OF OLEFINIC ACIDS 22 1 and on bromo-lactonisation ;51 but iodo-lact~nisation~~ succeeds only with the endo-isomer. Lactonisation During Carbonium-ion Rearrangements.-From the pre- ceding discussion it is apparent that protonation of olefinic acids leads to a carbonium ion which may yield a lactone directly or after an initial re- arrangement. It follows therefore that lactonisation should occur however the carbonium ion arises provided that a carboxyl group is suitably situated within the molecule. Thus the acid-catalysed ring opening of the cy~lopropane-~~ and cyclobutane-carboxylic (57 and 58) leads to the lactones (59) and (60) respectively.Other examples are among q H C 0 2 H HO2CJ0 H 0 2 C ( y ‘y03’ (57) (58) (59) (601 the bridged-ring terpenes. Thus the acid (61) in the presence of formic acid and sulphuric acid yields the lactone (62). CHJ *CO,H H+ l6l) - The nitrous acid deamination of (+)-y-aminovaleric acid55 yields (-)-y- valerolactone via the y-carbonium ion rather than by way of the y-hydroxy- .... OH (66) 51 Ver Nooy and Rondestvedt J. Amer. Chern. SOC. 1955,77 3583,4878; Braendlin 52 Sandberg Arkiv. Kemi 1960 16,255; Simonsen J. 1922 2298; 1929 305; Baeyer 53 Baeyer Ber. 1896 29 32 1907. 54 Battacharyya Science and Culture 1947 13 158; de Mayo “Mono- and Ses- 55 Austin and Howard Chem. and Ind. 1960 625; 1959 1413.Zielinski and McBee ibid. 1962 84 2109 and Gratieff Ber. 1896,29,2796. quiterpenoids” Interscience New York 1959. 222 QUARTERLY REVIEWS acid. Also formation56 of the lactone (64) on decarboxylation of the acid (63) may provide a further example as does the lactonisation with epimer- isation at C- 16 of 16a-hydroxy-3-oxo-20-isoeburico-7,9( 1 1)dien-2 1 -oic acid (65+66) in the presence of thionyl chloride.57 Cognate Cyc1isations.-As might be expected from the mechanism of lactonisation of unsaturated acids and esters a variety of analogous cyclisations occur. Thus unsaturated amides with bromine yield cyclic quaternary immonium salts ;58 unsaturated nitriles or a r n i d e ~ ~ ~ (e.g. 2,2-diethylpent-4-enonitrile and the corresponding amide) with sulphuric acid yield the imine (e.g.67; X=NH) which can be hydrolysed to the lactone (67 ; X= 0). Suitable unsaturated ketones49 under acid conditions yield cyclic vinyl ethers as illustrated by the partial formulae below. IFMe H'> IFMe ~ {p'L {%cH2 Cycloalkenone Formation.-Whereas lactonisation of an olefinic acid is an isomerisation the formation of a cycloalkenone from an olefinic acid (see p. 21 1) is an overall cyclodehydration reaction. As expected the more vigorous dehydrating agents or conditions lead to a higher propor- tion of ketone the reaction apparently being irreversible. As mentioned earlier sulphuric acid the most frequently used lactonising agent generally promotes ketone formation from alkenoic acid to only a small extent al- though under comparable conditions the intramolecular acylation of p- and y-arylalkanoic acids2 often proceeds in good yield.Intermediate between these two extremes is the action60 of sulphuric acid on 13-0- carboxyphenylcinnamic acid (68) which yields approximately equal amounts of ketone (69) and lactone (70). @ CO,H 0 0 (69) Johnson and his co-workers7 have given many examples of the conver- sion of olefinic acids into lactones by the action of hydrogen chloride in acetic acid whereas cycloalkenones may be obtained either from the lactones or directly from the acids by the action of zinc chloride in acetic acid-acetic anhydride presumably as a result of the greater de- 56 Tiemann Ber. 1896 29 2612. 57 Bowers Halsall and Sayer J. 1954 3070. 58 Craig J. Amer. Chem. SOC. 1952 74 129. 59 Raffauf J. Amer. Chem. SOC. 1952 74 4460.6o Marsili Ann. Chim. (France) 1961 51 237. ANSELL AND PALMER CYCLISATION OF OLEFINIC ACIDS 223 hydrating power of the latter reagent. A given reagent generally leads to a higher yield of ketone as the reaction temperature is increased. Thus,lC whereas oct-4-enoic acid with polyphosphoric acid at 50" yields mainly the y- and &lactones as the temperature is raised to 100" the yield of cyclo- alkenone increases at the expense of the &lactone. The action of hot acids especially polyphosphoric acid,9 on &lactones (e.g. 71) has been used in the preparation of cycloalkenones (e.g. 72). y-Lactones behave similarly when heated with polyphosphoric acid or phosphorus pentoxide.61 0 Lactonisation has been postulatedgc as an intermediate step in the forma- tion of cycloalkenones from olefinic acid since lactonisation occurs more easily (i.e.at a lower temperature) than ketone formation. This however seems unlikely as several examples are known7" of higher yields of ketones being obtained from the olefinic acid than from the corresponding lactone suggesting in fact that conversion of lactones into ketones proceeds via the unsaturated acid. A mechanism for this step (73+74) has been suggestedgasb but various other mechanisms involving carbonyl co-ordinated species13 are possible. # o - do - J-d;o.Po2 (74) - P0;O-PO2 H (73) H It has been suggested1a~61 that cycloalkenone formation occurs via an acylium ion presumably formed since13 protonation of carboxylic acid derivatives normally occurs at the carbonyl oxygen atom via the less favoured species (75),62 as is illustrated below for the formationla of 2-isopropylidenecyclopentanone (76) from 6-methylhept-5-enoic acid (77).la Thus in strong acid such as polyphosphoric acid the reactions of olefiaic acids and the corresponding lactones are qualitatively the same.(77 1 (75) Effect of Reagent and Reaction Conditions.-The intramolecular acylation of olefinic acids by polyphosphoric acid or sulphuric acid yields only five- 61 La Forge and Barthel J. Org. Chem. 1945 10 222; Gupta and Deshapande J. Indian Chem. SOC. 1953,30,23; Achmad and Cavill Proc. Chem. SOC. 1963,166. 62 Halsall Meakins and Swayne J. 1953 4139. 224 QUARTERLY REVIEWS and six-membered cyclic ketones irrespective of the relative positions of the carboxyl group and the double bond indicating that the double bond migrates freely both towards and away from the carboxyl group.la,blc Reactions at low temperatures (ca.50") indicate the preferenrial formation of exocyclic unsaturated ketones.lb$ It has been shown that isomerisation of the latter class of compounds to endocyclic unsaturated ketones occurs in acid solution at higher temperature^.^^ At room temperature the ultra- violet spectra of exo- and endo-cyclic unsaturated ketones are markedly differenP4 and very little if any interconversion occurs. At higher tempera- tures (ca. 100") in polyphosphoric acid formation of cycloalkenone is virtually irreversible and very little interconversion between rings of different size is apparent.lb Trifluoroacetic anhydride effects the intramolecular acylation of certain olefinic acids,12 probably via a mixed anhydride which breaks down to give the acylium ion of the olefinic acid RC02H + (CF3.C0)20 + RCO*OCO-CF + CH3COzH .1 RCO+ + -OCOCF3.No decision can at present be made as to the mechanism of the reaction but the presence of non-fluorinated anhydrides has been established.lb The reaction conditions are such that double-bond migration does not occur (trifluoroacetic acid a by-product of the reaction only promotes double-bond migration slowly and at elevated temperature@ ,65) and cyclisations have with one exception only been effected with alk-5-enoic acids (78) which when R=Me or Et yield mixtures of the corresponding 2-alkylcyclohex-2-enone (79) and 2-alkylidenecyclopentanone (80). With hex-5-enoic acid (78; R=H) only cyclohex-2-enone (79; R=H) is iso- (78) (79) (80) lated,l29lb probably due to the instability of 2-methylene~yclopentanone.~~ The cyclisation of hept-6-enoic acid1 with trifluoracetic anhydride yielded cyclohept-Zenone (10 %).The unstable66 isomeric 2-methylene- cyclohexanone was not detected. This is the first example of the formation of a seven-membered ring by the intramolecular acylation of an alkenoic acid. Ketones from Diolefinic Acids.-The direct conversion of a diolefinic acid (81) into a bicyclic ketone (83) has not been obser~ed,~' only the lactone (82) being isolated which could in turn be converted into the 1 3 ~ Brown Brewster and Schecter J. Amer. Chem. SOC. 1954 76 467. 64 Palmer and Urch J. 1963 174. 65 Peterson and Allen J. Org. Chem. 1962 27 1505. 66 Erskine and Waight J. 1960 3425. 13' Ansell and Ducker J. 1960 5219.ANSELL AND PALMER CYCLISATION OF OLEFINIC ACIDS 225 octalone (83). However the corresponding acid chloride on treatment with aluminium chloride yields the octalone (83) in low yield.67 (80 \ SOCL - AlCl Recently,68 the action of acetic anhydride containing small amounts of acidic compounds such as zinc chloride aluminium chloride stannic chloride or sulphuric acid has been shown to convert alka-2,4-dienoic acids (89) into phenols (90). This reaction can clearly be considered as the formation of a cyclohexadienone followed by enolisation. Rearrangements during Ketone Formation.-Just as carbonium-ion rearrangements occur in the lactonisation of olefinic acids they can also be observed in the formation of ketones from olefinic acids or the related lactones. Thus the actionlC of hot 50% sulphuric acid on y-methyl-6- hexanolactone (9 1) yields besides the expected 2,3-dimethylcyclopent-2- enone (92) appreciable quantities of 3-methylcyclohex-2-enone (93). The formation of the latter clearly involves the migration of a methyl group and is related to the rearrangements associated with 5-methylhex-4-enoic acid discussed on p. 219. 0 0 Ambreinolide (94) in the presence of hot 80% sulphuric acid yields69 the perinaphthene derivative (95). This reaction can be interpreted as illustrated below. 68 Chiusoli and Agnes Proc. Chern. Soc. 1963 310. g9 Buchi Saar and Eschenmoser Experientia 1956 12 136.

ISSN:0009-2681    

DOI:10.1039/QR9641800211

出版商:RSC    

年代:1964

数据来源: RSC