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QUARTERLY REVIEWS MEXHANISIVLS OF HYDROGEN CATALYSIS By D. D. ELEY PH.D. (NASH LECTURER IN BIOPHYSICAL CHEMISTRY UNIVERSITY OF BRISTOL) Introduction.-The building blocks of any theory of catalysis must be facts concerning reaction mechanism and in particular the details of reaction paths in a number of simple instances. The work of Rideal and his school Farkas Beeck and others has greatly advanced our knowledge of hydrogenation reactions and we shall discuss this and other work from this general viewpoint. In the first place we shall review recent knowledge of chemisorbed films that is unimolecular adsorbed films of gases held to the catalyst by forces so strong that they must be chemical in nature which Langmuir showed to be responsible for surface catalysis. Chemisorbed Films Hydrogen.-Because of the production of atomic hydrogen by clean suggested that the gas was dissociated and metal surfaces I.Langmuir held as atoms. If M denotes a surface metal atom H + 2 M -+ 2MH The very strength with which hydrogen is chemisorbed by metals is in favour of this hypothesis which however is not easy t o verify exactly owing to lack of knowledge of surface areas of metals. Working with tung- sten wires and assuming a reasonable roughness factor (ratio true apparent area) J. K. Roberts 2 has verified this to a first approximation. 0. Beeck A. E. Smith and A. Wheeler have shown that on evaporated films of nickel there is twice as much carbon monoxide chemisorbed as hydrogen and if we assume single-site adsorption for carbon monoxide CO + M + M*CO then the dissociation of hydrogen into atoms is very probable.W. G. Frankenburger has measured surface areas of tungsten powder by the B.E.T. method and has concluded that one hydrogen atom is adsorbed per tungsten surface atom. J . Amer. Chern. SOC. 1916 38 1145. ( a ) Proc. Roy. Xoc. 1935 A 152 445. Cambridge University Press London 1939. J . ,4?ner. Chem. Soc. 1944 66 1827 1838. ( b ) " Some Problems in Adsorption " Proc. Roy. Soc. 1940 A 177 62. 209 0 210 QUARTERLY REVIEWS The theory of chemisorption due to J. E. Lennard-Jones 5 starts from the model of a metal in which electrons move freely through Dht. periodic potential field due to the positive cores of the metal. The effect of an adsorbed atom on the surface of the metal is then to lead to a lowering of the potential energy of the electron in the surface of the metal in its immediate neighbourhood.Thus we have now two sets of essentially Zocalised energy levels for the electron one on the hydrogen atom and one on the surface of the metal. If the wave functions corresponding t o these energy levels overlap we may have the exchange phenomenon with the formation of an electron-pair bond the second electron for this bond coming from t'he conduction band of the metal. W. G. Pollard has developed this picture and considers in general that a two-electron bond will give an unstable charge distribution. Thus only one electron is available to form a bond to the adsorbed atom. We visualise the bond as covalent as in H2+ where the two levels are nearly equal or ionic as in Na+Cl- where I b n - Distance from metal FIG. 1 The potential energy curve for the chernisorption of hydrogen 2M + H -+ 2MH.the two levels are widely different in energy. Fig. 1 shows the potential energy curve for a hydrogen atom in the neighbourhood of a metal. The curve ua refers to the approach of a hydrogen molecule to the metal surface and is made up of the attractive London dis- persion forces and the usual repulsive forces arising from the int'eraction of closed electron shells. The value q corresponds to the heat of van der Waals adsorption and is about 2 kcals./g.-mol. for hydrogen. Curve bb gives the energy of the chemical bond between the hydrogen atom and the metal or rather twice this value. Q is the heat of chemi- sorption usually a t least 20 kcals./g.-mol. Where the two curves intersect a molecule held in the van der Waals layer may pass into a state of chemi- sorbed atoms the necessary activation energy being E.calculate similar potential curves making use of the Eyring-Polanyi treat- ment of the London equation for four electrons. Thus for nickel the ex- change and coulombic terms in the London equation are obtained by sub- dividing the Morse curve for NiH obtained from spectroscopic data. Whilst the method is restricted in so far as it regards the surface metal at.oms as isolated it does serve to bring out the possible importance of lattice spacing on the activation energy for chemisorption of hydrogen. Quantitatively the activat'ion energies obtained for hydrogen in general are too high. Nowadays we know from experiment that the activation energy of adsorp- H. Eyring and A. Sherman and A. Okamoto J.Horiuti and K. Hirota Trans. Faraday SOC. 1932 28 333. Physical Reviews 1939 56 324. Sci. Papers Inst. Phys. C?bem. Res. Tokyo 1936 29 223. J . Amer. Chem. SOC. 1933 45 2661. ELEY MECHANISMS OF HYDROGEN CATALYSIS 211 tion of hydrogen on t ~ n g s t e n ~ nickel and other metals 39 11 is close to zero. The activation energies for other gases such as oxygen nitrogen carbon monoxide etc. on these metals must also be small. The rapid chemisorption of the first two gases a t liquid-air temperature has been detected by constant potential work and of all three by the poisoning of the para-hydrogen conversion.1° Until recently many such chemisorptions on metals were believed to possess an appreciable activation energy (" activated adsorptions ") according to the hypothesis of Taylor to account for slow sorption.12 Whilst this hypothesis may hold for oxides and is not ruled out in principle for any solid slow adsorption on metals must in many cases be attributable to displacement of impurities 2 9 1 3 or sorption into the s01id.l~~ l5 The heat of adsorption of hydrogen on tungsten falls from a value of 45 kcals.for the bare surface to a value of 18 kcals. for the almost full surface. These measurements were made by Roberts 2 by measuring the minute temperature changes associated with chemisorption on fine tungsten wires and they have been checked by 0. Beeck 11 using evaporated films of 10,000 times the area of the wire. Beeck also finds a similar result for nickel. The powerful adsorption of hydrogen on transition metals is undoubtedly due to the partly empty d-band of this metal which gives rise to a high concentration of empty d levels in the surface much lower in energy than most of the s-levels.lG That the d-band in palladium is employed in bonding dissolved hydrogen has long been known from magnetic measurements and it seems natural to postulate a similar effect for adsorbed hydrogen.Incidentally it has been found possible to detect magnetic changes associated with the chemisorption of a monolayer of dimethyl sulphide on palladium,l* and the method is being extended to hydrogen and other gases. Hydrogen dissolved in palladium is positively charged but only. weakly so corresponding to about one-fiftieth of an electronic charge per atom.19r 2o We may suppose that while the hydrogen atom transfers an electron into the d-band of the metal the proton does in fact draw a large cloud of negative electricity around it.Such behaviour is necessary on energetic grounds.21 On platinum chemisorbed hydrogen has been found to be positive.22 On tungsten the contact potential of hydrogen yields a dipole R. C. L. Bosworth and E. K. Rideal Physica 1937 4 925. lo D. D. Eley and E. K. RideaI Proc. Roy. Soc. 1941 A 178 429. l1 0. Beeck Physical Reviews 1945 17 61. l 2 H. S. Taylor J. Amer. Ghem. SOC. 1931 53 578. 13A. J. Allmand and R. Chaplin Trans. Faraday Soc. 1931 28 223. l4 A. F. H. Ward ibid. p. 399. l5 0. Beeck A. W. Ritchie and A. Wheeler J. Coll. Sci. 1948 3 504. l6 D. D. Eley Research 1948 1 304. l7 N. F. Mott and H. Jones " Theory of Metals and Alloys " Oxford 1936. l8 M. H. Dilke D. D. Eley and E. B. Maxted Nature 1948 161 804.lS A. Coehn and W. Specht 2. Physik 1930 62 1 ; B. Duhm ibid. 1935 94 431. 2o P. H. Emmet and E. Teller " Twelfth Report of Committee on Catalysis " 2 2 C. W. Oatley Proc. Physical SOC. 1939 51 318. p. 77 New York 1940. 21 J . Franck Nachr. Ges. Wiss. Gottingen 1933 2 293. 21 2 QUARTERLY REVIEWS moment W+ - H- of 0.4 D. hydrogen being negative.23 This result has been confirmed by several other workers in Rideal's laboratory. In any event the dipole is a small one and the bond is essentially non-polar. Reverting t o the decrease of heat of adsorption with fraction of surface covered the approximately linear relation observed has been attributed to a repulsive potential between nearest neighbours,2 and the theory for both mobile and immobile chemisorbed atoms fits the experiments equally well.24 Since the activation energy for surface diffusion of atoms in a monolayer is usually about to Q of the bond strength of the atoms to the surface this should be about 20 kcals.for hydrogen on tungsten. Thus we should expect a mobile film a t 293" K. and 193" K. but an immobile film a t 77" K. From the decrease of heat of chemisorption with surface covered on porous metal films of area approx. 10,000 sq. em. Beeck 11 has concluded that hydrogen is mobile on nickel and iron at room temperature but not on iron at 90" K. Where an immobile film is formed by dissociation of gaseous molecules striking a surface it is a statistical requirement that 8% of the total sites will be " cut-off " as isolated single sites and be incapable of taking up a hydrogen atom.2 Roberts suggested that such sites might play a r61e in catalysis.Other Gases.-Langmuir assumed that oxygen dissociated into atoms on platinum and this was Roberts's original postulate for oxygen on tungsten.2 The heat of chemisorption is so high approx. 150 lmals. for a bare surface that the oxygen film is almost certainly immobile a t room temperature. Such an immobile atomic film should possess 8% of holes and indeed a second fdm of oxygen has been found to be adsorbed with a heat of 48 kcals. However the number of molecules in this film and the change of accommo- dation coefficient accompanying its removal are too large for the 8% of holes. If oxygen were adsorbed as mobecules then each molecule by its very size would exclude occupation of four adjacent sites on the 110 or 100 planes of tungsten leading to 50% of gaps t.hus allowing a relatively large uptake of gas in the secondary film as described by Roberts.%* 25 The W-WO contact potential of - 1.76 v.cannot properly be interpreted in the absence of knowledge concerning the structure of the film. If the film were atomic the dipole W-0 would be 0-66 D. the oxygen being nega- tive. For nickel Beeck has found that a t least a double monolayer of oxygen molecules is formed a t low pressures but one suspects that this and other metals may be complicated by formation of surface oxide. There is evidence for surface oxide on nickel catalysing the hydrogen-oxygen reaction.26 R. Juza and R. Langheim find that the n bond in the 0 molecule is ruptured on charcoal since on chemisorption the 0 molecule loses its paramagneti~rn.~' It is very likely that the n electrons are used in bonding the molecule to charcoal and also to metals.Nitrogen is chemisorbed rapidly 9 l o on tungsten a t 90" K. and its 2 3 R. C. L. Bosworth Proc. Camb. PJd. SOC. 1937 33 394. z 4 A. R. Miller and J. K. Roberts ibid. 1941 37 82. 28 D. R. Hughes and D. C . Bevan Proc. Roy. SOC. 1927 A 117 100. 27 2. Elektrochem. 1939 45 689. 25 Ibid. 1940 36 53. ELEY MECHANISMS OF HYDROGEN CATALYSIS 213 contact potential is negative - 1.38 v.9 On other metals such as iron it may be chemisorbed slowly. Beeck finds that nitrogen is not chemisorbed by nickel at 2 9 3 " ~ . l in contrast to hydrogen carbon monoxide oxygen and ethylene. Ethylene is chemisorbed rapidly by tungsten as is apparent from its effect in poisoning the para-hydrogen conversion.l6 According to BeeckY8 at saturation one ethylene molecule occupies four nickel sites which may be explained by his later observation 11 that on chemisorption on nickel an ethylene molecule dissociates into an acetylene complex and two hydrogen atoms. The heat of chemisorption for ethylene on nickel falls from 58 kcals. for a bare surface to 25 kcals. for a full ( ?) surface,ll but it is stated to be complicated to some extent by a side reaction (hydrogenation of a second ethylene molecule by the chemisorbed hydrogen atoms). So far a value for the contact potential has not been published. Beeck assumes that each carbon monoxide molecule occupies zt single site on nickel. Mixed Monolayers found that oxygen gas would throw an adsorbed monolayer of hydrogen off a tungsten wire in agreement with the known difference of 100 kcals.in their heats of chemisorption. Langmuir in his early papers 28 visualised the following two types of bimolecular catalytic reaction. (1) The two gases compete on more or less equal terms for sites in the chemisorbed monolayer and adjacent molecules or radicals interact. This mechanism which we might call the adjacent interaction (A.I.) mechanism has been applied particularly by C. N. Hinshelwood z9 and G. M. Schwab 30 to kinetic data. (2) Reaction occurs between chemisorbed A and a molecule of B colliding with it from the gas phase or van der Waals layer. This mechanism which we might call van der Waals-chemisorbed layer interaction (V.C.1.) has been applied recently by E. K. Rideal to certain simple exchange reactions of hydrogen.31 It may be diiiicult to decide between (1) and (2) on kinetic grounds.There seems little reason to doubt the applicability of the A.I. mechanism to many catalytic reactions a t high temperatures where both components give sparse monolayers. One can however make some quite general observations for reactions occurring at say T < 200" c. In these circum- stances many pure gases will tend to give full .chemisorbed layers even a t very low pressures. Consider two gases A and B pressures pA and pE heats of chemisorption LA LB (considered independent of surface fraction 6). Then we may suppose that t'he more strongly adsorbed gas A occupies 28 I. Langmuir Trans. Paraday SOC. 1921 17 607. 29 C. N. Hinshelwood " The Kinetics of Chemical Change in Gaseous Systems " a1 ( a ) Proc.Camb. Phil. SOC. 1938 35 130 ; ( b ) Sabatier Lecture Chem. and Ind. Scarcely any work has been done on mixed monolayers as yet. Roberts 3rd Edn. Oxford 1933. 1943 62 735. 30 G. 11. Schwab " Catalysis " London 1937. 214 QUARTERLY REVIEWS nearly all the surface that is eA - 1 and that OB can occupy the surface only by an expenditure of internal energy LA - LB ; since OA = 1 - OB Suppose we take the case of ethylene and hydrogen on nickel as an example. If we neglect the effect of surface covered on heats of adsorp- tion and take the L values for the full (0 = 1) single-component mono- layers l1 as relevant to the present calculation for equal pressures of hydrogen and ethylene we find a very small fraction of chemjsorbed hydrogen vix. oB/(I - oB) = pBe(LA-LB)/RT/pa It seems rather unlikely that the A.I.mechanism will hold when the second component has a chemisorbed concentration of only 10-7 unless the activa- tion energy for this mechanism is very much less than that for a V.C.I. mechanism. Rideal 31 has advanced a quite general reason why the V.C.I. mechanism should have a low activation energy for the case of monolayers containing " gaps ',. These gaps are supposed to furnish a particularly strong van der Waals adsorption and to supply a free valency which on general grounds will be expected to lower the inertia of the reaction. Thus for the exchange of deuterium with ethylene D2 CHZ-CH 1 CH2-CHZD D CH2-CHD HD I I + I ~ 1 - 1 I I I M M M M M 1LI M M M Steric considerations will probably lead to some gaps in the ethylene monolayer whether the monolayer is mobile or immobile.It is of course possible that the above considerations might be modified if the A-B repulsions in the monolayer were weaker than the A-A or B-B repulsions (or were even attractions). This might be the case where the two components had opposite dipoles in which case one would tend to get a higher concentration of hydrogen than that calculated above. C. W. Oatley has made a few observations of this kind.22 Generally speak- ing the electrostatic neighbour interaet'ions will be about 1 kcal. sufficient markedly to modify adsorption concentrations. D. D. Eley and Rideal lo have related the conversion of para-hydrogen in a partial film of oxygen on tungsten to this concept. Incidentally the decrease in heat of chemisorption of hydrogen with the surface covered is too large to be only a dipolar effect.It would require a W-H dipole moment of 1-94 D. in comparison with 0.44 D. observed. We would suggest that the clifferenee might be ascribed to a band-nature of the surface orbitals. The first d-orbitals taken up will of course be lower in energy than those occupied by the last amounts of hydrogen on the surface. Para-hydrogen Conversion The para-hydrogen conversion and the related hydrogen deuteride reaction because of their simplicity have been subjected to detailed study, ELEY MECHANISMS OF HYDROGEN CATALYSIS 215 originally by the pioneer workers K. F. Bonhoeffer and A. and L. Farkas and more recently by the Reviewer and his colleagues. Two mechanisms for the conversion exist a chemical and a paramagnetic mechanism.Since the latter involves a mere physical twisting of the spins of the hydrogen nuclei we shall no longer concern ourselves with it. The paramagnetic mechanism may usually be distinguished since in no case can it give the hydrogen deuteride reaction H +D2 -+ 2HD On the other hand the evidence is that the conversion of para-hydrogen and hydrogen deuteride goes through a chemical mechanism on the transition metals. The reactions occur with similar rapidity and have the same kinetics (on nickel and on tungsten). The position up to 1935 is set forth in the work by A. Farkasy32 and more recent work has been discussed by Eley in At present we shall only discuss features of general interest. I n the first place the mechanism was supposed to involve the dissociation of hydrogen molecules on the surface of the catalyst and the recombinat,ion of the atoms to give ortho-molecules.3~ p-H2 + 2M -j.2MH + 2M + o - H ~ This mechanism was advanced for platinum and other metals including tungsten where however it was challenged by Roberts,35 who pointed out that a t ordinary temperatures the hydrogen atoms form a saturated firmly- bound layer and do not recombine a t any appreciable rate. Rideal 31a suggested an exchange mechanism involving hydrogen molecules held over the 8% of gaps assumed to be in the chemisorbed film H H €€ / H ,,' '**. ... H H H H \ H ; - - + I ; - - ; I M M M M M 1 M Rideal pointed out that by making use of a free valency in this fashion one would expect an exchange reaction of low energy. More generally we can simply write M-H +p-H2 + MH + o - H ~ C.Wagner and K. H a ~ f f e ~ ~ working with palladium obtained the rate of recombination of hydrogen atoms from the rate of loss of dissolved hydrogen atoms measured by electrical resistance changes. This velocity was ten times less than the observed para-hydrogen conversion so they con- cluded that the conversion was largely due to some process other than the recombination of atoms vix. the exchange mechanism above or a para- magnetic conversion. 32 " Orthohydrogen Parahydrogen and Heavy Hydrogen " Cambridge University Press London 1935. 3 3 Cf. " Advances in Catalysis " 1 157 Academic Press New York 1948. 3 4 K . F. Bonhoeffer A. Farkas and K. W. Rummel 2. physikal. Chem. 1933 B 35 Trans. Faraday SOC. 1939 35 941. 36 2. Elektrochern. 1939 45 400. 21 226. 2 16 QUARTERLY REVIEWS It has been found p~ssible,~' by working with tungsten films of large area (400 sq.em.) to pre-adsorb hydrogen and then admit deuterium measuring directly the exchange reaction At 193" K. and 293" K. the whole of the chemisorbed hydrogen is exchanged with great rapidity. At 77" K. it is necessary for zero-point energy reasons to follow and one-quarter of the deuterium is exchanged rapidly with a speed com- parable with that of the para-ortho conversion. It was considered that a small difference in activation energy probably due to lattice-spacing effects slowed down the exchan'ge of the other three-quarters. It was concluded that the conversion reaction involved the exchange mechanism and A. and L. Farkas 38 came to the same result and conclusion for nickel. At 293" K.on palladium however the ratio of rates conversion surface exchange was 15 and for platinum 5. In this case Parkas and Farkas refer to a possible dissociation of hydrogen molecules into atoms on top of the chemi- sorbed film. This type of hypothesis really requires quantum-chemical investigation since it cannot be justified on the Langmuir surface-valency postulate. There is always of course the possibility that the combination mechanism is active on a small number of active spots not apparent in adsorption investigations. 39 The direct surface exchange work rules this out for nickel and tungsten. Also whilst we cannot lay too much weight on the experimental data available a t present the A factors found for platinum and palladium correspond to those one would calculate for surface exchange assuming that the whole surface is a c t i ~ e .~ O ~ Recent work by A. Couper 4Ob in collaboration with the Reviewer makes it unlikely that the exchange mechanism can involve actual empty sites. First the hydrogen film is probably mobile at approx. room temperature when the gaps should fill up but conversion activity remains unchanged. Secondly a chemisorbed hydrogen film on tungsten at 77" K. treated with hydrogen atoms which should fill the sites maintains its catalytic activity. However there seems no reason why the end of the hydrogen molecule should not interact with the metal between sites since the metal electrons are not localised. M-H + D + MD + HD M-D +H -+ MH +HD Exchange Reactions of Hydrogen Atoms It will be remembered that J. Horiuti and M. Polanyi 41 discovered that platinum black catalysed the exchange of atoms between deuterium gas and liquid water whilst A.and L. Farkas and Rideal 42 discovered the exchange between ethylene and deuterium on nickel. Subsequently 37 D. D. Eley Proc. Roy. SOC. 1941 A 178 452. 38 J . Amer. Ch,em. Soc. 1942 64 1594. 39 A. Farkas Trans. Furuday Xoc. 1939 35 943. 40a D. D. Eley ibid. 1948 44 216. 40b A. Couper and D. D. Eley unpublished. 4 2 A. Farkas L. Farkas and E. K. Rideal Proc. Roy. Soc. 1934 A 146 630. 41 Nature 1933 132 819. ELEY MECHANISMS OF HYDROGEN CATALYSIS 217 benzene was shown to exchange with deuterium gas,43 and at higher tempera- ture on active nickel catalysts exchange reactions were obtained with saturated hydrocarbons.44 In discussing these exchange reactions it would seem convenient to mention f i s t the general approach adopted by Farkas and Farkas for ordinary hydrides and later to discuss the special problems of unsaturated hydrocarbons.Much more requires to be done to fix the mechanisms of these reactions and one can only outline different points of view. Thus A. Farkas 45 assumes that the velocity of para-hydrogen conversion measured in the presence of the hydride RH gives the rate of dissociations of hydrogen molecules on the surface of the catalyst. * ( 1 ) H + H* +H* D + D* +D*}* - (2) RH $ R* +H* RD + R* +D*}' We shall in future use an asterisk as above to indicate a postulated valency bond to the catalyst. Thus where the rate of conversion was found equal to the rate of exchange the dissociation step (1) was supposed to limit the exchange reaction.I n the case where the conversion was more rapid than the exchange the rate of dissociation or recombination of hydride (2) was supposed to determine the reaction. which is of the second type the exchange is limited by D* +NH,* --f NH,D where the kinetics require NH,* to saturate the surface approximately and D* to be weakly held. However there is a possible alternative view of this reaction,33 an exchange reaction between NH,* and D held in a van der Waals layer NH,* + D -+ NH,D +D* The exchange reactions of saturated hydrocarbons are. of particular interest in connection with later considerations for unsaturated hydro- carbons. H. S. Taylor and his co-workers 44 48 have demonstrated an exchange between deuterium on nickel-kieselguhr and methane ethane or propane a t temperatures of 180° l l O o and 65" c .respectively. A. and L. Farkas 497 50 using platinised platinum foil have demonstrated exchange reactions of deuterium with n-hexane cydohexane propane and butane. The dehydrogenation equilibrium would of course lead to exchange of atoms C,H, + C,H + 3% but this is shown to be much too slow to account for the observed exchange. Thus we have Such might be the case for liquid ~ a t e r . ~ s Thus in the exchange with d3 J. Horiuti G. Ogden and M. Polanyi Trans. Paraday SOC. 1934 30 663. 4 4 K. Morikawa W. S. Benedict and H. S. Taylor J . Arner. Chem. SOC. 1936 58 46 D. D. Eley and M. Polanyi ibid. 1936 32 1388. 47 A. Farkas ibid. p. 416. 48 K. Morikawa N. R. Trenner and H. S. Taylor J . Amer. Chem. SOC. 1936 59. 49 Trans. Faraday Soa. 1939 35 917.1445 1795. 4 5 Trans. Paraday SOC. 1939 35 906. 1103. 5O Ibid. 1940 36 522. 218 QUARTERLY REVIEWS All the hydrogen atoms exchange and the activation energy for butane is 26 kcals. at 30" c. falling to 11 kcals. a t 80° whilst that for propane is 12-9 kcals. However exchange with the butane is 4-5 times faster than with propane. Again with n-hexane the activation energy is 17 kcals. at 30" c. falling to 9 kcals. in the region 55-124" C. The reactions are of zero order in n-hexane and cyclohexane. Farkas explains his results as above assuming a dissociative chemi- sorption We may regard this as a weak chemisorption since ethane does not inhibit the para-hydrogen conversion; 42 that is in the presence of H* from hydrogen molecules the above equilibrium is well to the left. However a high concentration of molecules in the van der Waals layer such as will be given by the larger hydrocarbon molecules will lead to a higher concentra- tion of radicals For a saturated van der Waals monolayer the activation energy for exchange will be that for the dissociation left to right.The experiments suggest that the true activation energy for this dissociation is approx. 25 kcals. As the temperature is raised we shall start to desorb molecules from the van der Waals layer in the order of increming temperature methane < ethane < propane < butane < hexane. The activation energy will then be 25 - q where q is the heat of van der Waals adsorption of the hydro- carbon. It might be asked if the exchange does not go through a reaction of the type D* + C,H + HD + C2H,* However K.Morikawa W. S. Benedict and H. S. Taylor have demon- strated exchange between CH and CD on active nickel catalysts a t 184" c. The activation energy is 19 kcals. which they attribute to the energy of desorption of methane i.e. CnJ&n+2 + CnH~n+l* + H* and a higher reaction velocity as shown. For propane we are probably already in the desorption range. CH,* + D* + CH,D Under the same conditions the methanedeuterium exchange is slower with an activation energy of 28 kcals. Here they consider that the strongly adsorbed deuterium has displaced the methyl radicals from the surface and under these conditions the slow process is CH -+ CH,* +H* in agreement with what has been stated above. In considering these theories it is remarkable that the chemisorptive rupture of C-H is more difficult than that of H-H since the bond energy of C-H is only 98.2 compared with 103.2 for H-H.51 The only explanation possible since steric effects are probably small is a low catalyst-carbon bond energy.This is confirmed by the additional observation by Taylor et at. that whilst on nickel ethane exchanges with deuterium at an appreciable rate at 110" bond scission producing heavy methane only occurs at 40" higher. C,H + D -+ 2CH,D 61 K. S . Pitzer J . Amer. Chem. Isoc. 1948 70 2140. ELEY RIECHANISMS OF HYDROGEN CATALYSIS 219 So on transition-metal catalysts the ease of bond rupture is in the order H-H > G H > C-C which is the reverse of their bond energies. There is however still some controversy on the carbon-bond energies. Exchange and Hydrogenation of Unsaturated Compounds The original papers demonstrated that the hydrogenation of ethylene 42 and benzene 43 on nickel was accompanied by an exchange of atoms C,H + D -+ C,H,D (hydrogenation) C,H $- D + C,H3D + HD (exchange) We shall at first sketch the original " associative " mechanism of Polanyi and Horiuti 52 for the case of ethylene.This postulates that ethylene is chemisorbed by opening of the double bond followed by CH,=CH -+ CH,*-CH,* a9" CH,D-CH,D (hydrogenation) > CH,*-CH*D + D* (exchange) CH,*-CH2* +D* + CH,*-CH2D/ Half - hydrogenated state The half-hydrogenated stmate by gain or loss of a hydrogen atom gives the This mechanism then Farkas 45 postulates that ethylene is chemisorbed by a dissociative hydrogenation or exchange reaction respectively. postulates an Jntimate connection between exchange and hydrogenation.mechanism as €or saturated compounds exchange following in the previous fashion by recombination of a C2H3" and a D* to give C2H,D. Polanyi and his co-workers 53 54 have made numerous experiments with benzene. Here the activation energies for both exchange and hydrogenation are about 10 kcals. suggesting a similar rate-determining process in each case. They find that while the hydrogenation velocity is of first order in hydrogenation pressure the exchange reaction is of about half-order which finds a ready explanation in terms of their mechanism above. For ethylene however the activation energy for exchange is much higher than for hydro- genation which suggests that two different mechanisms are involved in this case. Thus we shall find it convenient to discuss the two reactions separately.Exchange Reactions C,H* C2H3* + H* Ethylene.4. H. Twigg and E. K. Rideal 56 have carefully investigated the exchange with deuterium on an activated nickel wire. At 156" c. the reaction is proportional to the hydrogen and independent of the ethylene 5 2 Trans. Furaday Soc. 1934 30 1164. 58 R. K. Greenhalgh and M. Polanyi ibid. 1939 35 520. 5 p C. Horres R. K. Greenhalgh and M. Polanyi ibid. p. 511. 58 Proc. Roy. SOC. 1939. A 171 55. 220 QUARTERLY REVIEWS pressure. The hydrogenation has identical kinetics a t this temperature and the H + D reaction is completely inhibited. The observed activation energy is 18.6 kcals. md this falls off with temperature in the same way as for the hydrogenation reaction. Very similar results are reported for platinised foil by A.and L. Farkas,57 who obtain an activation energy of 22 kcals. The conversion of para-hydrogen is inhibited 5-6 fold by the ethylene but is not completely stopped the activation energy being 10 kcals. Twigg and Rideal postulate a modified half-hydrogenated state mechanism * * * * * CHZ-CH + D + CH2-CH2D + D* + CH,-CHD + HD where the first step determines the velocity. There is evidence that the if * if reaction D* + CH,-CH -+ H* + CHD-CH,* involving chemisorbed deuterium is very rapid. Farkas postulates the dissociative mechanism identifying the slowest step as C,H + C,H,* + H*. G. K. T. Conn and G . H. Twigg 58 find no exchange between ethylene and tetradeuteroethylene and conclude that this must rule out the dis- sociative mechanism. Farka~,~g however believes that this result will hold if the surface combina'tion of H* reduces its surface concentration to zero in the absence of added hydrogen gas.No test has so far been made for evolved hydrogen from an ethylene-treated nickel surface. Beeck's find- ings l* suggest that the amount of chemisorbed hydrogen will be low. In a further experiment Twigg 6o found that double-bond migration (D.B.M.) but-l-ene + but-2-ene occurred only in the presence of hydrogen and that if deuterium were used all the hydrogen atoms in the butene were exchangeable. About twelve butene molecules were isomerised per deuterium atom exchanged which suggests that the process involves chemisorbed D atoms rather than molecules. * * * CH2-CH-CH2-CH3 + D* + CH2D-CH-CH,-CH + * * CH,D-CH-CH-CH + H* A.Farkas's 45 explanation of D.B.M. in terms of his theory is not very convincing. The main fact that on nickel catalysts ethylene exchanges so much more rapidly with deuterium than does ethane is most strongly in favour of a special mode of adsorption of ethylene.66 Unfortunately quantitative comparisons of the two velocities on identical catalysts have not been made and whilst originally ethane showed only catalytic exchange at approx. 100" c. A. and L. Farkas 50 have now lowered this temperature to 72". This however is still higher than the - 80" at which catalytic ethylene exchange has been detected. Twigg and Rideal61 have added some weight to their general con- clusions by examining the packing of ethylene molecules on nickel surfaces. 57 J . Amer. Chem. SOC. 1938 60 22. Trans.Faraday Soc. 1939 35 941. 61 Trans. Faraday SOC. 1940 38 533. 58 Proc. Roy. SOC. 1939 A 171 70. 6O Proc. Roy. SOC. 1941 A 178 106. ELEY MECHANISMS OF HYDROGEN CATALYSIS 221 On this basis they predicted that the bulky methylethylenes would pack badly leaving space for chemisorbed hydrogen. They managed to detect this by showing the occurrence of the H + D reaction in these cases. While it is clear that no critical experiment has yet been made the bulk of the evidence lies in favour of the Twigg-Rideal mechanism. Beeck’s 11 work on hydrogenation raises one further possibility for the exchange reaction. Since no details have been published we cannot give a critical discussion. Beeck’s view is that ethylene is chemisorbed on nickel as acetylene plus two hydrogen atoms the latter being immediately removed by impinging ethylene as ethane.Thus his view is that the catalyst is largely covered by acetylenic complexes which are slowly removed by hydrogen. J. Sheridan’s 62 work makes it very certain that the main product of hydrogenation of adsorbed acetylene will be ethylene. Thus if Beeck is correct in his view of chemisorbed ethylene the exchange reaction will merely be the deuterogenation of the acetylenic complexes V i X . + + 9 CH,-CH -+ CI-I-CH + 2 H (rapid) 2H* + C,H -+ C,H (rapid) + Y D + CH=CH -+ CHD=CHD (slow) This suggestion would fit the kinetics but until further information is available it can only be regarded as a most tentative suggestion. The main argument against it is that the demonstrated activation energy for hydrogenatidn of acetylene vix.10.9 kcals. on nickel and 12-17 kcals. on platinum,62 is lower than that for the exchange reaction. But a t least if Beeck should prove correct in his view a reaction albeit a secondary one of this kind leading to exchange must be present. Benzene.-Benzene is not adsorbed so strongly or alternatively does not pack so well on nickel and platinum catalysts. This is shown by the relatively vigorous para-hydrogen conversion which goes on its pres- e r ~ c e . ~ ~ 5 4 9 63 The activation energy for exchange on both catalysts is about 9 keals. much lower than for ethylene. Whilst the hydrogenation reaction is always of first order with respect to hydrogen the exchange is often of one-half 539 54 or zero The half-order effect finds a natural explanation in terms of the original associative mechanism the step determining the rate.Because chemisorption of benzene in this way will break the resonance energy the benzene will be more weakly chemisorbed than ethylene (the difference in heats of adsorption will be approximately the benzene resonance energy minus the butadiene resonance energy i.e. 35 kcals.). The weak adsorption will allow an ample surface concentra- tion of chemisorbed D* and so the slow step will not involve D molecules 6 2 J. 1944 373; 1945 133 301 305 470. 63 A. Farkas and L. Farkas Tram. Furaduy SOC. 1937 33 827. P 222 QUARTERLY REVIEWS as with ethylene. This suggestion would bring the exchange reaction into line with H. A. Smith and H. T. Meriwether's observations on the hydro- genation reaction.64 One would expect that if benzene were chemlsorbed by dissociation it would be held as strongly as ethylene also so held.But on platinum the velocity of exchange is sensitive to the benzene pres~ure,~3 and the surface cannot be anything like saturated unlike the case with ethylene. Acetylene.-A. and L. Farkas G5 have investigated the exchange of deuterium and acetylene on platinum. Whilst ethylene reduces the velocity of para-hydrogen conversion on the catalyst by a factor 3 acetylene does so by a factor 15 showing a much stronger adsorption of the latter. The rate of exchange is much less than for ethylene and is comparable with that for ethane which is very little chemisorbed. In contradiction to Farkas we should regard these observations as directly against an identical " dis- sociative " mechanism for acetylene and ethane.Hydrogenation Ethylene.-The kinetics of hydrogenation of ethylene has been widely investigated on copper nickel and other catalysts,66 429 56 57 and the general picture that emerges is that at 100-150" c. the reaction is of f i s t order with respect to hydrogen and of zero order with respect to ethylene. An excess of ethylene will exert an inhibiting effect. These kinetics have also been found a t 0"c. on nickel films,3 although at these temperatures Twigg 6o has reported a tendency for the reaction to be of fractional order in both gases (e.g. but-l-ene a t 50" c. velocity = - pH:.5.pcpH,0*5). At high temperatures 200" c. it has been known since the work of U. Grassi that the reaction is of first order with respect to both gases. On the tran- sition-metal catalysts the activation energy is invariably low 5-10 kcals.for ethylene hydrogenation. A similar value has been reported for copper,66d but also a much higher value of 19 kcals.6Bg Becck l1 reports that for all transition metals E = 10.7 kcals. in k = Ae-E/RT and that the variation in activity is due to the A factor rhodium being most efficient. A well-known feature is the fall-off of activation energy with temperature reaching zero at 150" for nickel,6Ga but at much higher temperatures for nickel-silicon skeleton catalysts. 66h H. Zur Strassen has identified this effect with the desorption of ethylene but Twigg and Rideal56 prefer the desorption of hydrogen held over gaps in the film which starts a t 100" C. in their view a t which temperature the activation energy for exchange also starts to fall.8 4 J . Arner. Chern. Soc. 1949 71 413. 66 (a) U. Grassi I1 Nuovo Cimento 1916 11 147; (b) D. M. and W. G. Palmer Proc. Roy. Soc. 1921 A 99 402 ; (c) E. K. Rideal J. 1922 309 ; ( d ) R. N. Pease et al. J . Amer. Chem. SOC. 1923 45 1196 2297 ; 1927 49 2503 ; ( e ) F. H. Constable 2. Elektrochem. 1929 35 105 ; (f) C. Schuster 2. physikal. Chem. 1931 B 14 249 ; Trans. Faraday Xoc. 1932 28 406 ; (9) H. Zur Strassen 2. physikal. Ghem. 1934 A 169 81 ; ( h ) G. M. Schwab and H. Zorn ibid. 1936 B 32 169 ; 1935 A 171 421 ; (i) T. Tucholski and E. K. Rideal J. 1935 1701 ; ( j ) G. Rienacker and E. A. Bommer 2. anorg. Chem. 1938 236 263; 1939 242 302. 6 5 Ibid. 1939 61 3396. ELEY MECHANISMS OF HYDROGEN CATALYSIS 223 A. Parkas 45 has pointed out that hydrogenation leads to cis-compounds whether or not they are thermodynamically stable and considers that this must involve the simultaneous addition of two hydrogen atoms or a hydrogen molecule at one side of the acetylenic or ethylenic bond.R. P. Linstead et aLg7 have furnished nine further examples in the hydro- genation of phenanthrene and diphenic acid. R. K. Greenhalgh and M. Polanyi 63 have pointed out that this argument is erroneous and that cis-addition will result from the consecutive addition of hydrogen atoms through the half-hydrogenated state. The other argument advanced for direct addition of a hydrogen molecule or of two hydrogen atoms simultaneously is Twigg and Rideal's 56 finding that for ethylene both exchange and hydrogenation are of first order with respect to the hydrogen and that the activation energy for exchange is always 7-9 kcals.higher than for hydrogenation for a range of ethylenic compounds. Beeck 11 has recently stated quite new views on the hydrogenation of ethylene on nickel. He points out that since the overall heat of hydrogenation is 32.5 kcals. if we are to secure an energy balance only the hydrogen can be chemi- sorbed. Since ethane is not chemisorbed under these condi- tions if ethylene were also chemi- sorbed the heat of reaction must be at least 30 + 58 = 88 kcals. far in excess of the allowed value. However Beeck errs in taking the heats of chemisorption at 0 = 0. We are concerned with a largely complete ethylene film the heat of adsorption being about 25 kcals. I Ads I CZH6 C2H4+H2 ' Reaction path. - FIG. 2 The energy balance in ethylene hydrogenation.QE und Qa are the heats of chemisorption of ethylene and hydrogen respectively. EG and Es are the activation energies for the gaseous und surface reactions. Ethane i s assumed to be not chcmisorbed. The heat of chemisorption of hydrogen into this film will certainly not be more than the heat of chemisorption into a full hydrogen film that is 15 kcals. If the heat of rearrangement on the surface that is the activation energy is 7.5 kcals. we shall get an energy balance. The point is made clear in Fig. 2. Therefore on thermochemical grounds there is nothing against a reaction between chemisorbed hydrogen and chemisorbed ethylene (Polanyi mechanism) or van der Waals hydrogen molecule plus chemisorbed ethylene (Twigg-Rideal). However Beeck states briefly the results of two experiments which certainly necessitate a careful reconsideration of these mechanisms (a) a pre-adsorbed ethylene film on nickel held in his view as acetylenic complexes is only slowly removed by hydrogen ; and ( b ) a pre- 67 R.P. Linstertd W. E. Doering S. B. Davis P. Levine and R. R. Whotatone J . Amer. Chem. SOC. 1942 64 1985. 224 QUARTERLY REVIEWS adsorbed hydrogen film is very rapidly removed by gaseous ethylene. Beeck’s view is that the removal of the acetylenic complexes by hydro- genation leaves a small part of ‘the surface perhaps one part in lo6 free to chemisorb hydrogen. The actual ethane formation is then due to a very rapid reaction between colliding ethylene and chemisorbed hydrogen which he takes to be the mechanism in experiment ( b ) above. The overall hydrogenation velocity however is determined by the slow removal of acetylenic complexes.Beeck then argues that the portion of surface covered by hydrogen is proportional to pH,/pGIH and that the velocity is then proportional to this fraction times the pressure of ethylene so that velocity = k .PC,H( .PH,/PC,H = k .PH A critical discussion of these interesting conclusions must await the detailed publication of the experimental data. So far this most studied of bimolecular catalytic reactions still evades final solution. Benzene.-Recent workers 533 549 63 employed nickel and platinum catalysts. The reaction is of first order with respect to the hydrogen and for gaseous benzene,63 of zero order with respect to the benzene. The activation energy is about 7 kcals. as for ethylene. Smith and Meri- wether 64 have pointed out that the resonance energy of benzene must be suppressed on the catalysts otherwise the activation energy for hydrogena- tion would be much bigger.This notion as we have developed it above is probably the best existing argument for the associative chemisorption of benzene. It is very likely that hydrogenation involves the consecutive addition of two hydrogen atoms as argued by Greenhalgh and Polanyi but of course their kinetics do not necessarily rule out addition of a molecule. One may however suppose that where chemisorbed hydrogen atoms can exist they will hydrogenate rapidly. Only where the substance is chemi- sorbed as strongly as ethylene is it really necessary to invoke the direct action of hydrogen molecules. Smith and Fuzek 68 make the interesting observation that since furan has an activation energy higher than that of benzene the slower hydrogcna- tion of benzene must be due to a decreased A factor.This is the second piece of evidence pointing to the importance of A factors in hydrogenation. A careful investigation of the hydrogenation of phenyl-substituted aliphatic acids in solution O9 leads them to the view that the adsorption of hydrogen on the catalyst is a slow process sterically hindered by the adsorbed sub- strate. This is not dissimilar to the views of Twigg and Rideal. Acetylene.-The hydrogenation of this substance 62965 is of first order with respect to hydrogen and of zero or negative order with respect to acetylene on the usual catalysts. The reaction is slower arid the activation energy is higher than for ethylene.The main product is ethylene and ethylene has absolutely no effect as a diluent. However whilst the hydro- genation of acetylene is slower than that of ethylene so far as one can judge it would seem to be much faster than Beeck’s reaction l1 between hydrogen 68 H. A. Smith and J. F. Fuzek J . Amer. Chern. SOC. 1949 ‘71 415. H. A. Smith D. M. Alderman and F. W. Nadig ibid. 1945 6’7 272. ELEY MECHANISMS OF HYDROGEN CATALYSIS 225 and pre-adsorbed ethylene which would suggest that normally cheniisorbed acetylene is different from Beeck’s acetylenic complexes. Conclusion It will be seen that in spite of so much work with the probable exception of the para-hydrogen conversion all the hydrogenation mechanisms are a t present controversial. It is hoped that this review will help towards their solution.The weight of evidence still lies in the Reviewer’s opinion in favour of associative chemisorption at the double bond followed by reaction with chemisorbed hydrogen atoms (for benzene) or hydrogen molecules where the substance (like ethylene) is so strongly chemisorbed that i t displaces the hydrogen atoms. So far as the basic theory of catalysis goes two trends of work are apparent. The first a continuation of the work of A. A. Balandin,70 seeks to link catalyst activity with the fitting of substrate molecules to lattice spacings. This approach is quite decisively limited a t present by our knowlcdge of reaction paths. Thus the favourable action of the 110 plane of nickel on the hydrogenation of ethylene has been associated with its suitability for chemisorbing on the one hand hydrogen and on the other hand ethylene.Recently experimental results have started to appear linking the electronic band structure of the solid with the activation energy for reaction. In the past this field of activity has been mainly cultivated by J. E. NJTOP,~~ whose enthusiastic correlations of activity with ionisation have been spoilt unfortunately by his misuse of the physical theory.72 Then G. M. Schwab 7 3 has demonstrated that the height of the filled electron levels in certain alloys determines their catalytic activity in the dehydrogenation of formic acid. A. Couper and D. D. Eley 74 have demonstrated that filling the partly empty d-band of palladium by alloying it with gold destroys its catalytic activity. From their results they argue that the exceptional behaviour of transition metals as catalysts lies in their possession of vacant d-orbitals of low energy and high numerical density.D. A. Dowden and P. W. Reynolds 75 have also discussed correlation between electronic structure and catalyst activity. Work of this kind still calls for a know- ledge of reaction paths but not perhaps so urgently as does the lattice- spacing concept. From the fundamental physical viewpoint of course the two approaches are complementary and the separation is made simply for the benefit of the experimentalist. 70 2. physikal. Chem. 1927 A. 126 267. 71 “ The Catalytic Action of Surfaces ” London 1937. 72 P. H. Emmet and E. Teller “ Twelfth Report of tho Committee on Catalysis ” 7 s Trans. Faraday SOG. 1946 42 689. 7 4 Nature (in the press) and Chemical Society Meeting London March 17th 1949. 75 Chemical Society Meeting London March 17th 1949. p. 68 New York 1940.

ISSN:0009-2681    

DOI:10.1039/QR9490300209

出版商:RSC    

年代:1949

数据来源: RSC

摘要:

HYPERCON JUGATION By V. A. CRAWFORD M.Sc. PH.D. ( WHEATSTONE PHYSICS LABORATORY UNIVERSITY OF LONDON KING'S COLLEGE *) SINCE the review by C. L. Deasy the concept of hyperconjugation has become well established and furthermore has been extended and widely applied ; consequently the present more extensive review was undertaken. Historical Introduction and Definition of the Term.-If electron displace- ment in alkyl groups resulted solely from the operation of the inductive effect then the order But > Pri > E t > Me should always be obtained for those reactions which require accession of electrons to the reaction region. Experimental evidence that this order is not universally applicable was first provided by J. W. Baker and W. S. Nathan,2 who studied the rate of reaction of various alkyl-substituted benzyl bromides with pyridine.It was found that all alkyl substituents increased the reaction rate while with a single p-alkyl substituent the rate decreased in the order Since the reaction studied is one facilitated by electron accession towards the side chain it follows that the accelerating effects of alkyl groups must be related to their capacity for electron release and furthermore that the relative magnitude of this electron release diminishes in the order indicated in (a) which order is exactly the reverse of that expected on the basis of the inductive effect of alkyl groups. The methyl group it would seem is there- fore capable of permitting additional electron release by some mechanism which is either greatly reduced or inoperative in the more highly alkylated groups.Since an anomaly of this kind is not found in every type of system containing alkyl groups Baker and Nathan pointed out that the additional mechanism of electron release by the methyl group in p-methylbenzyl bromide is to be associated with the presence of the attached conjugated system of the aromatic nucleus. It was therefore suggested that when a methyl group is attached to a conjugated system the pair of electrons forming the C-H bond in the methyl group are appreciably less localised than are those in a similarly situated carbon-carbon bond. Hence a methyl group attached to the requisite system permits electron release by what is essentially a type of electromeric effect Me > Et > Pri -N But . - ( a ) This new effect acts in addition to and in the same direction as the inductive effect but since it depends on the number of C-H bonds its magnitude clearly will diminish in the order Me > Et > Pri > But.Hence 1 Chena. Reviews 1945 36 145. * Now Lecturer in Theoretical Chemistry The Durham College University of 226 J . 1935 1844. Durham. CRAWFORD HYPERCONJUGATION 227 bromide the methyl group by this new mechanism electron release at the G-Br bond in p-methylbenzyl permits additional and therefore further facilitates the anionisation of the bromine atom. On the other hand in p-tert.-butylbenzyl bromide CH C"&+H.-Br - CH3 although the inductive effect of' But is greater than that of methyl the postulated electron-release mechanism is no longer possible for the inter- calation of the tertiary carbon atom prevents the additional methyl groups from forming part of the necessary conjugated system.Although the presence of a conjugated system was emphasised it was assumed that the effect could also function in those systems where electron- pair displacements arc rendered possible by fission of a group with its bond electrons as for example If however it requires separation as an anion of a group or atom which normally jonises as a cation the effect is assumed not to occur. The mobility (kl + k,) of the prototropic change in tho azomethine system ki P-RC,H,.CH :N*CH2Ph + P-RC,H,.CH~*N :CHPh Jca which is known t o be facilitated by electron withdrawal from the triad ~ y s t e m ~ was found 4 to decrease in the order But > Pri > Me indicating that the retarding effect of a p-methyl group is. much greater than that of more highly alkylated groups.This result provides good evidence that the Baker-Nathan effect is of a mesomeric character for it is well established that a mesomeric effect is capable of retarding a reaction which is facilitated by electron recession from the region of reaction. This result was confirmed ti by an investigation of the equilibrium p-Rc,$,*CHO + HCN f p-RC,H,*CH(OH)*CN for the stabilities of the aldehydes relative to the cyanohydrins diminished in the order Me > Et > Pri > But > H. This is an observation to be expected if the Baker-Nathan effect predominates over the inductive effect for in the freo aldehyde conjugation extends to the side-chain carbonyl group so that the new effect stabilises the free aldehyde to a greater extent than it would the cyanohydrin.a C . W. Shoppee J . 1933 1117. 4 J. W. Baker W. S. Nathan and C. W. Shoppee J . 1935 1847. J. W. Baker and M. L. Hemming J . 1942 191. 228 QUARTERLY REVIEWS This power of alkyl groups to conjugate with multiple bonds (either double or triple) is known as hyperconjugation i.e. an additional conjugation beyond that ordinarily recognised. Since groups which conjugate with unsaturated systems act as electron donors single and multiple bonds are known as acceptor and donor bonds respectively. Some Theoretical Considerations. -T he decrease of el e c tr on-re pulsive character in ascending the homologous series of alkyl groups had been deduced by N. G. Burkhardt and 31. G. Evans,s on the basis of Mulliken's theory of united atoms. As a united atom group the methyl substituent has the electronic con- figuration ( 1 ~ 2 2 ~ 2 2 ~ 3 5 ) ) which is the same as that of fluorine (except that all the orbitals are more or less deformed) and falls into line with other o,p- directing groups all the electrons (except the carbon 1s) making up the electronic shell of the group.Replacement of the hydrogen atoms of the CH by other alkyl substituents results in (1) destruction of the united atom system with consequent reduction of the nuclear charge and (2) removal to some extent of eloctrons which were available and taking part in the bond when the electronic configuration of ths group resembled that of fluorine. This decrease in the availability of electrons it was suggested might contribute to the apparent decrease in o,p-directing power of substituted methyl groups.More recently R. S. Mulliken C. A. Rieke and W. G. Brown 7 studied the phenomenon quantitatively using the method of molecular orbitals. In this method of treatment it is natural to write the methyl group as -CrH, and to compare it with such groups as -C=N and -C=CH. In general a quasi-triple bond is defined as one consisting of three ordinary single bonds from a carbon atom to any three other atoms whenever there is opportunity for conjugation across an intervening carbon-carbon single bond with a second quasi- or ordinary multiple linkage. Since the quasi- triple bond CEH is much more saturated than the C=C triple bond its conjugating power is smaller but that it is still considerable is shown by certain physical data. Second-order conjugation or first-order hyperconjugation involves one quasi- and one true multiple bond whereas third-order conjugation or second-order hyperconjugation involves two quasi-multiple bonds ; ordinary conjugation fits into the scheme as first-order conjugation.The following molecules provide examples of conjugated systems in this gsneralised sense } Ordinary or first-order Conjugation. HCL,C-C=CH NEEC-CEN H,,C-C=CH Second-order conjugation or H,_C-C=N } first -order hyperconjugat'ion. H,~C-C%H Third-order conjugation or second-order hypercon j ugation. 6 Mem. Proc. Manchester Lit. Phil. SOC. 1933 77 37. 7 J . Amer. Chem. SOC. 1941 63 41. CRAWFORD HYPERCONJUGATION 229 On this basis nearly all saturated organic molecules are stabilised by Thus propane third-order conjugation of rather a complicated character.may be written as either H,=C-CFH,Me or MeH,EC-C_H the two ways of writing the formula indicating two different possibilities for hyperconjugation both of which contribute to the stability of the molecule. The three hydrogen atoms of CH have three valency electrons just like the nitrogen atom in the CEZN group. If therefore the three hydrogen atoms of the methyl group can be treated like a pseudo-atom and suitabIe H group molecular orbitals be formulated the procedure for studying N_C-C=N theoretically may likewise be applied to H,=C-C-H,. In this connection it is recalled that a single bond is always formed by a pair of electrons each in a o orbital and in the molecular orbital (M.O.) approximation both electrons occupy a single bonding o M.O. which is symmetrical with respect to rotation about the axis of the molecule.A multiple linkage consists of a o bond and one or more n bonds. Thus the triple bond in acetylene is composed of a o bond together with two pairs of electrons in n A.O. or M.O. If axes x y and x are taken with the x axis dong the symmetry axis of the molecule then each n M.O. occurs in two forms which may be called nz and q, and these will hereafter be denoted simply by x and y. If the three hydrogen atom 1s atomic orbitals of the H group are denoted by a b and c and located a t the vertices of an equilateral triangle group orbitals of two types may be constructed as follows. (i) The linear combination (a + b + c ) may be formed and this has approximate symmetry around an axis passing perpendicularly through the centre of the triangle.This group orbital may therefore be described as a quasi-rr M.O. and can be expected to interact with other a orbitals. Delocalisation of this type is known as o-hyperconjugation and since CJ electrons are relatively tightly bound the resulting energy of delocalisation is likely to be very small. Consequently a-hyperconjugation is generally ignored. (ii) There may also be constructed the linear combination a - +(b + c ) and b - i ( c + a) and these have the same type of symmetry as the dumb- bell orbital of an isolated carbon atom. Pictorial representations of these two types of group orbital are given by C. A. Coulson.8 The three hydrogen atoms of the methyl group can therefore function as a pseudo-atom with a n-type orbital which can interact with other orbitals of the same symmetry such interaction being known as n-hyper- conjugation.Sinco the C-H bonding orbital i s not of the right symmetry the electrons of a single C-H bond clearly cannot be involved in n-hyper- conjugation. If CH is attached to a benzene ring there will be overlapping between the TC orbitals of the ring and that of the H group (via the C atom of CH,) Quart. Reviews 1947 1 144. 230 QUARTERLY REVIEWS resulting in a M.O. in which electrons from the methyl group can migrate into and out of the ring. The resulting delocalisation energy in toluene is small and stabilises the molecule by approximately 1.5 kcals. It might be noted that the hyperconjugation here is analogous to the ordinary con- jugation which occurs in styrenes where the group -CH=CH instead of CH is substituted in the ring.Hypercon jugation in Molecules containing Double Bonds.-Butadiene is the simplest example of ordinary or first-order x conjugation the x axis being taken perpendicular to the plane of the nuclear framework. In propylene H3~C-CH=CH, the C-C single bond can act as an acceptor bond for second-order x conjugation between the x electrons of the C-C double bond and the x electrons of the C-H quasi-triple bond. The y electrons of the CrH bond are inactive in second-order conjugation ; hence for this purpose the CGH bond acts like a double bond. In cyclopentadiene (I) there is a combination of first-order x conjugation as in butadieno and second-order x conjugation as in propylene the molecule having two second- Taking axes as shown with the x axis per- pendicular to the plane of the ring tho two ;% - kTJ-y I I order and one first-order acceptor bonds.c g 6 C II H2 I ;2 hydrogen atoms of the CH group are located one above and the other below the plane of the I 8 I ring. The quasi-double bond between the car- (1.) (11.) bon atom and the pseudo-atom H, involves one o bond and one x bond with M.O. formed by combination of a cr and x carbon A.O. respectively with the following quasi-A.O. of the H group [a] = (a + b ) / 2 / 2 ; [XI = (a - b ) / 2 / 2 The quasi [XI orbital it will be noticed is antisymmetrical with respect to the plane of the ring and consequently two of the four C-H bonding electronsin the CH group must be allocated inM.0. approximation to a quasi- unsaturation M.O. i.e. to a M.O. whose symmetry permits it to interact with the unsaturation M.O.of the ring carbon atoms. The nature of the hyperconjugation here is made clearer by comparing cyclopentadiene with fulvene (11). The quasi-double bond like the C=CH ordinary double bond in fulvene contains one pair of electrons which can enter into conjugation with the two pairs of unsaturation electrons associated with the two double bonds in tho ring. However since the conjugation in cydopentadiene involves ordinary double bonds and one quasi-double bond the resonance here will be less intense than in fulvene where ordinary double bonds only are involved. Third-order Con jugation. -In ethylene only third-order conjugation is present. For each H group there are quasi-A.O. as follows [o] = (a + b ) / d 2 ; [y] = (a - b ) / 2 / 2 a and b being located on opposite sides of the xx plane.CRAWFORD HYPERCONJUGATION 231 The y electrons of the two CH groups of ethylene can give third-order conjugation across the C= C bond which here acts as acceptor in the same way that the [z] and the [y] electrons of the two CH groups conjugate across the single bond of ethane. In both cases the acceptor bond takes on triple-bond character. Bond Orders.-With regard to the analysis of bond lengths and the character of bonds W. G. Penney and C. A. CoulsonlO introduced the term bond order taking values of 1 2 and 3 respectively for the orders of the central bonds in ethane ethylene and acetylene and drew curves relating bond length to bond order. Since there is no conjugation in acetylene the value for the C r C bond in this molecule is correct but in ethylene and ethane however there is third-order conjugation and when allowance is made for this the orders of the central bonds in these molecules become 2-12 and 1.12 respectively.The excess bond order above the usually assumed values of 1 and 2 for normal C-C and C=C may be expressed by saying that normal C-C and C k C bonds contain a certain yo of triple-bond character the values being 6% and 12% respectively. As a result of the inclusion of hyperconjugation the curve relating bond order to length does not reach bond order 1 at 1.54 A. but a t about 1-58 A. Although this value was interpreted as being the normal bond length for an isolated or ideal C-C single bond the analysis does not in general definitely indicate that ethane should have a greater C-C bond length than the normal value of 1-54 A.found in saturated molecules. Effects to be expected if Hyperconjugation occurs.-Hyperconjugakion might be expected to reveal its occurrence in any molecule by its effects on the following properties first-order hyperconjugation being anticipated to produce larger effects than second-order hyperconjugation. The electron-diffraction studies by L. Pauling L. 0. Brockway and J. Y. Beach 11 l2 have shown that first-order or ordinary conjugation causes considerable shortening of single bonds. Hyper- conjugation it might be expected would produce similar changes but since it is weaker in its effects than ordinary conjugation appreciable changes are not likely to be produced. (ii) Dipole moments. Conjugation alters the electron distribution in a molecule and so affects its dipole moment.Whether the moment because of conjugation is increased or diminished depends on whether the con- jugation or resonance moment is in the same or the opposite direction to the normal moment. Dclocalisation whenever it occurs stabilises the system and therefore less heat will be evolved in the hydrogenation process than in the corresponding system not stabilised by hyperconjugation. Furthermore progressive alkylation of a double bond may be expected to result in its progressive stabilisation and this should be reflected in a progressive diminution in the heat of hydrogenation. (i) Bond length. (iii) Heats of hydrogenation. Proc. Roy. SOC. 1937 A 158 318. l1 J . Amer. Chern. Soc. 1935 57 2705. lo Ibid. 1939 A 169 419. l2 Ibid. 1937 59 1223. 232 QUARTERLY REVIEWS (iv) Spectroscopy.It is found empirically that a compound with conjugated double bonds absorbs light of longer wave-lengths than an analogous compound with isolated double bonds. Moreover as the number of double bonds in the conjugated system increases the absorption is progressively shifted to longer wave-lengths. Conjugation causes (a) the unoccupied antibonding M.O. to become less strongly antibonding and (b) a raising of the occupied bonding M.O. and hence absorption occurs at longer wavc-lengths an effect which increases with progressive conjugation. Since hyperconjugation is generically similar to ordinary conjugation it too might be expected in general to result in displacement of absorption bands towards longer wave-lengths when alkyl substituents are attached to unsaturated atoms. Highly unsaturated substituents may be expected to exert pronounced effects but with alkyl groups where the unsaturation is small the system itself may be expected to play a controlling part.This it will accomplish by enhancing the unsaturation in the substituent by means of its own unsaturation either as this exists permanently or as it is developed during the transition state of the reaction. l3 The clectromeric effect therefore becomes of importance in those transition states which require an enhanced alkyl conjugation and in such reactions the pheno- menon will manifest itself. The remainder of this review will be devoted to tho presentation of experimental evidence confirming these expectations and also to such other experimental work in which the concept of hyperconjugation has becn applied.Bond Length.-It has been pointed out that hyperconjugation should produce detectable changes in the lengths of acceptor bonds. The results of electron-diffraction studies by L. Pauljng H. D. Springall and K. J. Palmer l4 are shown below (in A.). (v) Kinetic studies. Single bond Single bond bond. bonds. adjacent to triple between two triple H,=C-CrCH . . 1.46 H 3 ~ C - - - C ~ C - C ~ C - C H 3 . . 1-47 1.38 HC=C-C=uH . 1.36 H 3 r C - C ~ C - C ~ H 3 . . I 1-47 It is seen. that in those compounds in which the C-C single bond is adjacent to the C-C triple bond its value is appreciably lcss than the normal value of 1.54 A. and this has been regarded as being due partly to con- tributions to the normal state from hyperconjugated structures. H+ H H* H I - I C=C=C-H C=C=C-H I H I H The carbon-carbon distance in acetaldehyde is approximately 0.04 A.1 3 M. L. Dhar E. D. Hughes C. K. Ingold A. M. M. Mandour G. A. Maw and L. I. Woolf J . 1948 2095. l4 J . Amer. Chem. SOC. 1939 61 927. CRAWFORD HYPERCONJUGATION 233 less than the normal value,15 and here again the shortening has been attributed to hyperconjugated structures. The shortened bond length in methylacetylene has been confirmed by the spectroscopic studies of R. M. Badger and S. H. Bauer and also of G. Herzberg F. Patat and H. Verleger,l7 and according to R. S. Mulliken,ls when the formula is written as H,=C-C=CH it would indicate hyper- conjugation almost as strong as the conjugation in N%2--C=N where the C-C length is 1.43 A. It has also led to the conclusion that the methyl group conjugates more with C=C than with C-C.Indeed two conjugated triple bonds can interact through either their x or their y orbitals so that for this system conjugation twice as great as for two conjugated double bonds might be expected. However in the molecules C2H6 C2H, and C2H2 in which there is no conjugation (ignoring third-order conjugation in C,H6 and C2H,) there is an abrupt shortening on passing from C-C to C=C to C%C. Thus part of the observed shortening is no doubt due to changes of hybridisation.18a Recently X-ray analysis l9 of the di-isoprene derivative geranylamine hydrochloride (111) has revealed a shortening of the central bond accom- CH3 CH3 I 1-51 1-44 1-51 I CH~-C=CH~CH,~CH~-C=CH-CH~-NH,Cl (111.) panied by the planar arrangement of the adjacent groups and L. Bateman and G.A. Jeffrey 2O have suggested that this unique bond character which simulates ordinary conjugation is the result of hyperconjugation. Hyperconjugation has also been invoked to account for the partial double- bond character of the C-C bond in dibenzyl 21 which is a similar system. In this molecule (IV) the three formally single carbon-carbon bonds have lengths of 1.523 1.510 and 1.523 A . ~ ~ ~ and make angles of 115" with each other.22 Aniongst other causes the observed shortening in these two molecules may be due to third-order conjugation between the two quasi-double bonds but this is hardly likely to produce decreases in bond length of the observed magnitude. Further- more just such third-order conjugation occurs in paraffin hydrocarbons and in these molecules no such decrease in bond length has been obtained.The angle abc is 115" and this is intermediate between 109" 28' and 120° the values required by 8p3 and sp2 hybridisation respectively. The l5 P. Stevenson H. D. Burnham and V. Schomaker J . Amer. Chem. Xoc. 1939 61 2922. l7 J . Physical Chem. 1937 41 123. lSa C. A. Coulson Victor Henri Memorial Vol. Desoer LiBge 1948 16. 2o Nature 1943 152 446. 21 M. Szwarc Faraday SOC. Discussions 1947 No. 2 39. 21a E. G. Cox and D. W. J. Cruickshank Acta Cryst. 1948 1 921. 2 2 G. A. Jeffrey Proc. Roy. XOC. 1947 A 188 222. @ \ (IV. ) l6 J . Chem. Physics 1937 5 599. Is J . Chem. Physics 1939 '4 339. G. A. Jeffrey Proc. Roy. SOC. 1945 A 183 388. 234 QUARTERLY REVIEWS bond bc therefore has more s character and is consequently stronger than the normal C-C single bond.23 This is a sufficient and probably the sole explanation of the observed shortening in these two molecules.Application of the concept of hyperconjugation with caution is therefore indicated. Dipole Moment.-Dipole-moment measurements have yielded significant evidence as to hyperconjugation in unsaturated molecules. Some pertinent values for the vapour state 2 4 7 25 are shown in Table I. Dipole moment D. j/ Substance. I-____- ~______I_ 2.27 Crotonaldehyde . . . 2.72 1 -Methylbutadhe . 2.73 2-Methylbutadiene . . 2.72 2 3-Dimethylbutadiene TABLE I Dipole moment D. 3.67 0.68 0.38 0-52 ________ Substance. Formaldehyde . . . Acetaldehyde . . . Propaldehyde . . . n-Butaldehyde . . . If the large increase in moment from formaldehyde to acetaldehyde is due solely to the inductive effect it might be expected to have produced a further increase from acetaldehyde to propaldehyde.That this does not occur was regarded as indicating that resonance is partly responsible for the increase from formaldehyde to acetalde- I - hyde. Thus structures of the type (V) among others would be expected to contribute to the observed increase in moment. The observed shortening of the C-C bond by 0.04 A. indicates that this bond contains about 8% of double-bond character. The solution value of acraldehyde is 0.4 D. greater than that of acetalde- hyde an increase due to the transfer of charge to the oxygen atom facilitated by the conjugation of the molecule I3 H H H+H H H+- H H-C=C-O I (V.1 H I I I - I I - H-C+-C=C-O H-C-C-C-0 The effect of hyperconjugation is more strikingly shown in the large rise in moment of nearly 0.6 D.in tmns-crotonaldehyde. In addition to polar structures analogous to those which have been written for acraldehyde three further highly polar ones (as in VI) can be written for crotonaldehyde. H+H H H I I I - H-C=C-C=C-O I H H+H H I I I I H-C=C-C- H H (VI4 (VII.) In like manner the moments of propylene and but-l-ene are accounted 23 A. D. Walsh Faraday SOC. Discussions 1947 No. 2 18. 2 4 E. C. Hurdis and C. P. Smyth J . Amer. Chern. SOC. 1943 65 89. a 5 N. B. Hannay and'C. P. Smyth &id. p. 1931. CRAWFORD HYPERCONJUGATION 2 35 for by polar structures of the kind written for aldehydes. Thus for propylene may be written three structures of the type (VII). The moments of the methylbutadienes provide further evidence of the polarity resulting from hyperconjugation.Thus the moment of 2-methyl- butadiene is experimentally indistinguishable from that of propylene suggesting participation in the resonance hybrid of structures of the type (VIII). However the extent to which structures of this type contribute H H H I I I I II I c-c=c -C- H C H H / \ H H+ (VIII. ) should be rather less than in the case of propylene for the amount of double- bond character of the central butadiene bond should reduce that of the bond to the methyl carbon. In l-methylbutadiene (penta-1 3-diene) polarity should arise from polar structures analogous to those proposed for propylene and 2-methylbutadiene but here the negative charge instead of being displaced three carbon atoms away from the methyl hydrogens is displaced five carbons away to give (IX).H+H H H H H+ I3 C1 I I I I (X. 1 c=c c1- H C1 Since three such structures are the principal source of the moment of the molecule as in 2-methylbutadiene the moment of the molecule should be to that of 2-methylbutadiene approximately as the charge separation in the l-mothyl is to that in the %methyl. Measurement of tho molecular models shows that the ratio of these distances is approximately 1-5 if l-methylbutadiene is cis with respect to the central single bond and 14 if it has the more probable trans-structure. Tho fact that the ratio of the two moment values is 1.8 is considered by Smyth to give striking evidence in support of the validity of hyperconjugation as applied to unsaturated compounds. The argument however is of rather doubtful validity because in 2-methylbutadiene the conjugation is crossed whereas in l-methylbuta- diene it is not ; hence the 2-methyl isomer should have a lower moment.Hyperconjugation has also been invoked to account for the large increase in moment obtained on passing from chloroform to methylchloroform 26 for which there are possible nine resonance structures of the type (X). Similar considerations have been applied to account for the moments of acrylonitrile 26 E. C. Hurdis and C. P. Smyth J . Amer. Chem. SOC. 1942 64 2829. 236 QUARTERLY REVIEWS and isocrotyl chloride,25 some unsaturated aldehydes ethers and halogen compounds.27 The observed zero moments of all cyclohexanes combined with the known absence of a moment in benzene itself show that the moments of the corre- sponding alkylbenzenes must arise from electron H+ H+ displacement in the aromatic electronic system stimulated by the polar effect of the alkyl sub- stituent.Thus hyperconjugated structures (XI) and (XII) contribute to the small moment of -0 0 toluene.24 However the observed sequence of moment values relating to the vapour state28 in- creases from toluene to tert.-butylbenzene which is the order of the inductive effect-a result ex- plained by J. W. Baker 29 on the basis of the simultaneous operation of both electron mechanisms. Heats of Hydrogenation.-In Table I1 are recorded the heats of hydro- genation of certain relevant unsaturated That the heat of hydrogenation of cyclopentadiene is 50.9 kcals. per mole as compared with 57.1 for butadiene is regarded by Mulliken l 8 as evidence suggesting an H*-C-H H-C-H - (XII.) Substance.Substance. Ethylene . . . . Propylene . . . . isoPropylethyleno . . tert.-Butylethylene . . But-2-ene (trans) . . , (cis) . . . Trimethylethylene . . Heat at 356' K. in kcals. TABLE I1 Tetramethylethylene . cydopenta-1 3-dieno . Benzene . . . . . Ethylbenzene . . . Mesitylene . . . cycZoHcxa-1 3-dime . o-Xylene . . . Heat at 355" I(. in licals. 26.6 55.4 50.9 49.s 48-9 47.6 47.3 32-8 30.1 30.3 30-3 27.6 2S.G 26.9 added stabilisation of cyclopentadiene by hyperconjugation. However it was also pointed out that the low heat of hydrogenation may be due equally well to instability of the saturated alicyclic five-membered ring as to the stability of the unsaturated compound. For 1 3-cycZohexadiene where the aliphatic six-membered ring would be expected to have normal stability the value 55.4 kcals.would indicate that hyperconjugation has a smaller though appreciable stabilising effect. The progressive substitution by methyl groups of the hydrogen atoms of ethylene diminishes the heat of hydrogenation of the resultant com- pounds the diminution being greatest for tetramethylethylene for here 6.2 kcals. less heat is evolved than in the case of ethylene. However the diminution from ethylene to propylene is 2-7 kcals. whereas that from trimethyl to tetramethylethylene is only 0.3 kcal. thus indicating that the a7 M. T. Rogers J. Arner. Chem. SOC. 1947 09 1243. 28 J. W. Baker and L. G. Groves J . 1939 1144. a. J. B. Conant and G. B. Kistiakowsky Chem. Reviews 1937 20 181. 29 Ibid. p. 1150. CRAWFORD HYPERCONJUGATION 237 effect of progressive substitution of hydrogen atoms of ethylene by methyl groups is not additive.I n the alkylbonzenes substitution likewise results in diminished heats of hydrogenation although hero the stabilisation produced is lower than in ethylene. This is probably duo to the large resonance stabilisation already present in the aromatic ring.31 Thermal data show therefore that double bonds are considerably affected by the nature of the groups attached to them. Since the n electrons are tho ones involved in the hydrogenation process it follows that such substitutions affect their energy states and hence it might be expected that such changes will be reflected in the spectra and ionisation potentials of these electrons. Absorption Spectra and Ionisation Potentials.-(A) BZkyZethyZenes. The work of E.P. Carr and her collaborators 32p 33 clearly shows that progressive substitution in ethylene by methyl groups results in progressive shift to longer wsve-lengths of the bands in the Schumann region. Some of their results are shown in Fig. 1 whore the wave-number of the first band is plotted against the number of alkyl groups. The diminution in heat of hydrogenation and the long wave- length shift show parallel effects. Thus the largest fall in heat of hydrogenation (2.7 kcals.) occurs in passing from ethylene to pro- pylene and this also corresponds to the greatest long wave-length shift (3500 cm.-l) while the smallest diminution in heat of hydrogena- I I I 42' 0 ; 2 3 4 Number of Me groups around the doubje bond FIG. 1. tion (0.3 kcal.) occurs in passing from trimethyl- to tetramethyl-ethylene corresponding to the smallest shift (2000 cm-l).Table I11 shows the experimentally observed term values of methyl- substituted ethylenes for both the ground and the excited state.34 It will be noticed that the term values of both states decrease with progressive methylation. TABLE I11 8-75 8.3 No. of Me groups . . . . . . . 1 2 ' 0 Term values Ground state . . . . 10.45 ev. {Excited state . . . . 3.04 1 :& 1 29:328 ~ 2.04 1 1-72 31 W. C. Price Chem. Reviews 1947 41 258. 32 E. P. Carr and M. K. Walker J . Chem. Physics 1936 4 751. 33 E. P. Carr and H. Stiicklen ibid. p. 760. 34 R. S. Mulliken Rev. Mod. Physics 1942 14 265. Q 238 QUARTERLY REVIEWS These results are explained as being due to (i) charge transfer or inductive effect of the methyl group and (ii) hyperconjuga,tion of the CH group and the double bond.(i) Since the inductive effect is short-ranged it is likely to be of greater importance in small molecules and in terms of hybridisation may be described in the following way.,5 When the less electronegative CH group replaces a hydrogen atom in ethylene the bond linking the carbon atom of CH contains more s character and therefore the cr bond of the C=C double bond contains more p character. The cr electrons of the C-C double bond therefore become less tightly bound and consequently the repulsion between them and the n electrons is increased whence removal of the latter is more easily effected i.e. the inductive effect results in a raising of the ground-state orbital. However the excited state must also be considered F I G .2 Result on excited state of (a) inductive effect and (b) hyperconjugation efSect. Result on ground state of ( c ) inductive effect and (cl) hyperconjzigation effect. for the wave-length a t which an ab- sorption band appears depends upon the energy difference between the ground and the excited state. Com- putation shows that the inductive effect results in a decrease in the excited term value the change in the excited however being smaller than that brought about in the ground state. (ii) Hyperconjugation of the CH group with the ethylene double bond not only raises the ground state but also causes an increase in the excitcd- state torm value. The hyperconjugation and induc- tive effects are therefore opposed in the excited state and when allowance is made for the greater effect of charge t,railsfer the observed net decreases in excited-state term values are accounted for.Since hyperconjugation raises the ground state 0.14 e ~ . ~ ~ of the total drop in ionisation potential (0.80 ev.) on passing from ethylene to propylene is regarded as being due to this cause. Fig. 2 shows the influence of the two effects (acting independently) on the ground and the excited state. (B) AZkyEbenzenes. F. A. Matsen W. W. Robertson and R. L. Chuoke 36 have compared the near ultra-violet spectra of toluene ethylbenzene iso- propylbenzenc and tert.-butylbenzene and found that the bands representing transitions allowed by the lowered symmetry relatively to benzene due to migration of charge into the ring become stronger and shift to longer wave-lengths on passing from tert.-butylbenzene to toluene This result was ascribed to increase of hyperconjugation between the ring and the side chain as the latter changes from tert.-butyl to methyl.a 5 A. D. Walsh Ann. Reports 1947 44 32. 36 Chem. Reviews 1947 41 273. CRAWFORD HYPERCONJUGATION 239 In Table IV are recorded the ionisation potentials of some alkylbenzenes. That hyperconjugation is more important here than in the ethylene system would seem to be indicated by the smaller lowering in ionisation potential on passing from benzene to toluene (0.32 ev.) compared with (0-80 ev.) on TABLE IV I Substance. Benzene . . . Toluene . . . Ethylbenzene . . isoPropylbenzene . Ionisation potential ev. 11 Substance. 1 Ionisation potential ev. 1 9.24 8.92 8.75 8.6 fed-Butylbenzone . 1 o-Xylene . . . I . . . . I 8.5 8.3 8.3 s.3 passing from ethylene t o propylene.Indeed if the inductive effect were equally important in larger molecules then as R. N. Jones 37 points out it is difficult to understand why the absorption spectra of the ions of aromatic amines should resemble those of the parent hydrocarbon so closely. The polar effect of the positive charge on the nitrogen atom must produce a greater electron displacement in the -C-NH linkago than the relatively weak dipole displacement in -C-CH, yet in several cases the NH sub- stituent has been observed to produce shifts in aromatic hydrocarbons no larger than those produced by the at introduction the same position; of a methyl e.g. substituent the wave- 9 3 + c L - @ length and intensities of the maxima of 3-aminopyrene hydrochloride (XIII) and cyclohesadiene show marlredly and 8.4 ev.respectively) and absorb at relatively long wave-lengths compared with open-chain dienes and it was to explain this characteristic that Mullikcn independently invoked the concept of hyperconjugation. However T. M. Sugden and A. D. Walsh 38 obtained values of 8.71 and 9.02 ev. respectively for the ionisation potentials of s-cis- and s-trans-forms of butadiene and pointed out that the problem arose mainly because of the comparison of cyclic dienes with the spectra of predominantly s-trans-butadione. Walsh maintains that when the more satisfactory comparison of cyclic dienes with s-cis-open-chain dienes is made the changes in the ground states of the cyclic dienes may be explained as being due entirely to strain and charge-transfer effects.Further applications of the concept of hyperconjugation to spectra are discussed by E. A. Fehnel and M. Carmack 39 and by I. M. K l o t ~ . ~ * + ‘7 and 3-methylpyrene (XIV). / lower first ioilisation potentials (8.62 (XI11 .) (XIV.) \ (C) Cyclic dienes. cycZoPentadiene 37 Chem. Reviews 1943 32 1. 38 Trans. Faraday SOC. 1945 41 76. 40 Ibid. 1944 66 88. J . Amer. Chem. Xoc. 1949 71 84. 240 QUARTERLY REVIEWS 6 C- I H* H M. Szwarc 41 determined the C-H bond energy in toluene and the xylenes from pyrolysis experiments and found the weakest bond to be the C-H bond in the methyl group. The energy of that bond was found to be 77.5 Bcals. for toluene and m-xylene 75 for p-xylene and 74 for o-xylene. The data were taken to indicate that hyperconjugation in p-xylene decreases the C-H bond energy in the CH group by 2.5-3 kcals.the weakening of the C-H bond in the methyl group of p-xylene as compared with that in toluene being expected on the basis of hyperconjugation occurring for example in the p-x~dyl radical as in (XV). Tho pyrolysis of the three fluorotoluenes gave a value for the C-H bond energy very nearly equal to that obtained for toluene.42 Since therefore the field effect has very little in- fluence on the bond energy this result was regarded as sup- porting the suggestion that hyperconjugation is responsible for the weakening of the C-H bond of the CH in p-xylene. Recent kinetic studies have indicated that hyperconjaga- .I tion might be of some significanco in the oxidation of hydro- (xv.) c a r b o n ~ . ~ ~ R. S. Mulliken and C. C. J. Roothaan 4 4 9 45 have given a theoretical discussion of the twisting frequency and barrier height for free rotation in ethylene and find that a 90" rotation of the two parts of the molecule does not entirely destroy tho TC bond.When the two CH groups have been twisted through 90" relatively to each other a kind of hyperconjugation must exist between the x unsaturation electron of each CH group and a pair of quasi-unsaturation electrons y involved in C-H binding in the other CH group. Tho unsaturation electrons which would have no bonding power in " perpendicular " ethylene contribute somewhat to the bonding because of this hyperconjugation and the barrier to free rotation must be lowered thereby. As far as the methylothylenes are concerned it might be thought that hyperconjugation would tend to stabilise certain orientations of the CH group bLit this is probably not so ; for only tho four conjugating electrons in propylcne H,=C-C-C being considered and other electrons and also the other hydrogen atoms being neglocted tho energy of the system is unchanged for successive rotations of the CH group by 60" around its axis and is probably very little if at all changed for rotations through intermediate angles.In ethane also hyperconjugation has been shown to have little or no direct effect in rostricting free rotation.ls Molecular Refractivities.-In the alkylbenzenes there is a progressive increase in exaltation with increase in the number of methyl groups and furthermore the exaltation increases with increasingly symmetrical dis- tribution of the methyl groups. In aliphatic systems e.g.substituted 4 1 J. Ghem. Physics 1948,16 128. 4 2 M. Szwarc and J. S. Roberts ibid. p. 609 4 3 C. F. Cullis C. N. Hinshelwood and M. F. R. Mulcahy Proc. Roy. SOC. 1949 45 R. S. Mulliken Physical Rev. 1932 43 301. A 196 160. 4 4 Chena. Reviews 1947 41 219. CRAWFORD HYPERCONJUGATION 241 butadienes changes in exaltation may be due to changes in the shapo of the molecule but since this is hardly possible here R. S. Mulliken 46 regards hyperconjugation as being the constitutional cause of the observed increasing exaltation. It has been shown that equilibrium and kinetic studies provide chemical evidence for the tautomeric electron displacement in alkyl groups and it was in the field of kinetics that the first unambiguous evidence such as a well-spaced and complete sequence of rate constants substantiated by corresponding activation energies was obtained by E.D. Hughes C. K. Ingold and N. A. Taher.47 Furthermore it was pointed out why the effect had not clearly been observed before in such studies. Thus in reaction rate for example the initial and the transition state must be treated in a differential manner and since each will be subject to both inductive and mesomeric displacement the net result dopends upon a com- bination of four effects. In this way the clectron displacement is classified as a polarisation Le. a mesomeric effect if the unsaturation permitting alkyl conjugation is present in the initial state but as a polarisability i.e. an electromeric effect when the conjugation is either only present or enhanced in tho transition state.Because of these complications it becomes clear why the complete inversion of the inductive-effcct order had not previously been obtained. Indeed the extreme rate range H Me was only 1 1-66 in Baker and Nathan's experiments and moreover since the reaction was a bimolecular nucleophilic substitution witJh only a small electron demand because of mutually accommodating electron transfers no clear result should have been obtained. Hughes Ingold and Taher studied the unimolecular hydrolysis of p-alkylbenzhydryl chlorides which is a strongly eloctron-demanding reaction involving only a single electron transfer. Thus the factor involving alkyl conjugation swan ps the others and the following unequivocal results were obtained x = - ~~ lo% . . E kcals. . I I I I €1. 1 Me. 1 Et. ~ Pri. 1 But.2.82 63.5 62.6 46.95 35-9 21.0 1 18.9 I 19-4 1 19-8 1 20.05 More recent rate data concern the competitive bromination of toluene and tert.-b~tylbenzene,*~ which yielded relative rates of 4 1 a result readily explicable on the basis of hyperconjugation. The relative rates of chlorination of a series of alkylbenzenes also show the operation of hyperconjugation. 49 For tert.-butylbenzene and benzene 60 tt relative rate of 115 1 was obtained a result which cannot be accounted for by hyperconj ugation involving hydrogen atoms and therofore the 46 J . Chem. Physics 1939 7 356. 47 J . 1940 949. 48 E. Berliner and F. J. Bondhus J. Arner. Chem. Suc. 1946 68 2355. 49 P. B. do la Mare and P. W. Robertson J. 1943 279. 5O E. Berliner and F. J. Bondhus J. Amer. Chem. SOC. 1948 70 854. 242 QUARTERLY REVIEWS difference in rates was explained as being due to release of electrons from the tert.-butyl group through structures of the type (XVI) contributing to the resonance hybrid.The experimental data however are not sufficient to warrant such a postulate for partial rate factors were not determined. Furthermore the order of electron release by this mechanism is the same as that due to the inductive effect and thus would be created the difficulty of two opposing orders based on the same effect.50 Anionotropy.-The concept of hyperconjugation has also been used by A. G. Catchpole E. D. Hughes and C. K. Ingold 51 to account for equilibrium in anionotropic systems that isomer being preferentially formed in which the methyl group is hyperconjugated with the double bond.The crude approximation being made that only the energy of hyperconjugation makes a direct contribution to the free energy of one of the isomerides and this being talien to be 3 kcals. for the gem-dimethyl group the system ( CH3),C:CR-CH,I3r + (CH,),CBr*CR:CH + H,C-C-cH C"3 - 6 (XVI.) a t 300" K. should contain 1 % of the aa-dimcthylallylbromide. present is known to bo experimentally undetectable. A similar calculation being made for the system The amount CH,*CH:CH*CH,Br + CH,-CHRr.CH:CH which has been experimentally investigated the following results were obtained 7 :; 1 12 13 :i 1 15 Temp. OK. 273 373 In this table y and a refer to y- and a-methylallyl bromide respactively. the agreement being fortuitously better than could have been expected. Prototropy.-With regard t o t hI ee-carbon prototrop y hyper conjugation has been used to account for the following facts.52 A single activating group in three-carbon systems does not much influence the equilibrium between a@- and By-unsaturated forms whereas alkyl substituents in certain positions do quite markedly influence the ratio in which the isomers are formed.For instance in the case of an unsubstituted y-position the ap-unsaturated form is the one present a t equilibrium. As a result of introducing a methyl group in the y-position the equilibrium is shifted to the side of the By-isomer which is now the important form and 51 J . 1948 8. 5 2 P. B. ds la Mare E. D. Hughes and C. IS;. Ingold ibid. p. 17. CRAWFORD HYPERCONJUGATION 243 y-Substituent. H. 1 GH,. Et. I 1 - ____. Py-Form present at equilibrium yo 1 0 1 9 3 67 .CHNe,. 51 Also pentenoic acids and their methyl derivatives show that an a-methyl substituent does not favour the production of &forms which is the opposite effect to that produced by a y-mothyl substituent. This is well illustrated in the following table Acid. -~ Py-Form at equilibrium yo . _______ 94.4 - 24.6 19.3 The abovo facts may be explained as follows. The ap-form preponderates in the non-y-substituted compounds such as (XVII) for in that isomer there CH,*CR :CHCO,H CH,CH,*CR:CH*CO,H (XVII. ) (XVIII. ) is conjugation between the olefinic bond and the activating group whereas there is no such conjugation in the &form. Consequently hyperconjuga- tion between the @-substituent and the double bond produces only second- order effects. I n the y-methyl compounds (XVIII) the hyperconjugation energy of the y-methyl group with the double bond in the &-form compensates quite appreciably the conjugation energy in the ap-form.That the methyl group should be more effective is characteristic of alkyl conjugation. The data relating to pentenoic acid and its methyl derivatives are readily explained for there is hyperconjugation between the y-methyl and the By-double bond whereas the hyperconjugation between the ap-double bond is offset by conjugation between that bond and the activating group. Ingold has also explained on the basis of hyperconjugation data relating to the proportions in which ap- and ad-dihydro-compounds are formed by reduction of vinylacrylic acid and certain methylated derivatives. Halogen Addition.-Recently halogen additions to conjugated un- saturated systems have been explained on the basis of hyperconjugation as exemplified by the systems n n C=C-C-HaL a n d H-C-C-HaL For instance the equilibrium mixture obtained when bromine is added to 244 QUARTERLY REVIEWS butadiene contains 80% of the ccd-dibromide a result regarded as a mani- festation of the doubled bromine hyperconjugation in this compound whereas in the ap-isomeride there is only a single halogen hyperconjugation CH,Rr*C€I:CHCH,Br + CK,Br*CHBrCH:CH (80%) (20%) In 8- and y-allrylated butadienes alkyl hyperconjugation is also of importance and so in /3-methylbutadioneY since yb-addition is excluded by C initiation only the ab-compound CH,X-CMe :CH*CH,X of remaining possible addition compounds maintains the hyperconjugation of the methyl group.Since halogen hyperconjugation reinforces alkyl hyperconjugation the ab-dihalide may be expected t o preponderate a t equilibrium a conclusion verified by experiment .53 Elimination.-The Saytzeff rule namely that in elimination reactions of sec.- or tert.-alkyl halides with alkali the most alkylated ethylene is formed is readily explained as being govorne'd by electromeric electron displacement. In the transition state the quasi-unsaturation electrons of the alkyl group hyperconjugate with the unsaturation electrons of the developing double bond. Since such conjugation creates a system of lower energy the elimination is facilitated by the development of the latent hyperconjugation. l3 Experimental investigations in which alkyl groups are conjugated with unsaturated olefinic bonds have normally been confined to cases where there is conjugation between C-H bonds and an aromatic nucleus.No such restriction is applicable theoretically and so conjugatioii of a methyl group with a double bond should result in increased reactivity of a-methylenic hydrogen atoms. Recently J. W. Baker 64 carried out the Prins reacbion with propylene and obtained as products (i) the diacetate of n-butane- 1 3-diol (64%) (ii) the cyclic formal of this diol (la%) (iii) 4-acetoxy- tetrahydro-y-pyran (22%). Acid-catalysed addition to the double bond resulted in the first two compounds being formed whereas the third involves the direct reaction of the hydrogen of the methyl group activated by its conjugation with the olefinic linkage. Further evidence for such cc-methylenic activity in olefinic and poly- olefinic systems will be found in references 55 56 and 57. I am indebted to Prof. C. A. Coulson for suggesting to me the investigation of the subject here reviewed and for friendly discussions. Prof. Coulson also read the manu- script and made valuable criticisms and suggestions which have helped to overcome certain infelicities of presentation. Dr. D. H. R. Barton checked the manuscript and this service as well as beneficial comment is warmly acknowledged. 6 3 W. J. Jones and H. G. Williams J . 1934 829. 5 4 J . 1944 296. 5 5 E. H. Farmer Trans. Paraday SOC. 1942 38 340. sE E. H. Farmer G. F. Bloomfield A. Sundralingham and D. A. Sutton ibid. p. 348. 67 E. H. Farmer {bid. p. 356.

ISSN:0009-2681    

DOI:10.1039/QR9490300226

出版商:RSC    

年代:1949

数据来源: RSC

摘要:

NATURALLY OCCURRING PEPTDDES By R. L. M. SYNGE B.A. PH.D. (THE ROWETT RESEARCH INSTITUTE BUCKSBURN ABERDEENSHIRE) General THIS review is intended to bring together information about a class of substances of which until recently very few examples were known in Nature. The substances discussed contain peptide bonds of the general type *CHR*CO*NH*CHR'* yield a-amino- or a-imino-carboxylic acids (amino-acids) on hydrolysis and differ from the proteins in that they have smaller molecules or that they embody a much smaller variety of amino- acid species. The upper limit for molocular weight has been arbitrarily placed a t 10,000 (about 100 amino-acid residues). There is in fact no sharp dividing line between peptides and proteins and it is thus accidental that such low-molecular proteins as cytochrome-c,l ribonuclease,2 and the dis- aggregated form of insulin are excluded.However the 10,000-molecular- weight limit corresponds roughly with a liinitat'ion of the power to pass through Cellophane on dialysis and is therefore convenient since dialys- ability is often the only evidence as t o molecular weight cited by authors. The most markod difference between peptides and proteins is the greater tendency of the latter to undergo denaturation. Typical denaturation phenomena are not shown by any of the substances discussed here and some of the proteins of lower molecular weight are also extremely resistant to denaturing agents. Otherwise the general physical and chemical properties of the peptides are only different from those of proteins in ways that can be directly cdrrelated with smaller molecular size.The alternative criterion-lack of variety of amino-acid residues-is employed mainly on account of the polymer of D-glutamic acid found in BaciWus anthracis etc. (see below) which is a high-molecular substance although it is not usually referred to as a protein. No other such substances have yet been described. The absence of one or of a few amino-acid species from a protein? as with gelatin or zein has not led to inclusion. Whilst some of the substances described below contain a small variety of amino-acids no evidence as to their molecular weight has been presented; it is therefore desirable to retain the criterion. The study of peptides has scarcely advanced beyond the descriptive stage; the structures of few of the compounds mentioned have been thoroughly elucidated and classification according to chemical structure or biological function would seem premature.The substances have been arranged in three main groups according to biological origin (a) found in animals or higher plants ; ( b ) found exclusively in fungi or related micro- J. Wymtbn jun. Advances i n Protein Chemistry 1948 4 410. I. Fankuchen ibid. 1945 2 387. H. Gutfround Biochem. J . 1948 42 544. 245 246 QUARTERLY REVIEWS organisms; and ( c ) found exclusively in bacteria. An outline is given of the salient chemical and biological features for each t,ype with key references to the literature. No attempt has been made to deal with the important and extensive subject of the experimental techniques used in studies of peptides. However studies of natural peptides are not confined in interest to the substances themselves but are felt to have very important implications for the study of proteins and of the metabolism of nitrogenous compounds generally.have been shown to be large molecules possessed of the most diverse biochemical activities which are fundamental to the continuance of life. They are made up from a very limited number of species of amino-acid residue joined for the most part in peptide linkage. Only about twenty different species of amino-acid have yet been discovered in proteins isolated from the whole range of living organisms,s 9 and most of these are present in evory protein. This implies a degree of standardisa- tion of the “working parts ” of living organisms that has usually been explained in terms of highly specific geometrical arrangements of the chemical groupings in protein molecules leading to highly specific “ sccon- dary valency ’’ interactions with other molecules large and small.10 The study of simpler peptides has already helped in elucidating tho nature of interactions of this kind,lO and has further implications for the study of proteins that are discussed below.Peptides as Model Substances in the Study of Proteins.-Peptides of known structure and purely synthetic origin have been extensively used in studies of the specificity of proteolytic enzymes in electro-chemical investigations bearing on proteins and so forth. These lie outside the scope of this article. The structures of the majority of known natural peptides have not yet been elucidated so they are less useful for these purposes.However owing to their relatively simple molecular structure they have already proved of value in testing methods of amino-acid analysis and methods of determining the mode of linkage of amino-acid residues. Methods for the latter purpose have so far depended mostly on identification of free functional groups 11-16 Proteins 4 5 9 6 C. L. A. Schmidt “ The Chemistry of the Amino Acids and Proteins ” Thomas ti D. Jordan Lloyd and A. Shore “ Chemistry of the Proteins ” Churchill London 7 Thorp’s Dictionary of Applied Chemistry Vol. X. Article on “ Proteins and 8 H. B. Viclrory and C. L. A. Schmidt Chem. Reviews 1931 9 169. 9 A. J. P. Martin and R. L. M. Synge Advances in Protein Chemistry 1945 2 1. 10 K. Landsteiner “ The Specificity of Serological Reactions ” Harvard University l1 R.M. Herriott Advances in Protein Chemistry 1947 3 169. 12 H. S. Olcott and H. Frasnkel-Conrat Chem. Reviews 1947 41 151. l3 8. W. Fox Advances in Protein Chemistry 1945 2 155. l4 F. Sanger Biochem. J . 1945 39 507. l5 R. R. Porter and F. Ssnger ibid. 1948 42 287. l6 F. Sanger ibid. 1949 44 126. Springfield Ill. 1938. 1938. Peptides ” Longmans London in the press. M. V. Tracey “ Proteins and Life ” Pilot Press London 1948. Press Cambridge Mass. 1945 ; L. Pauling p. 275. SYNGE NATURALLY OCCURRING PEPTIDES 247 and of products of partial hydrolysis. Studies of partial hydrolysates of proteins have not so far yielded very much structural information. The subject has been reviewed in detail to 1941.l' Since then not many structural studies have been made of " true '' proteins although concrete evidence has been obtained of the extreme complexity of their partial hydrolysates.Natural peptides are proving to be very valuable substances with which to test bhe methods used in such studies. Glutathione 1 3 and gramicidin S There are grounds for attaching greater significance to products of partial hydrolysis effected with acid or alkali than to those obtained enzymically. Peptides obtained in the former ways are excluded from consideration here. Of the few peptides arising by enzymic hydrolysis of proteins that have been characterised at all only casein phosphopeptone hypertensin and strcpo- genin seem to merit detailed description here. As the chemical structure of the simpler peptides becomes known it will become possible to interpret their physical properties in coiisiderable detail and this will give much greater significance to future interpretations of the structure of true proteins based on their physical properties.This applies to such properties as 'solubility osmotic pressure sedimentation diffusion electrophoretic migration and so forth. l9 However perhaps the most promising field for such developments is that of X-ray crystallography. We can hope that X-ray studies of peptides of known chemical structure will lead to the elucidation of complete crystal structures and that know- ledge of such simpler crystal structures will help towards the elucidation of the more complex structures of true proteins. Many of theso occur as crystals giving beautiful X-ray diffraction pictures that can as yet be inter- preted only in the most general terms.2 2o The complete crystal-structure analysis of benzylpenicillin by Crowfoot and her colleagues,21 with its implications for the chemical formulation of that group of substances is an important step forward.Finally it should be pointed out that many of the peptides occur in Nature as mixtures of related compounds similar in general structure but differing in detail. The ergot alkaloids gramicidins tyrocidines and penicillins are well-established examples of such families and many more will be found. Such mixtures often behave in many respects as though they were pure homogeneous substances and the establishment of their heterogeneity has tried existing techniques to their limits. Workers in this field tend therefore to be highly sceptical of claims to have isolated " pure homogeneous proteins " since by analogy with the peptides proteins might also be expected to occur in families.The molecule being so large detection of heterogeneity would require an altogether higher order of technique. l7 R. L. M. Synge Chem. Reviews 1943 32 135. I s R. Consden A. H. Gordon A. J. P. Martin and R. L. M. Synge Biochem. J. l9 E. J. Cohn and J. T. Edsall " Proteins Amino Acids and Peptides " Reinhold 2o J. Boyes-Watson E. Davidson and M. F. Perutz Proc. Roy. Xoc. 1947 A are perhaps a t present the best examples. 1947 41 596. Publishing Corp. New York 1943. 191 83. 21 D. Crowfoot Ann. Review Biochem. 1948 17 115. 248 QUARTERLY REVIEWS Meanwhile the critical application of existing techniques raises serious doubts as to the validity of most claims so far made for the homogeneity of protein preparations.6c Abnormal ’’ Structural Features of Peptides and their Possible Occur- rence in Proteins.-Peptides may also because they have lower molecular weights and because their study is technically simpler provide the first examples of structural features that subsequently prove to be present in proteins. (However reasons are given in the following section why peptides so far studied may not be representative in this respect.) Strepogenin hypertensin and casein phosphopeptone are in fact derived from proteins by enzymic breakdown and other peptides such as the posterior-pituitary hormones and various of thc antibacterial substances from Bacillus spp. may be products of breakdown of proteins brought about by the somcwhat violent conditions of extraction.Nevertheless many of the amino-acid species present in natural peptides have not been revealed in hydrolysates of proteins studied by appropriate methods. This is true for @-alanine ay-diaminobutyric acid ornithine penicillamine etc. Amino-acid residues possessing abnormal steric con- figurations occur in many of the fungal and bacterial products described below. Attcmpts to demonstrate the occurrence of D-amino-acid residues in proteins have not however been generally successful and have been complicated by racomisation. 2 2 s 23 Reference should however be made to claims for the occurrence of D-ainino-acids in hydrolysates of Lactobacillus arabinoszcs z4 and Bacillus brevis. 25 Many of the peptides described below seem to have a “ normal )’ open unbranched peptide-chain structure.Others however present “ abnormal ” features whose occurrence in proteins deserves serious consideration. Thus gramicidin X and the components of tyrothricin are closed-chain ‘‘ cyclopeptides ”. Glutathione and probably the pteroyl peptides and the capsular substance of Bacillus anthracis contain glutamic acid residues linked through their y-carboxyl group. Gramicidin embodies a residue or a precursor of ethanolamine in such a way that its basic properties are masked ; lycomarasmin and the ergot alkaloids may contain a-hydroxy-a- amino-acid residues. Similar types of linkage of these and other hydroxy- amino- and thiolamino-compounds should be searched for in proteins. Penicillin similarly illustrates ways in which an aldehyde group (e.g. of a sugar residue in a glycoprotein) could combine with thiol groups or with an enolised peptide bond.Numerous peptides embody acyl residues that are not amino-acid in character (e.g. pteroylglutamic acid and related com- pounds penicillins polymyxins and ergot alkaloids) and could serve as models for the linking of acyl groups in some of the conjugated proteins. 2 2 A. Neuberger Advances in Protein Chemistry 1948 4 207. 23 Riochem. SOC. Symposia No. 1 “ The Relation of Optical Form to Biological 24 M. S. Dunn M. N. Camien S. Shankman and H. Block J . Biol. Chem. 1947 Z 5 A. S . Konikova and N. N. Dobbert Biokhimiyu 1948 13 115. Activity in the Amino-acid Series ” University Press Cambridge 1948. 168 43. SYNGE NATURALLY OCCURRING PEPTIDES 249 Casein phosphopeptone has already proved useful in elucidation of the linking of phosphoryl residues in phosphoproteins.Distribution and Physiological Function of Peptides.-It is still very difficult to generalise with any confidence about the biological r61e of pep- tides. A very high proportion of those so far described possesses some notable physiological activity-as hormones poisons growth promoters or inhibitors and so on. This is the result of researches aimod spocifically at concentrating substances possessing particular biological activities. Such active substances have been found in a wide variety of chemical classes and none of these types of biological activity has so far been found to be invariably due to peptides. However of those antibiotic substances about whose chemical structure sufficient has been published to permit classifica- tion a high proportion appears to be peptides or peptido derivatives.Work and Work 26 suggest that some of these may act as specific inhibitors of stages in protein synthesis and prove to be valuable aids for elucidating that process in micro-organisms. The occurrence in toxic substances fi.om bacteria and fungi of stereo- isomers of substances commonly found in higher organisms calls for special comment in this connection. Possible interpretations of the r61e of D-amino- acids have been discussed in a special p~blication.2~ Attention should also be drawn to the occurrence of antibiotic substances which though not peptides are sufficiently closely related in structure to be possible antagonists for reactions in which peptides aro involved. Certain of the antibiotics from Fusarium s p p ~ .~ ~ and aspergillic acid 2 8 p Zgn closely simulate poptides whilst streptomycin and gliotoxin are rather more remotely analogous. What is manifestly needed is objective study of peptides in living organisms irrespective of any imniediately obvious physiological activity that they.possess. Very little work on such lines has been done and much of that such as the earlier studies of peptidzmia is technically inadequatte. However techniques for studying peptides have much improved in recent years. A very large proportion of the reliable physiological information so far available is due to the critical and caroful studies of H. N. Christensen and his colleagues making use of the Van Slyke ninhydrin CO determina- tion. Peptides without doubt occur in great variety during enzymic broak- down of proteins as in tissue autolysis in the intestinal tract in breakdown of proteins by bacteria and so forth.Over-all properties of such mixtures have been extensively studied but very few chemical individuals from them have been charscterised. It is noteworthy that at most it small proportion of the peptides present in the mammalian intestine are absorbed as such ; 26 T. S. Work and E. Work " The Basis of Chemotherapy " Oliver and Boyd Edinburgh 1948 p. 227. 27 A. H. Cook S. F. Cox T. H. Farmer and M. 5. Lacey Nature 1947 160 31 ; A. H. Cook S. F. Cox and T. H. Farmer ibid. 1948,162 61 ; P. A. Plattner U. Nager and A. Boller HeZv. Chim. Acta 1948 31 594 ; P. A. Plattner and U. Nager ibid. p. 665. Many of their data are referred to below. 28 J. D. Dutcher J.Biol. Chern. 1947 171 321 341. 28rr G. Dunn G. T. Newbold and F. 8. Spring Nature 1948 162 779. 250 QUARTERLY REVIEWS the gut wall seems selectively to transmit free amino-a~ids.2~ 30 Mixed peptides obtained by cnzymic or acid hydrolysis of proteins when injected into the blood-stream are rather slowly utilised by the tissues and a con- siderable proportion of them is excreted through the kidneys.31 However peptides derived from different proteins behave differently in this re~pect.~2 The blood-plasma contains only small amounts of pep tide^.^^ 34 Peptides are liberated when blood cl0ts.~4 Red blood corpuscles muscle liver and other tissues contain much larger concentrations of peptides. Of these a high proportion is accounted for by the ubiquitous glutathiono (together with anserine and carnosine in muscle).Other peptides however are certainly pre~ent.2~9 33 35 363 37 Peptidcs are normally present in urine in addition to such conjugates as benzoylglycine (hippuric acid).31 32 38 39 I n a recent study of pathological urines some individual peptides were observed one of which may be ~erylglycylglycine.~0 “ Bound ” amino- acids or peptides have likewise been reported in extracts of plant tissues 417 42 43a 43b and in culture media after the growth of bacteria.43 All the tissues and fluids mentioned above contain also free amino-acids and it is naturally surmised that the peptides may be intermediate stages in the interconversion of amino-acids and proteins which Schoenheimer and his colleagues showed to occur at a very considerable rate. Unfortunately tho more actively an intermediate takes part in a chain of reactions the lower is its concentration.It is even possible to visualise protein synthesis or breakdown occurring as an enzyme reaction without detectable inter- mediates as in the breakdown of amylose to maltose by P - a m y l s ~ e . ~ ~ When techniques have been devised for the isolation and characterisation of peptides present in living tissues isotopic tracers should prove of great J . Clin. Invest. 1947 26 853. ibid. p. 333. 29 H. N. Christensen D. C. Docker E. L. Lynch T. M. Mnckenzie and J. H. Powers 3O C. E. Dent and J. A. Schilling Biochem. J. 1949 44 318 ; H. N. Christensen 31 H. N. Christensen E. L. Lynch and J. H. Powers J . Biol. Chem. 1946 166 649. 32 €I. N. Christensen E. 1,. Lynch D. G. Decker and J. H.Powers J. Clin. Invest. 33 H. N. Christensen and E.’ L. Lynch J. Biol. Chena. 1946 163 741. 3 4 Idem ibid. 1946 166 87. 35 H. N. Christenson J. A. Streicher and R. L. Elbinger ibid. 1948 172 515. 36 I€. N. Christensen J. T. Rothwoll R. A. Sears and J. A. Streicher ibid. 1948 175 101. 37 H. Borsook C. L. Deasy A. J. Haagen-Smit G. Keighley and P. H. Lowy ibid. 1948 1’94 1041 ; ibid. 1949 179 705. 38 H. E. Sauberlich and C. A. Baumann ibid. 1946 166 417. 39 R. D. Eckhardt A. M. Cooper W. W. Faloon and C. S. Davidson Trans. N.Y. 40 C. E. Dent Biochem. SOC. Symposia No. 3 ‘‘ Partition Chromatography ” 41H. B. Vickery J. Biol. Chem. 1925 65 657 and previous papers. 4 2 P. Haas T. G. Hill and B. R. Wells Biochem. J. 1938 32 2129. 4 3 H. Proom and A. J. Woiwod J. Cen. Microbiol. 1949 3 319.43u P. Hans and T. G. Hill Biochem. J. 1931 25 1472. 43B T. Ohira J . Agr. Chem. SOC. Japan 1939 15 370; 1940 16 1 293. 4 4 M. A. Swanson J . Biol. Chem. 1948 172 805. 1947 28 849. Acad. Sci. 1948 Ser. 2 10 284. in the press; Biochem. J . 1947 41 240. SYNGE NATURALLY OCCURRING PEPTIDES 251 value in assessing their metabolic function. Some data of this kind exist for glutatlnione.45 The syntheses of leucylglycine from glycine 46 and of peptide material from leucine 37 have also been demonstrated using tracer techniques. Nevertheless the first requirement still is for non-discrimin- atory study of the peptides occurring in living organisms. Whether or not such studies will throw much light on protein synthesis and breakdown they may have other important results that cannot a t present be predicted.Descriptive Peptides from Animals and Higher Plants.-Casein phosphopeptones (Zactotyrines) are obtained by treating casein with trypsin with or without previous peptic digestion. They have been studied in considerable detail by a number of authors and there seems to be substantial unanimity that each molecule contains eight or more amino-acid residues belonging almost exclusively to the species glutamic acid serine and i ~ o l e u c i n e . ~ ~ - ~ ~ The numerous phosphate residues are esterified to hydroxyl groups of serine. The larger peptides in this group may additionally contain aspartic acid residues.51 Partial acid hydrolysis of casein phosphopeptones yields phosphoserylglutamic acid which probably furnishes the free amino-group of the polypeptides.51-s3 The data on phosphopeptone thus suggest that there is a high degree of localisation of tho isoleucine and phosphoserine residues within the structure of the casein molecule.There is evidence that the phosphoric estor groupings may prevent trypsin from attacking the phosphopeptone.50~ 54 Hypertensin (sometimes also called nngiotonin) is a blood-pressure- raising substance obtained by allowing a proteolytic onzyme (renin) from the kidney to act on serum-protein of the globulin fraction. A similar reaction occurring in vivo may be responsible for thc clinical condition of renal hypertension which may be roalised experimentally by promotion of renal ischemia. P. Etlman 55 has carried the purification of hypertensin further than other workers and has givcn a full review of the literature.A somewhat similar product (“ pepsitensi.12 ”) result’s from the action of pepsin on serum-protein. Edman’s hypertensin preparations were dialys- able through Cellophane and diffusion measurements suggested a molecular weight of the order of 3000. The material yielded much histidine on hydrolysis and of the usual amino-acids appeared to lack arginine cystine methionine threonine and phenylalanine and to contain little tyrosine. 4 5 K. Bloch and H. S. Anker J. Biol. Chem. 1947 169 765. 46 F. Friedberg T. Winnick and D. M. Greenberg ibid. p. 763. 47 M. Damodman and B. V. Ramachadran Biochem. J. 1941 35 122. 48 J. Lowndes T. J. R. Macara and R. H. A. Plimmer ibid. p. 315. 49 C. Rimington ibid. p. 321. 50 T. Posternak and H. Pollaczek Helv. Chirn. Acta 1941 24 921. 51 Idem ibid.p. 1190. 5 2 P. A. Lovene and D. W. Hill J. Bid. Chem. 1933 101 711. 53 B. H. Nicolet and L. A. Shinn. 5 4 T. Posternak and S. Grafl Helv. Chirn. Acta 1945 28 1258. 5 5 Arkiv Keini. Min. Geol. 1945 22 A No. 3. See Ann. Review Biochem. 1947 16 236. 252 QUARTERLY REVIEWS The methods of purification used both by A. A. Plentl and I. H. Page 56 and by Edman 55 are similar to those used in isolations of histidine. E. Cruz-Coke 57 has provided somo data on the beliaviour of hypertensin with ion-exchange adsorbents. Taken together these results suggest a considerable localisation of the histidine residues within the parent molecule of serum-protein. Hypertensin appears to be stable to boiling water or for a few hours to boiling dilute hydrochloric acid but is readily inactivated by alkali and various other agents including proteolytic enzymes.Plentl and Page 58 have studied the attack by various enzymes as a possible approach to elucidating the chemical structure. It seems probable that like the phosphopeptones hypertensin and pepsitonsin are not chemical individuals but families of similar peptides. Xtrepogenin is the name given to a growth-factor for certain bacteria first shown to be present particularly in tryptic digests of various pr0teins.5~ Subsequent work indicated that concentrates of this principle could stimulate the growth of animals. It appeared to be peptide in character and to promote growth additional to that obtained with the best available mixtures of free amino-acids. 6o Woolley has put forward evidence that strepogenin is a derivative of glutamic acid and has found that various peptides of glutamic acid possess strepogenin activity although not to a degree com- parable with concentrates of strepogenin.61 Phenylalaniiie and lysine were absent from Woolley 's concentrates.G2 From diffusion measurements the molecular weight of strepogenin was estimated a t 300-500. 63 Evidence was further obtained that strepogenin may embody a free amino-group of a glycine residue and be derived by tryptic or acid hydrolysis from the neigh- bourhood of terminal glycyl groups l4 in intact insulin or trypsinogen though not in casein.62 Recently some possible amino-acid sequences at such points in the insulin molecule liave been mentioned.64 It is of particular interest that the activity of strepogenin can be in- hibited competitively by the plant-wilting substance lycomarasmin (see below) and that conversely strepogenin competitively inhibits the activity of lycomarasmin.The same relations oxtend on the ono hand to the synthetic glutamic acid derivatives possessing strepogenin activity and on the other to a group of liomologous aspartic acid derivatives which appear to be structurally related to lyc~marasmin.~~~ 65 This seems to be the clearest example of biochemical antagonism of structurally analogous substances yet revealed in the poptide series. This group of basic substances found associated with nucleic acid in the sperm of certain fish has been subjected to intensive Protamines. 6 6 J . Biol. Chem. 1945 158 49. 58 For refs. see A. A. Plentl J. H Page and F. R. Van Abeele J . Biol. Chem. 59 H. Sprince slid D.W. Woolley J . Amer. Chem. SOC. 1945 67 1734. 6O D. W. Woolley J . Bid. Chem. 1946 162 383. G1 Idem ibid. 1948 172 71. G3 Idem ibid. 1946 166 783. 6 4 Idem Fed. Proc. 1948 7 200 ; J . Biol. Chem. 1949 179 593. 6 5 Idem J . Biol. Chem. 1948 176 1299. 57 Ciencia 1945 6 101. 1946 163 49. 62 Idem ibid. 1947 171 443. SYNGE NATURALLY OCCURRING PEPTIDES 253 study for much longer than any of the other substances described here. Apart from the interest of their biogenesis Kossel considered that they had interest as especially simple members of the group of proteins suitable for model studies. Subsequent work has fully confirmed that they possess a much simplified qualitative amino-acid composition and there is further physical evidence that some of them have molecular weights less than 10,000.66~ 67 Kossel 68 reviewed the whole subject in great detail to 1927.Since that date interest has largely been concentrated on clupein (from herring) and saZmine (from salmon) the latter of which has found application for combining with insulin for injection. These protamines both contain arginine as their only basic amino-acid and there are two residues of arginine per residue of neutral amino-acid present. The neutral amino- acids alanine valine proline and serine are common to and proline appears in each case to be the terminal residue possessing a free basic group."? 60 719 72 isoLeucine 7O9 7 l and glycine 7 l are present in salmine but do seem not to have been demonstrated in clupein and the converse is true for hydroxyproline.69-7l Differences have also been noted in behaviour on methylation 7 3 and treatment with alkali.74 However the two protamines appear to be so similar as to merit more detailed comparative study.The Reviewer l 7 has summarised structural studies by partial hydrolysis especially of clupein and suggests that the structural con- clusions reached by Felix and his colleagues by actual isolation of peptides from partial hydrolysates made with acid are preferable to those of Waldschmidt-Leitz and his colleagues which were reached by non-isolative studies of enzymic digests. The last-mentioned authors have since described some interesting work involving isolations from tryptic digests which does not however call for reversal of this judgment.75-77 Particularly in view of the demonstration by Felix's school that clupeiii is heterogeneo~s,7~ it seems best to regard the protamines like so many other peptides not as pure substances but as families of compounds closely related in chemical structure.The hishones 68 embody a much wider variety of amino-acid residues and there is little evidence that their molecular weight is low ; they are therefore not considered in this article 6 * E. Waldschmidt-Leitz F. Ziegler A. SchBffner and L. Weil 2. physiol. Chem. 67 V. PIaskAev N Yarovenko and A. Passynski Compt rend. Acad. Sci. U.R.S.S. 6 8 A. KosseI " The Protamines and Histones " Longmans Green & CO. London 70 R. J. Block and D. Bolling Arch. Biochem. 1945 6 419. 71 G. R. Tristrstm Nature 1947 160 637. 7 2 R. R. Porter and F. Sangor Biochem. J . 1948 42 287. 73 S. Edlbacher 2. physiol. Chem. 1919 107 52.7 4 J. Roche and M. Mourgue Compt. rend. 1946 222 204. 75 E. Waldschmidt-Leitz and F. Turba J . p r . Ghem. 1940 156 55. '* E. Waldschmidt-Leitz J. Ratzer and F. Turba i b i d . 1941 158 72. 77 E. Waldschmidt-Leitz Beiheft 2. Ver. dtsch. Chern. No. 45 ; Chemie 1942 55 62 1045 1931 197 219. 49 580. 1928. 69 K. Felix and A. Mager 2. physiol. Chem. 1937 249 111. K. Felix and K. Dim 2. physiol. Chem. 1929 184 111. I1 254 QUARTERLY REVIEWS Pituitary hormones. An interesting position exists with regard to the hormones of the posterior lobe which has been reviewed at some length by G. W. Irving and V. du Vigneaud y9 and by B. F. Chow.80 The two gener- ally recognised principles the so-called pressor and oxytocic hormones have been isolated and separated from one another by chromatographic separa- tion on ion-exchange adsorbents.81 The substances responsible appear to be peptides of molecular weight 600-1300,82~ 83 and both contain much cysteine and tyrosine ; the pressor principle contains also arginine.82 The resulting difference in basicity probably accounts for the separation by adsorption and the differences in electrophoretic behaviour. For a long time it was held particularly by Abel that both activities were due to a single large molecule and recently extracts of the gland have yielded an apparently homogeneous protein of molecular weight approx. 30,000 possessing both the activities to about the same extent (molecule for molecule) as the individual active peptide preparations. Since the latter are isolated from the gland by somewhat drastic procedures it i s reasonable to assume that they are liberated from the " parent protein " during the isolation.This does not yet seem to have been established by direct experiment but it is suggestive that the two active peptides are resistant to digestion by pepsin whereas the " parent protein '' is digested by pepsin though without loss of either activity. An enzyme inactivating the pressor but not the oxytocic substance has recently been described.84 85 appear to be protein in character although material possessing adrenocorticotrophin activity may be ultra-filtered through Cell~phane,~~ 86 and there is evidence that the thyrotrophin may also have a rather low molecular weight.85 Secretin. This hormone that stimulates pancreatic secretion has been isolated from intestine in a crystalline state (for bibliography see refs.87 88). It appears to have molecular weight of approx. 5000. Recently some new data on its amino-acid composition have become available. g9 This tripept ide y-L-glutamyl-L- cystcinylglycine with the corresponding -S*S- compound is a very widely distributed tissue com- ponent often occurring in high concentrations. Its structure has been established by s y n t h e s i ~ . ~ ~ ~ 91 Its metabolic function is by no means Most of tho hormones of the anterior lobe Glictathione . 79 G. W. Irving jun. and V. du Vigneaud A n n . N.Y. Acad. Sci. 1943 43 273. 80 B. F. Chow Advances in Protein Chemistry 1944 1 153. 8 1 A. M. Potts and T. F. Gallagher J . Biol. Chem. 1944 154 349. a 2 Idem ibid. 1942 143 561. s3 M. Rosenfeld Bull. Johns Hopkim Hosp. 1940 66 398.S 4 H. Croxatto W. Badia and R. Croxatto Proc. SOC. Exp. Biol. Med. 1948 69 422. s5 A. White Physiol. Reviews 1946 26 574. 8 6 Dr. C. J. 0. R. Morris private communication. 87 G. Agren and E. Hammarsten J . P h p i o l . 1937 90 330. 88 G. Agron ibid. 1939 94 553. 89 P. Edman and G. Bgren Arch. Biochem. 1947 13 283. 90 C. R. Harington and T. H. Mead Biochem. J. 1935 29 1602. 9l V. du Vigneaud and G. L. Miller J . Biol. Chem. 1936 116 469. SYNGE NATURALLY OCCURRING PEPTIDES 255 clear despite a vast amount of experimentation since its discovery by F. G. Hopkins more than twenty years ago. These well-known compounds respectively p-alanyl-L-histidine and p-alanyl-1 -methyl-L-histidine are major components of muscle tissue for which likewise no metabolic function has been clearly established.Recent work has helped to elucidate the chemical nature of certain growth-factors for animals and micro-organisms belonging to the " B complex " and their relation to p-aminobenzoic acid a known growth-factor and naturally occurring antagonist of sulphonamide drugs Although several of these products have only been isolated from micro-organisms i t is convenient to consider all together. A very full review has been published by T. H. Jukes and E. L. R. S t o k ~ t a d . ~ ~ ~ Carnosine and anperine. Derivatives of pteroic and p-aminobenxoic acid. The structure of ptcroic acid has been established by degradative 92-g4 and synthetic 95-g8 studies. Pteroyl-L-glutamic acid has been isolated from liver 99 lo0 and synthe- ~ised.~5-98 The pteroyl derivative of what appears to be a peptido com- posed of three residues of glutamic acid has been isolated from a bacterial fermentation product (Corynebacterium sp.).92 93 101 It can be degraded to pteroylglutamic acid by partial alkaline hydr0lysis,~2 and synthetic studies lo3 suggest that it may have the structure pteroyl-y-glutamyl-y- glutamylglutamic acid.103@ The pteroyl derivative of what may be a hepta- peptide embodying seven glutamic acid residues has been isolated from yeast.104 It is inactive towards micro-organisms but acts as a growth- factor for chicks.It appears to be closely related with a polypeptide also isolatod from yeast 105 and embodying ten or eleven residues of L-glutamic acid one of unidentified a-amino-acid per residue of p-aminobenzoic acid and one amidic NH,. The molecular size of all these compounds othcr than pteroylglutamic acid must be regarded as uncertain until more physical data have been published.Other acyl derivatives. Although they are not strictly peptides mention should be made of certain acylamino-acids. Pantothenic acid (ay-dihydroxy- O l a Physiol. Reviews 1945 28 51. 9 2 E. L. R. Stokstatd ef al. J . Amer. Chem. SOC. 1948 'SO 5. B3 13. L. Hutchings et al. ibid. p. 10. 95 C. W. Waller et al. ibid. p. 19. B7 R. B. Angier et al. ibid. p. 25. gg J. J. Pfiffner et al. ibid. 1947 69 1476. lod E. L. R. Stokstad B. L. Hutchings and Y. SubbaRow ibid. 1948 70 3. lol B. L. Hutchings et al. ibid. p. 1. lo2 J. H. Mowat et wl. ibid. p. 1096. lo3 J. H. Boothe et al. ibid. p. 1099. 103a This seems t o have been conclusively established by J. Somb et al.(ibid. 1949 lo5 S. Ratner M. Blanchard and D. E. Green J . Biol. Chem. 1946 164 691. B4 J. H. Mowat et al. ibid. p. 14. B6 M. E. Hultquist et al. ibid. p. 23. B8 J. H. Boothe et al. ibid. p. 27. 71 2310). lo4 J. J. Pfiffner et al. ibid. 1946 68 1392. 256 QUARTERLY REVIEWS N-p’/3’-dimethylbutyryl-p-alanine otherwise pantoyl-B-alanine) is a vitamin and a growth-factor for micro-organisms that is very widely distributed among living organisms. Evidence has recently been presented that it may occur in a conjugated form in tissues.lo6 Other well-known acyl derivatives are found in urine and have beencalled “ detoxicationproducts ” ; e.g. benxoylglycine (hippuric acid) ad-dibenxoylornithine (ornithuric acid) phenylacetylglutamine etc. 6-Acetylornithine has been isolated from Corydalis.lo’ Miscellaneous. Cobalt- and phosphorus-containing substances have recently been isolated from liver possessing very high activity for the treat- ment of pernicious ansmia and also growth-factor activity for certain bacteria. 108-112 The molecular weight appears from X-ray and diffusion measurements to be 1500-3000. It is not yet clear whether these sub- stances are peptides.l1Za E. L. Smith’s earlier preparations were associated with peptide material and varied considerably in amino-acid composition. Enzymic attack permitted purification by fractionation procedures already employed suggesting that the active material was unchanged whereas associated contaminants had been digested. Among less well characterised substances possibly peptides mention should be made of a trypsin inhibitor from pancreas,l13 a pepsin inhibitor and other peptides from pepsinogen,l14peptide material from milk,114n a basic compound from wheat (possessed of some antibacterial activity and con- taining much arginine cystine and tyrosine 115 116) “ allergens ” from plant pollens,ll’ 118 and components of bee venom 119 l20 and cobra venom.121-122 Peptides from Fungi.-Penicillins. No attempt is made to review the extensive data on these antibacterial substances (see refs. 123 124). The complicated condensed ring-system of the penicillin molecule can be regarded as arising from reactions of the side-chains in the corresponding dipeptide acyl-L-a-formylglycyl-D-p-mercaptovaline (penicillamine). The D-configura- 106 G. D. Novolli N. 0. Iiaplan and F. Lipmnnn J. Biol. Chem. 1949 177 97.l o 7 R. H. F. Manske Canad. J. Res. 1937 15 B 84. l o 8 E. L. Smith Nature 1948 161 638. lo9 Idem ibid. 1945 162 141. 110 E. L. Rickes et al. Science 1948 107 396. 111 Idem ibid. 1948 108 134. 112 E. L. Smith and L. F. J. Parker Biochem. J. 1948 43 viii. 112a But see N. G. Brink et al. J. Amer. Chem. SOC. 1949 71 1854. 113 M. Kunitz and J. H. Northrop J. Gen. Physiol. 1936 19 991. 114 R. M. Horriott ibid. 1941 24 325. 114a R. Aschaffenburg J. Dairy Res. 1946 14 316. 115 A. K. Balls W. S. Hale and T. H. Harris Cereal Chem. 1942 19 279. 116 A. K. Balls and T. H. Harris ibid. 1944 21 74. 1 1 7 H. A. Abramson D. H. Moore and H. H. Gettner J. Phpical Chem. 1942 118 G. E. Rockwoll J. IminunoZ. 1942 43 259. 119 R. Havemann and K. Wolff Biochem. Z. 1937 290 354. 120 W. Fassbender ibid. 1944 317 246.1 2 1 F. Micheel and H. Emde 2. physiol. Chem. 1940 265 266 and previous papers. 122B. N. Ghosh and D. K. Chowdhuri J. Indian Chem. Soc. 1943 20 22. 1 2 3 A. H. Cook Quart. Reviews 1948 2 203. 1z4 E. Chain Ann. Review Biochem. 1948 17 657. 46 192. SYNGE NATURALLY OCCURRING PEPTIDES 257 tion of the penicillamine moiety appears to be essential for the biological activity. Neither of the amino-acids has so far been found elsewhere in Nature but L-a-formylglycine (aminomalonaldehydic acid) can be regarded as a simple oxidation product of L-serine whilst the finding of p-mercapto- valine suggests that past claims to have isolated hydroxyvaline should be taken more seriously. Ergot dlcaloids. Members of this family extracted from ergot of rye (Claviceps purpurea) have proved difficult to separate owing to mixed- crystal and molecular-compound formation.A. Stoll 125 has given a useful review of the whole subject together with a classification of the alkaloids. Por details reference should be made to papers from the schools of Stoll 126 and W. A. Jacobs.127 All of the family embody one residue per mol. of the complicated condensed-ring indole derivative lysergic acid (R*CO,H) . In ergobasine (ergometrine) this occurs as 2-N-lysergamidopropim-1-01 The natural compound has proved of great value in obstetrics and this substance and its optical stereoisomers have been partly synthesised. The other ergot alkaloids have not found so much application. On hydrolysis each molecule yields one molecule each of lysergic acid ammonia D-proline an a-keto-acid (pyruvic acid or @-dimethylpyruvic acid) and an a-amino-acid (L-leucine ~-valine or L-phenylalanine).Stoll 125 on alkaline hydrolysis of ergocornine obtained lysergamidc and dimethylpyruvoylvalylproline. Full experi- mental dotails do not yet seem to have been published ; meanwhile taking account of the suggestion of W. A. Jacobs and L. C. Craig 12* that the keto-acids and ammonia arise by breakdown of a-hydroxy-a-amino-acids (see also data on lycomarasmin below) the following formula seems reason- able for ergocornine R-CO-NH-CPri- 0-CO-CH-CH I I )CH2 CO-NH-CHPri-CO-N-CH This interesting structure is only partly peptide in character and has affinity with other products of Fusarium spp. 27 besides lycomarasmin. This substance was isolated from Fusarium lycopersici and has powerful leaf-wilting effects on tomato plants.129 P.A. Plattner and his colleagues hydrolysed lycomarasmin and isolated glycine aspartic acid pyruvic acid and ammonia. They suggested a provisional formula which they recognised to have defects.130 Recently D. W. Woolley has produced strong evidence in support of the following formulation 131 Zycomarasmin. CH,-CO-NH CH3 I I I I C0,H CH-NH-CO-CH2-NH-C-OH C0,H lZ5 Experientia 1945 1 250. lZ6 F. Troxler Helv. Chim. Acta 1947 30 163 and previous papers. lZ7 F. C. Uhle and W. A. Jacobs J . Org. Chem. 1045 10 76 and previous papers. lZ8 7 . Biol. Chem. 1938 122 419. lZo P. A. Plattner and N. Clauson-Kaas Experientia 1945 1 195. I3O P. A. Plattncr N. Clauson-Kaas A. Bollor arid U. Nagor Helv. Chini. Acta 1948 31 860. 131 J . Biol. Chem. 1948 176 1291.258 QUARTERLY REVIEWS The configuration of the or-hydroxyalanine residue is unknown. However Woolley synthesised a mixture of the diastereoisomers which possessed the appropriate biological activity and closely resembled lycomarasmin in a number of other respects. He also found that a number of derivatives of aspartic acid possess leaf-wilting properties. 65 The antagonism between some of these substances and strepogenin and its analogues has been referred to above. Amanitine and phalloidine. These two highly toxic heat-stable sub- stances have been isolated in a crystalline state from the well-known fungus Amanita phaZbides (Death Cap). Phalloidine has been investigated in the greater It yielded on hydrolysis two compounds not previously found in Nature-a substance which seemed to be a hydroxytryptophan and L-albhydroxyproline.The latter was identified with the original synthetic material of Leuchs. The hydroxytryptophan has special interest as a possible metabolic intermediate in the degradation of tryptophan to k y n ~ r e n i n e . ~ ~ ~ Cystine and alanine were also found in the hydrolysate. No free amino- or carboxyl group was found and no physical evidence as to molecular weight was presented. Amanitine 134 likewise appeared to be peptide in character and probably to contain hydroxytryptophan. Furnaryl-DL-ahnine has been isolated from Penicillium recticulosum. 134a Peptides from Bacteria.-yrothricin. R. J. Dubos and R. I). Hotchkiss isolated peptides having antibacterial activity from strains of BaciZZus brevis. Those appeared to be in some way associated with bacterial protein being set free by autolysis extraction with acid alcohol or by proteolytic enzymes against which tho peptides are resistant.The material so obtained is called tyrothricin. Simple fractionation of tyrothricin with solvents serves to separate the neutral gramicidins from the basic tyrocidines. These groups of substances are totally different in the nature of their antibacterial action. R. D. Hotchkiss 135 has given a very full arid clear review of the earlier data on the isolation and chemistry of these peptides and has also discussed their mode of action on living cells. The gramicidin fraction from tyrothricin yields on repeated crystallisation from acetone a product with constant nmino- acid composition. 136 The mother-liquors contain material differing somewhat in amino-acid composition and optical rotation.136* 138 For a long time it appeared as if an individual substance had been obtained by crystallisation (e.g.ref. 139). However J. D. Gregory and L. C. Craig lgO Grarnicidin A B etc. 132 H. Wieland and B. Witkop Annalen 1940 543 171. 133 A. Butenandt W. Weidel and E. Becker Nnturwiss. 1940 28 447 ; E. Becker 134 H. Wieland R. Hallermeycr and 'CV. Zilg Annalen 1941 548 1. 134u J. H. Birkinshaw H . Raistrick and G. Smith Biochern. J . 1942 36 829. 135 Advances in Enzymology 1944 4 153. 136 A. H. Gordon A. J. P. Martin and R. L. M. Synge Bioclzem. J . 1943 37 86. 137 R. L. M. Synge ibid. 1949 44 542. 13* R. D. Hotchkiss J . Biol. Chem. 1941 141 171. 139 R. L. M. Synge and A. Tiselius Acta Chem. Xcand. 1947 1 749. 140 J .Biol. Chem. 1948 172 839. ibid. 1941 29 237. SYNGE NATURALLY OCCURRING PEPTIDES 259 showed that such preparations are heterogeneous. They recognised a major component ( A ) a minor component (B) containing phenylalanine 140a in addition to the residues present in ( A ) and other minor components (C D etc.) containing tyrosine. The following facts though established with mixed material are probably mostly true a t least for gramicidin A . The molecular weight appears to be in the range 3000-5000 from measure- ments of diffusion 141 142 and vapour-pressure lowering ; 142 cryoscopic study gave results difficult to interpret. l 4 2 Acid hydrolysis yields L-trypto- phan and D-leucine together with D-valine L-valine L-alanine glycine 135 and ethanolarnine,lP3 in amounts accounting adequately for all of the carbon and nitrogen; 137 free amino- and carboxyl groups are absent.138 This implies some unconventional mode of linkage for the ethanolamine.Whatever this linkage is it seems necessary to postulate a cyclopeptide structure. The following peptides have been isolated from mixed grami- cidins by partial hydrolysis with acid ~-valylglycino,l44~ 145 D-leucylgly- cine L-alanyl- D - v a h e L-alanyl- D -leucine L-val yl-L-valine and D -valyl- ~ - v a l i n e . l ~ ~ 147 It seems unlikely from the yields that the first two can be derived from tho same gramicidin. The data seem to suggest that residues both of D- and of ~-valine exist in the molecule preformed but Lhere are possibilities of epimerisation that cannot be ignored. Gramicidin possosses free hydroxyl groups which have been acetylated.138 The action of sulphating 14* 149 and phosphorylating 150 agents has also been studied. A less toxic and more water-soluble product has been obtained by treating gramicidin with formaldehyde; 151 the chief reaction appears to be with the indole g r 0 ~ p s . l ~ ~ Solubility has also beon increased by preparing the half-succinic ester with and without additional formaldehyde treatment. 153 The biological activities of these and other derivatives have been These are a family of basic peptides of which some at least differ in their content of tryptophan,l39 a difference which permits their separation by adsorption on charcoal.139 156 The fractions so obtained do not differ much in other properties. J. D Gregory (private communication) has fractionated tyrocidine by counter-current distribution.155 Tyrocidines. l40@ Private communication from Dr. J. D. Gregory. 141 K. 0. Podersen and R. L. M. Synge Acta Ghe~n. Scand. 1948 2 408. 142 1\1. Tishler J. L. Stokes N. R. Trenncr and J. B. Conn J . Biol. Chem. 1941 144 Idem ibid. 1944 38 285. 146 H. N. Christensen J . Biol. Chem. 1943 151 319. 14' Idem ibid. 1944 154 427. 148 H. C. Roitz et al. J . Amer. Chem. SOC. 1946 6S 1024. 160 R. E. Ferrol R. S. Olcott and H. Fraenkel-Conrat ibid. 1948 'SO 2101. 151 J. C. Lewis ct al. Science 1946 102 274. 15% H. Frnonkol-Conrat B. A. Brandon and H. S . Olcott J . Biol. Chem. 1947 153 H. S. Olcott et al. Arch. Biochem. 1946 10 553. 154 H. Fraenkel-Conrnt et al. Proc. Xoc. Exp. Biol. Med. 1946 63 302. 155 0. Schalcs and G. E. Maim Arch.Biochem. 1947 13 357. 156 R. L. M. Synge and A. Tiselius Acta Chem. Scand. 1949 3 231. 141 197. l P 3 R. L. M. Synge Biochem. J . 1945 39 355. 145 I d e m ibid. 1946 39 351. Idem ibid. p. 1031. 168 99. 260 QUARTERLY REVIEWS The amino-acid species found in tyrocidine aro L-ornithine L-valine L - leucine D - p hen y lalanine L- pr o h e L - t yr osine L- tr ypt op han L - glu t ami c acid and L-aspartic acid. The two last are probably present as glutamine and asparagine an equivalent amount of ammonia being liberated in hydrolysis.135~ l 5 7 These account for substantially all of the carbon and nitrogen of tyrocidine. The molecular weight appears from diffusion experiments,141 to lie in the range 1900-5100. The 6-amino-groups of ornithine and the phenolic groups of tyrosine are free,158 but it is doubtful if carboxyl groups occur free.138 A cyclopoptide structure without other abnormalities can therefore be postulated.The occurrence together of the first five amino-acid species listed above suggests a close relationship with graniicidin X (see below). Grumicidin S. This antibacterial substance is produced by different strains of BuciZZus brevis and was discovered by G. F. Gause and M. G. Brazhnikova. 169 The material is readily crystallisable and recrystal- lised Soviet material appears substantially homogeneous by chemical ana1ysis,l6O diffusion,141 adsorption,139 and counter-current-distribution studies.161 The last method revealed the presence of several related com- pounds in an American preparation of gramicidin 8. Gramicidin X yields on hydrolysis equimolar amounts of L-ornithine L-valine L-leucine L-proline and D-phenylalanine.160 One free amino-group is present per stoicheiometric minimum unit and this is the 6-amino-group of the ornithine residue. 162 A cyclopeptide structure is therefore postulated embodying only a-peptide linkages. X-Ray studies of the hydrochloride and other salts and of the N-acetyl derivative etc. impose a maximum molecular weight corresponding to two stoicheiometric minimum units. In this case a two-fold axis of symmetry must be present in the molecule. This could reasonably occur only by ropetition twice of the same sequence of five amino-acids. The alternative possibility is a cyclopentapeptide molecule.fs3 An extensive study of the products of partial hydrolysis by acid produced strong evidence for the amino-acid sequence being -a - (~-valyl) -L-ornithyl -L-leucyl -D -phenylalanyl-L-prolyl- though no differentiation was made between the cyclopentapeptide and cyclodecapeptide formulae.l 8 Evidence from cryoscopic 164 and diffusion l 4 l measurements is in favour of the latter. J. I. Harris and T. S. Work 165 have synthesised some analogous open-chain peptides. At present grami- cidin ~!5' appears to be the most complicated natural peptide of which the structure can be formulated with reasonable certainty. PoZymyxins (uerosporin). What appear to be members of a family of 15' H. N. Christensen L. Uzman and D. M. Hegsted J. BioE. Chem. 1945 158 279. 158 H. N. Christensen ibid. 1945 160 75. 159 P. G. Sergiev (ed.). " Sovyetskii Gramitsidin i Lochoniye Ran " Medgiz Moscow 1943.160 R. L. M. Synge Biochem. J. 1945 39 363. Dr. L. C. Craig private communication. l6zF. Sanger Biochem. J . 1946 40 261. 163 D. Crowfoot and G. M. J. Schmidt unpublished. 1 ~ 4 A. N. Belozersky and T. S. Paskhina Biokhintia 1945 10 344 ; Lancet 1944,11 716. 165 Nature 1948 161 804. SYNGE NATURALLY OCCURRING PEPTIDES 261 peptide-like antibacterial substances have been isolated from strains of Bacillus polymyxa.166g 1679 cf* lSs Polymyxins A B C D and E have so far been described. The data and publications are concisely summarised in three recent abstracts 169-171 and more fully in a forthcoming publica- tion.172 The polymyxins are recognised as individuals on the basis of distinctive chromatographic behaviour or of yielding distinctive hydrolysis products or of both. So far most of the strains studied have each yielded polymyxin of a single type.On acid hydrolysis L-aydiaminobutyric acid D-leucine phenylalanine L-threonine serine and an optically active saturated fatty acid have been identified. For data on molecular weights see P. H. Bell et al. (ref. 172 p. 187). A component of culture filtrates of Bacillus licheniformis active against Mycobacterium tuberculosis was first described by R. K. Callow and P. d'A. Hart,l73 who reported a positive Sakaguchi reaction and diffusion through Cellophane (see also refs. 174 175). R. K. Callow and T. S. Work (private communication) state that licheniformin has been resolved into three biologically active peptides of similar amino-acid composition. This material isolated from a strain of the BaciElus subtilis group and possessing antibacterial act,ivity was discovered by B.A. Johnson H. S. Anker and F. L. Melene~.l~~ cf. 177 G. T. Barry J. D. Gregory and L. C. Craig 178 have produced evidence of the homogeneity of the active material in a commercial preparation. After hydrolysis they found phenylalanine leucine isoleucine glutamic acid aspartic acid lysine histidine cystine and an unidentified basic amino-acid. Methionine valine threonine serine proline and arginine were absent. A dipeptide of phenylalanine and isoleucine and a peptide containing phenylalanine and ornithine were also isolated from the hydrolysate. Some D-amino- acids were present. The permeability of membranes to bacitracin 177 suggests a molecular weight less than 2000. A number of other antibacterial preparations from BaciEEus subtilis have been described under this 179-185 The main 166 G.C. Ainsworth A. M. Brown and G. Brownlee Nature 1947 160 263. 16' P. G. Stansly R. G. Shepherd and H. S. White Bull. Johns Hopkins Ho.yn. 168 R. G. Shephord et al. J . Amer. Ghem. Soc. 1948 'SO 3771. 169 G. Brownlee and T. S. G. Jones Biochem. J . 1948 43 xxv. 170 T. S. G. Jones ibid. p. xxvi. 171 J . R. Catch T. S. G. Jones and S. Wilkinson ibid. p. xxvii. 172 P. H. Long et al. Ann. N . Y . Acad. Sci. 1949 51 853-1000. 173 Nature 1946 157 334. 17p R. K. Callow R. E. Glover and P. d'A. Hart Biochem. J . 1947 41 xxvii. 175 R. K. Callow et d. Brit. J . Exp. Path. 1947 28 418. 176 Science 1945 102 376. 17' 13. S. Anker et al. J . Bact. 1048 55 249. 178 J . Biol. Ghem. 1948 175 485. 179 E. F. Jansen and D. J . Hirschmann Arch.Biochem. 1944 4 297. 180 J. C. Lewis et al. ibid. 1947 14 416. 181 J. J . Stubbs et ul. i b i d . p. 427. 183 K. P. Dimick et al. ibid. 1947 15 1. lB4 R. E. Feency H. D. Lightbody and J . A. Guribuldi ibid. p. 13. 185 C. H. Hassall Nature 1948 161 317. Licheniformin. Bacitracin. Subtilins. 1947 81 43. 182 J. C. Lewis et al. ibid. p. 437. 262 QUARTERLY REVIEWS evidence as to their peptide nature is their dialysability and loss of bio- logical activity with proteolytic enzymes. In this last they differ from polymyxins. A number of other such bacterial products has been d e s ~ r i b e d ~ ~ ~ - ~ ~ ~ some of which may be peptides e.g. colistatin diplococcin nisin eumycin bacillin (and an inhibitor thereof ) actinorubin lavendulin streptollin etc. Capsulated strains of BacilZus anthracis and related Bacillus spp.produce this substance in their capsules and culture rnedia.lg1 lg2 H. N. Rydon lg3 has made a useful review of work on this topic. On acid hydrolysis the substance appears to yield only D-glutamic acid.lg4 The material in the capsules has a molecular weight of 50,000 or more when undegraded but breaks down rather readily and the material in the media has a molecular weight of a few thousand. There is evidence for the occurrence of both a- arid y-linkages. The y-linkages appear to be fewer in number and more labile. The peptide has been osterified with lg6 and through the ester converted into the amide polyglutamine. lg6 Studies have been made of the reactivity of the peptide and these derivatives with sulphnting agents,148 149 isocyaiiate~,~~~ and phosphorylating agents.150 W.E. Hanby S. G. Waley and J. Watson 198 have synthesised a polymeric a-peptide of L-glutamic acid possessing properties similar to those of the capsular substance. Peptide material has been repeatedly observed in tuber- culin preparations and some of the larger peptide molecules present have biological activity similar to that of the tuberculin proteins.lg9 2oo Other bacterial products. So far despite fairly extensive studies of biologically active materials from pathogenic bacteria few if any products falling within the scope of this review have been reported.201 The claim that scarlet-fevcr toxin has a low molecular weight 202 has been made doubtful by subsequent work. 201 Other antibiotics. Capsular substance of Bacillus anthracis and related species.Tuberculin. 188 P. G. Stensly and N. H. Anenenko Arch. Biochem. 1947 15 473. 187 R. G. Benodict and A. F. Langlykke Ann. Review Microbiol. 1947 1 193. 188 J. H. Bailey and C. J. Cavallito ibid. 1948 2 143. 189 J. Miller and D. Rowley Brit. J . Exp. Path. 1948 29 452. I90 T. L. Xu ibid. p. 473. 191 G. Iv&novics and V. Bruckner 2. Immun. Fomch. 1937 90 304. 192 V. Bruckner and G. Ivhovics 2. physwl. Chem. 1937 247 281. Riochem. Soc. Symposia No. 1 " The Relation of Optical Form to Biological Activity in tho Rmino-acid Series " p. 40 University Press Cambridge 1948. ls4 W. E. IIanby and H. N. Rydon Biochem. J . 1946 40 297. 195 H. L. Fraenkel-Conrat and H. S. Olcott J . Biol. Chem. 1945 161 259. 196 I€. L. Fraenliel-Conrat M. Cooper and H. 8. Olcott J . Amer. Chem.SOG. I g 7 Idem ibid. p. 314. 198 Nature 1948 161 132. ID9 F. B. Soibert Uact. Reviews 1941 5 69. 2oo Idenz Chem. Reviews 1944 34 107. 201 A. M. Pappenheimer jun. Advances in Protein Chenzistry 1948 4 123. 202 E. S. G. Barron G. P. Dick aid C. M. Lyman J . Biol. Chem. 1941 137 267. 1945 67 950.

ISSN:0009-2681    

DOI:10.1039/QR9490300245

出版商:RSC    

年代:1949

数据来源: RSC

摘要:

THE TERRESTRIAL DISTRIBUTION OF THE ELEMENTS By DAVID T. GIBSON D.Sc. (SENIOR LECTURER IN CHEMISTRY GLASGOW UNIVERSITY) GEOCHEMISTRY may be described as the Natural History of the chemical elements. The crystal form of naturally occurring compounds has long served to direct attention t o minerals of economic value. Only since the detailed structure of such crystals has become known has it been possible to elicit the laws which govern the formation and disintegration of minera'ls. Consequently although many relevant data had been accumulated no comprehensive account of the principles on which the segregation and aggregation of terrestrial elements take place could be made until someone equally experienced in mineralogy petrology and X-ray crystallography could turn his attention to tha problem.The scientific antecedents which enabled Victor Moritz Goldschmidt to make this study and thereby establish the science which will always be associated with his name will be found in the Memorial Lecture delivered by J. D. Berna1.l It is all the more regrettable that Goldschmidt did not live to produce a comprehensive account of his work since so much of it was published in comparatively inaccessible journals. Important references to the work of other investigators will be found in the Faraday Society discussion on igneous rocks,2 Berg's " Vorkommen der mineralischen Rohstoffe," Vernad- sky's " Geochemie ",4 Holmes's " Age of the Earth ",5 and in the discussion on the minor elements in Xoil Science.6 E. S. Larsen' gives a historical sketch covering the last 50 years and G. W.Tyrrell8 summarises a number of recent papers. The Three Major Partings of the Elements Originally a uniform gas mixture the constituents of the Earth in cooling pass through the phases I I1 111 Gas -+ Liquid -+ Solid -+ Solution Each change of phase is governed by a characteristic property of the atom and represents a major parting of the elements some undergoing the change before others. I. T h e First Parting.-This was originally pictured by V. M. Gold- Schmidt 6* as resulting in the condensation of the gaseous substance of the Earth into an iron core an intermediate sulphide-oxide zone a siliceous envelope and an atmosphere. He showed that similar artificial melts J . 1949,2108. See also C. E. Tillcy Obituary Notices of Fellows of the Royal Society 1948 0 51. G. Rcrg " Vorkommoii tier mincrnlischen Rolistoffo " Lcipzig 1929.W. J. Vernndsliy " Goochemie " Akacl. Verlag Leipzig 1930. A. Rolmes " Ago of the Earbh " Nelson 1937. V. hl. Goldschmidt et al. Soil Sci. 1945 60 1 . Amcr. Gcol. Xoc. Ann. vol. 1941 391. a Xci. Progress 1948 30 506. V. M. Goldschmidt 2. Electrochem. 1922 28 411. Trans. Paraday Soc. 1925 20 413. 263 264 QUARTERLY REVIEWS separated during cooling into three phases and that individual elements were concentrated preferentially in one or two of these phases. He classified the elements as siderophil (core) chalcophil * (intermediate zone) lithophil (crust) and atmophil. As may be seen from the accompanying Periodic Table (Table I) these designations correspond to typical electronic con- K Ca Sc Ti V Cr Mn Fe Co Ni Cs Ba ::; Hf Ta JV lithophil He Ro 0 s Ir Pt siderophil C N O F N e chalcophil or - C1 A Br Kr I xe - - At Rn lithophil ntn1 ophil These allocations are of course not rigid.As a result of variability of valency mutual solubility of layers and change of partition coefficients with temperature many elements enter more than one phase. The classi- fication of elements as “ lithophil ” etc. is not in any way invalidated by recent Swiss work discussed below. The solidification of the crust is really fractional crystallisation on a cosmic scale over a considerable time and is governed by ionic size. Cations which may be compactly incorporated in a silicate lattice are accepted early in the crystal- lisation those less suitable are accepted later or rejected completely. (‘ Stable ” and I‘ unstable ” are terms related to environment.Rocks formed at high temperature (500-1200”) and pressure fall ready victims to the effect of ice air and carbon dioxide. 111. The Third Parting.- Weathering. Formation of sedimentary rocks. Igneous rocks are now subject to selective dissolution leaving silica and alumina as residue. From this solution the other constituents are severally removed by hydrolysis adsorption biological action or even by crystal- lisation. These accumulated residues and precipitates form sedimentary rocks and the residual solution is sea water. The third parting is governed by the intensity of ionic charge. For comparative purposes this may be expressed as charge/radius of the ion. The connexion between weathering and biological activity has been extensively investigated in Russia by Vernadsky,4 A.P. Vinogradov l1 and A. E. Fersman.12 Much interesting information has also been accumulated by 0. Baudisch,13 E. J. C~nway,~* and M. D. Kamen,15 but as yet no 11. The Second Purting.-Formution of igneous rocks. lo S . J. Shand “ Eruptive Rocks ” Rlurby London 1947. l1 Nature 1943 151 659. l3 Soil Sci. 1945 60 173. l5 Bull. Amer. Museum Nat. Hist. 1946 87 109. * Shanci’s term “ thiophil ” is much botter.1° l 2 See S. Tomkeieff ibid. 1944 154 814. la Quart. Rev. Bid. 1943 18 337. GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 265 answer can be given to the question What specific character of the atom makes it suitable for participation in biological reactions ? IV. Metamorphic Roc1cs.-As is well known the compacted products of physical and chemical weathering when subjected to heat and pressure are recrystallised.This transformation is not merely physical involving the loss a t most of simple volatile materials like carbon dioxide and water but has been shown by D. L. Reynolds l6 to involve the replacement of certain ions by others. Metamorphic rocks are of course similarly formed (and to a very much greater extent) by the same forces acting directly on igneous rocks. In fact a number of geologists (see A. Holmes 17) envisage granitisation as a basic " front " invading pre-existing rock. Ahead of the front will be found mobile ions expelled by the process followed by the less mobile which have taken their place. There will be a ' I chromato- graphic " effect in which sorting in terms of ionic mobility l8 will have taken place.A fuller understanding of the process will reveal novel segregations but work on these lines is only just beginning 21 and will not be considered further in this review. In this connexion the behaviour of silver iodide quoted by Wells,lg is probably relevant. At the transition point (146") the iodine ions suddenly re-arrange to form a new stable lattice but the lattice of the silver ions breaks down. From 146" to the melting point (555") the silver is in a fluid form in the solid iodine lattice. The process is evidently one of solid diffusion. The First Parting Goldschmidt's conception of the segregation of the constituents of the Earth into three concentric zones has formed the basis of geochemical work for the last 25 years. It was well-known t o metallurgists that a melt con- sisting of iron sulphide soda sand and (insufficient) coke would form on cooling three layers slag ore and metal.G. Tammann 2o and V. M. Goldschmidt further showcd that those elements which react with oxygen more exothermally than iron will tend to accumulate in the slag. Those which react less exothermally than iron will concentrate in the metallic phase. V. M. Goldschmidt 21 was able to determine the zonal affinity of many elements by adding them to such artificial melts and then finding the distribution between the zones by actual analysis. He found that meteoric iron and the native iron of Disco (Greenland) are much richer than ordinary pig iron in the precious metals because they have concen- trated the precious-metal content of a large body of surrounding rock during formation.The enrichment factor yo M in meteoric iron/% M in ordinary iron is large (Pt 4000 Ir 5000 Pd 1000 Au 1000 Ag 50). The most important consequence of this conception is that lithophilic l6 Proc. Royal Irish Acad. 1943 B 48 231. l8 P. Lapadu-Hargues quoted by D. L. Reynolds Sci. Progrese 1947 35 212 ; cf also D. L. Reynolds Qeol. Mag. 1946 102 389. l9 A. F. Wells " Structural Inorganic Chemistry " Oxford 1945 p. 165. l7 Nature 1943 155 412. 8. anorg. Chenz. 1924 134 276. 21J. 1937 655. 266 QUARTERLY REVIEWS elements enjoy a seeming abundance simply because their locus is ours also. Conversely those which are not lithophilic seem to us rarer than they actually are because their main depot is inaccessible but no element is completely excluded from the lithosphere because at the temperature of segregation silicate sulphide and metal phases have an appreciable mutual solubility.I n this connexion the “Abundance of the Elements in the Earth’s Crust ” originally determined by F. W. Clarke and H. S. Wasliington,22 is of very great interest. Their figures formed the basis of Goldschmidt’s work and in turn were greatly amplified by him. A recent study from the Geophysical Laboratory in Washington 23 concludes that “ whilst it is quite possible that these figures will have to be changed when more accurate data are obtained i t is doubtful if this change will significantly alter the orders of magnitude ’,. This conclusion is confirmed by the more restricted but very thorough work of S. R. Nockolds and R. L. Mitchell ; 24 however in the case of two very important elements fluorine and potassium the accepted values have already been called in question.25 26 The following Table of abundance of elements in the Earth’s crust is adapted from data published by Berg.3 Relative abundance of the elements in the Earth’s crust Oxygen .. . . . . . . 50% Potassium . . . . . . . 2.4% Silicon . . . . . . . . 26 Magnesium . . . . . . . 1.9 Aluminium . . . . . . . 7.5 Hydrogen . . . . . . . 0-9 Iron . . . . . . . . . 4.7 Titanium . . . . . . . 0.6 Calcium . . . . . . . . 3.4 Chlorine . . . . . . . . 0.2 Sodium . . . . . . . . 2.6 Phosphorus . . . . . . . 0.1 Total 99.5y0 The abundance of tho rcmsihing elements is mom conveniently expressed in g./ton of the crustal rock less than 1 kg./ton (1 kg./ton = 0.1 74) ; Mn C S Ba Cr N F Zr Zn Ni Sr V less than 100 g./ton ; Cu Y W Li Rb Hf Ce Pb Th Ntl Co B (total 0-0470) less than 10 g./ton ; Mo Br Sn Sc Bo La .. . As A Ge (total O*Olo/b) less than 1 g./ton ; Se . . . Nb Sb U Tn Ga Tn T1 Ctl mg./ton; I P t metals Ag Ri Hg To Au rare gzwes mg./1000 tons; Ra Ac Po. A number of interesting points is revealcd by these data. (a) Many of the “ common ” metals e.g. Cu Hg Sn and As are really very scarce. Their familiarity is due to their ores being conspicuous con- centrated in a few favoured localities and readily reduced. ( b ) On the other hand many elcrnents not naturally concentrated are regarded as scarco even though they are abundant in the aggregate. ( c ) Some of the lighter elements are surprisingly scarce. This may be due to the sensitivity of their atoms to disruption the consumption of their atoms in the synthesis of more complicated atoms or more prosaically to their inclusion in the 22 U.S.Geol. Svy. 1924 Professional paper 127. 23 E. G. Zies Amer. J. Sci. 1938 35 385. 26 Trans. Roy. SOC. Edin. 1948 61 533. 25 E. S. Shepherd Amer. J. Sci. 1940 38 117. z6 J. H. J. Poole Nature 1948 162 775. (total 045%) GIBSON TERRESTRIAL DISTRIBUTION OF TIIE ELEMENTS 267 analyses of more abundant elements e.g . beryllium included with aluminium or strontium with calcium. (d) There is a marked falling off in abundance of all elements having an atomic number greater than nickel. ( e ) Elements of even are more abundant than those of odd atomic number. This is not because the even-numbered elements are more accessible. The question may most suitably be examined using the lanthanons.In them we have a chemically coherent group of elements in which no geochemical process operates to effect Selective enrichment. There are minerals in which the abundance rises (or falls) with rising atomic number but in all of these the elements show an alternating abundance. It may also be mentioned that the abundance of a rarer element is estimated not by summing the contents of those few favoured localities where mineralogical treasures containing a high proportion of the rare element exist but by determining into which common minerals the rare element preferentially enters and in what proportion it exists therc. W. Kuhn and A. Rittmann’s Views.-In view of the wide acceptance of Goldschmidt’s three-zone theory of the Earth it is surprising that quite recently two Swiss workers have put forward what seems a t first sight to be a completely different hypothesis.Kuhn and Rittmann 27 point out that as there is general agreement that the Earth is derived from the Sun the initial material of the new planet must have been very rich in hydrogen. Under its relatively tiny gravitational force the early history of the planet must have been dominated by the loss of hydrogen and resultant superficial cooling. The mechanism of this escape had been discussed by Niggli 28 who indicated that a selective transport of less volatile materials to the surface would be effected thereby. The result would be the formation of a composite crust the outer part rich in the more volatlile silicon and aluminium compounds the inner in the less volatile compounds those of iron and magnesium.Thus far the new view is in agreement with the older ideas. The essential novelty is the insistence that with the high temperature and viscosity obtaining and the decreasing efficacy of gravity as the centre of the Eartti is approached there will be no force adequate to bring about the separation into definite zones of the whole material of the Earth. In particular the idea of an iron core must be abandoned. Just inside the crust Kuhn and Rittmann suppose that a silicate-metal zone exists as an undifferentiated emulsion. The depth of this transition zone is small and within a distance from the centre corresponding to half the Earth’s radius there is only undifferentiated solar material. They regard the loss of hydrogen and resultant cooling as an entirely superficial phenomenon.The contrast between the old and the new con- ceptions will be seen in Fig. 1. The first parting as regards the segregation of the elements of the Earth as a whole is therefore and not gas + siliceous segregate + undifferentiated material gas + three concentric phases 27 Geol. Rundschau 1941 32 215 ; W. Kuhn Experientia 1946 2 10. 28 P. Niggli 2. anorg. Chem. 1912 ‘75 162. 268 QUARTERLY REVIEWS Kuhn and Rittmann are careful to point out that this drastic recon- sideration of Goldschmidt’s original ideas does not in any way invalidate them so far as preferential concentration or elimination of elements in the lithosphere is concerned; nor does it affect any surface phenomena which bring molten sulphide silicate and metallic phases into contact. The most important geochemical consequence of the Swiss view is that it points to a completely different set of values for the abundance of elements in the Earth as a whole.Several workers notably F. W. Clarke,29 H. S. Wash- ingt~n,~O and H. N. Russell 31 have attempted to arrive a t such values. They all agree in allotting a very subordinate position to hydrogen. It is K- FIG. 1 O,?d and new conception of the interior of the 8 a r t k . Old (left) solid crust K ; silicate shell M ; intermediate zone Z ; iron core. New (right) solid crust K ; magma zone M composed of molten silicates with increasing Fe-Blg-H content downwards ; practically unaltered solar material S. the essence of the new view that a large core of completely undifferentiated solar material containing approximately 30% of hydrogen exists in the Earth.Not only in this but also in their deductions from the weighed average composition of meteorites Kuhn and Rittmann differ from previous workers. The relative abundance and the composition of iron meteorites have frequently been used as an index of the size and actual composition of the Earth’s core. Kuhn and Rittmann point out however that the iron meteorites being far more conspicuous and durable than the stony meteorites have exerted a correspondingly exaggerated influence on the estimates of the internal composition of the planetary bodies and hence 29 U.S. Geol. Svy. 1924 132 D 76. s1 Science 1941 94 375. so Amer. J . S c i . 1925 9 351. GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 269 We are not here concerned with their extremely interesting The cosmic implications of the Swiss view have of the Earth.geophysical conclusions. since been briefly considered by R. Wildt.32 The Second Parting This parting may be further subdivided into three stages (a) The first crystallisation (b) the main crystallisation and (c) the final crystallisation and the residual liquors. To understand the second parting igneous-rock formation (cf. P. Niggli,33 Shand lo) must be considered. Although very many silicates and alumino- silicates are known igneous rocks are composed of a very small number of mineral species of which pyroxenes amphiboles micas felspars and quartz are by far the most important. These represent however only a part albeit a very large part of the original magma. Volatile constituents which may represent only 1% of the whole exercise a quite disproportionate effect on the behaviour of the mass in lowering viscosity and the temperature range of crystallisation.The crystallisation of silicates results as W. L. Bragg 34 has shown in the formation of one- two- or three-dimensional giant anions in which the Si-0 ratio may be 1/4 1/3 2/5 4/11 etc. The negative charges on the lattice are balanced by cations of size suitable to the interstices in the Si-0 pattern. The fundamental distinction between the chemist’s and the geologist’s viewpoint is that the former regards such crystals as pure only if the cationic points are occupied by two or three cations at most in molecular proportions. To the geologist it is the Si-0 pattern which is critical. The negative charges on the lattice will be balanced by cations A B C .. . but they may be present in any proportions say lA mB nC . . . so long as the total charge is neutralised. The over-riding considera- tion is the economical use of space. Goldschmidt 21 has provided detailed evidence of the operation of this principle. The Conditions of the Second Parting.-The progress of crystallisation is conveniently divided into three stages corresponding to three substantially distinct physical environments in which it takes place. The First Crystallisation.-In the first crystallisation the parent magma -homogeneous at greater depths-wells up to fill subsidiary reservoirs a t perhaps 2-3 miles depth. In these differentiation must be effected by crystallisation of the higher-melting constituents from the excess of molten silica. These crystals sink and if they melt again a t lower levels they do not mingle again perfectly with the melt from which they separated.Thus is explained the composite character of the crust a comparatively infusible “ sial ” outer layer in which silica and aEumina are predominant and an inner crustal “ fema ” layer made up of ferromagnesian minerals. These two layers are separated by a comparatively more fusible “ sima ” layer. Further differentiation may be produced by subterraxan movement of the 32 Month. Not. Roy. -A&. Soc. 1947 107 97. 33 “ Ore Deposits of Magmatic Origin ” Murby London 1929. 34 Nature 1927 120 410. Their fate will be considered later. S 270 QUARTERLY REVIEWS partly crystallised magma resulting in a filterpress action the molten material being strained off from the crystalline resulting in the formation of an acid magma (the melt) and a basic one (the crystals).The immediate cause of igneous action is relief of pressure rather than supply of heat ; 35 the ultimate cause is the potential energy of the hydrogen of the Earth’s c0re.~7 The natural silicates are generally completely miscible not only when molten but also in the solid state. As G. W. Tyrrell 36 has pointed out it is unusual for their cooling curves to show an eutectic minimum. Such curves have been extensively investigated by N. L. Bowen 37 whose well- known “ reaction principle ” indicates that if a mixture of silicates cools sufficiently slowly to establish true equilibrium the solid will be homo- geneous and of the same composition as the melt. However since movement is so often associated with cooling it is evident that equilibrium will often not be established.It is therefore important to know in what sequence and in what combinations the major components of the magma will separate when conditions are favourable for differentiation. Further it is of interest to know how the minor constituents will be distributed throughout the mass. The Main Crystallisation.-This takes place when the magma is either (a) injected into fissures comparatively near the surface forming intrusive rocks e.g. granite or (b) ejected at the surface forming extrusive rocks e.g. basalt. The essential difference is that the intrusive rocks cool slowly under pressure retaining their volatile constituents and consequently forming large crystals. Extrusive rocks cool rapidly and volatile con- stituents are lost.The consequent fine-grained character of the product is much less efficient in parting the elements. The first and the main crystallisations may he summarised in the following scheme. BASIC I INTERMEDIATE ACID ROCKS I Refractory oxides ’ Amphiboles Olivines 1 Pyroxcnos I Micas Pelspars I 4- - -QUARTZ- - - - - - - - -+ I I 1 ‘ R,SiO I in which the predominant cations R are corresponding to the chemical types R,O A1,0, GSiO I I R,SiO Ca Na K I I I Fe Mg I Fe Mg Cs Harker 35 and Niggli 38 give a series of graphs detailing the variation in percentage of all these cations with increasing “acidity” of various types of rock. From these graphs it is evident that the formation of the different silicates during the crystallisation overlaps extensively. 35 A. Harker “ Natural History of Igneous Rocks ” Methuen 1909.36 Trans. Paraday SOC. 1925 20 418. 37 “ The Evolution of Igneous Rocks ” Princeton 1928. 3* Y. Niggli ref. 33 p. 15. GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 27 1 I n the first and the main crystallisations the solvent is molten silica whilst the solutes are iron oxide magnesia lime alumina soda potash and a small admixture of supercritical gases CO, SO, HF B,Os H2S H20 etc. Niggli 33 has emphasised that in the cooling of an intrusive magma containing such volatile materials " the process of consolidation cannot terminate with the formation of ordinary igneous rocks. The residual solutions continuously change in composition and the melt gradually assumes the character of a pegmatitic pneumatolytic,* and finally hydrothermal solution while progressively decreasing in quantity." So much silica and basic oxides have separated that water previously a minor solute has accumulated in the residues t o such an extent as to become the effective solvent.I n supercritical water silica is not only soluble but actually volatile.39 Thanks to this dissolved silica the vapour Muin crysta/l;.sation I I - - Concentration o f Pressure volatile materials in residual solution FIG. 2 pressure of the system in spite of the temperature (500") remains com- paratively low. Below this the solubility of silica falls off so rapidly that the vapour pressure of the system rises with falling temperature. Such a state of affairs has been called by Niggli 40 " retrograde boiling ". It has the very important consequence that the residual liquors of crystal- lisation of the magma are forced into every cranny which cooling is producing in the surrounding rocks.The minerals which then separate are a minute fraction of the original intrusion but by reason of accessibility characteristic appearance and efficient differentiation they are of the utmost economic 39 C . J. van Nieuwenburg and P. M. van Zon Rec. Traw. chim. 1935 54 129. 40 P. Niggli " Die leichtfluchtige Bestandtcile im Magma " Teubner Leipzig 1920. * Shand lo suggests that this term should now bo discarded having been used in different senses by Bunsen Brogger and Niggli. He prefers "high and low temperature hydrothermal ". 272 QUARTERLY REVIEWS and scientific importance. The geologist is primarily concerned with the products of the first and main crystallisations the mineralogist and chemist with the final one; the geochemist finds them all significant.The Products of the Second Parting.-The three cnvironments in which solidification takes place give rise to distinct types of mineral Above 1200"-fist crystallisation-refractory oxides and pyrites. 1200-500" main , ortho- meta- and alumino-silicates ; predominant cations Fe Mg Ca Na K. and anions similarly rejected. Below 600' final ? 9 cations too large or too small to be accepted earlier Shand distinguishes between the formation of sulphide and oxide ores. He considers that while there is ample evidence for the claim that the ores of such outsize cations as tin tantalum tungsten and beryllium are deposited in the final crystallisation the very low solubility of the sulphides in silicate melts shows that they must be eliminated as a liquid phase during the first crystallisation.On the other hand their solubility is not nil. There must be some remaining in solution to give rise to a second crop of sulphide ores a t the very end of the final crystallisation. The amount then deposited will make up in accessibility what it lacks in quantity. The First Crystallisation.-This accordingly affords two distinct types (a) Refractory oxides-magnetite chremite ilmenite spinel apatite and to a certain extent olivine [(FeMg)SiO,]. Owing to their density these sink and become accessible only in special circumstances. Ferruginous sands of this origin give such viscous slags that as yet they defy economic exploitation; but for this very reason chromite finds its major use as a material for furnace linings.( b ) The pyrites of the first crystallisation probably separates as a liquid phase and becomes trapped in the mass of more refractory crystals. The importance of this phase depends on the impurities which it carries par- ticularly copper and nickel. It is interesting that whilst gold occurs in pyrites from both the first and the final crystallisations platinum occurs in the former only. The Main Crystallisation.-The subsequent process of crystallisation may be described as consisting essentially of the adjustment of the composition of the magma to that required for the separation of the felspars. Accord- ingly in the main crystallisation the first minerals to separate are pyroxenes [(Mg,Ca)SiO,] followed by lime soda and potash felspars.Whilst these together with quartz make up approximately four-fifths of the Earth's crust other minerals are important and widespread (a) Those which arise when the parent magma is poor or rich in tho ions which make up pyroxenes and felspars. Thus an excess of aluminium results in its being accepted into the metasilicate crystals with the formation of augite (4 for MgSi). An even greater excess of aluminium may result in its separation in the first crystallisation as spinel. A deficit of aluminium is correspondingly compensated by the formation of alkali-augite. These are in a sense compensatory minerals. (b) Those which have remained in suspension until the residual melt GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 273 Average . . . . . Economic . . . . has a composition substantially different from that in which they were produced.Thus olivines may a t a lower temperature react further with silica. At the other end of the main crystallisation by the separation of the principal silicates the volatile constituents become more significant components of the magma and accordingly silicates with hydroxyl and fluoride groups are produced e.g. mica and tourmaline. The conditions for such components have been extensively studied by B~wen.~’ Such variants differ substantially from the minerals from which they are theoretically derived. They involve much more drastic modifications of the lattice than the quantitatively less extensive replacements of common ions by rarer ions which occur at the same time. These formed the theme of Goldschmidt’s 31 Lecture to the Chemical Society in 1037 and are described shortly below.sio,. j Al,O,. Fe,O,. CaO. NaCl. __-/ -_ -____ _____ - ____ 50-80 15-20 7 5 5 98-99 40-60 50 50-55 90-95 TABLE I1 Average percentage composition of rocks compared u d h the economic requirement of proJitable minemls It is evident from Table I1 that only in rare cases are common elements concentrated by igneous action to an extent sufficient to form economic sources of supply. Nevertheless the partial concentrations which do take place at this stage exercise a profound influence on the course of weathering. As will be seen later the products of weathering provide the main sources of our economic minerals. Quite apart from the direct utilisation of the products of igneous action an understanding of the principles by which the common elements are accumulated provides an insight into the fate of those elements which are present in amounts too small to yield character- istic minerals.It also enables a distinction to be made between those which are hard to procure because they are nowhere concentrated and those which are intrinsically in short supply a t the Earth’s surface. The distribution of those minor constituents is no longer the academic question it might seem for they are the source of the trace elements indispensable to fertile soils. 21 The Crystal Sieve.-Table I11 21 gives a list of the ions of the elements arranged according to their radii. The rarer elements fall into two categories. (a) Those whose ionic radius is very much greater or much less than that of Mg Fc Ca Al Na or K.Such elements cannot be accommodated in any of the main lattices and they accumulate in the final liquors often forming typical compounds with rarer anions which have been rejected for similar reasons. ( b ) Those whose ionic radius is approximately equal to that of one or other of the major The Fate of the Rare Elements in the Xecond 274 QUARTERLY REVIEWS TABLE I11 Radius A. 0.1-0.3 . . . 0.3-0.5 . . 0.5-0.7 . . . 0'7-0.9 . . . 0.9-1*1 . . . 1.1-1.4 . . . 1.4-1.7 . . . Ions. B3+ C4+ N5+ SS+ Be Si4+ Ge P5+ Mos+ V5+ W6+ A1 Ga Fo3+ Cr3+ V3+ Ti4+ Nb5+ Ta5+ Li Ni Co Fez+ Zn Sc In Zr 3 Sn Mn Na Ca Cd Y Gd-Lu Co Th U K Sr La Eu Rb TI Cs Ba R a constituents. Such rarer ions crystallise in the appropriate fraction of the common minerals as will now be described. "The formation of crystalline minerals involves the building up of space lattices of ions .. . depending on the size of the individual ions. Into such a lattice only particles which are of a sizc appropriate to the lattice spacings can enter. Therefore the crystals act as a kind of sieving mechanism allowing certain particles to enter excluding others of unsnitable size." Consequently minor constituents tend to enter the lattice most suited to their size and valency and to act as deputies for the major constituent as far as the amount available will allow. Thus in the crystallisation of the felspars cations are incorporated in order of increasing size Ca Na K. It is observed that barium is to be found mainly in the potash felspars and this is entirely in keeping with the relative ionic radii of the four elements Na/Ca = 0.98/1.06 = 0.925 K/Ba = 1.33/1.43 = 0.93 Goldschmidt 21 terms this " the crystal sieve ".The deputy is however seldom both the right size and of the right valency and Goldschmidt considers three possibilities (1) The deputy has the right valency but is slightly too large and will therefore enter the later-forming fractions of a'mineral. It may even be found in the outer layers of large crystals. Thus rubidium will be found preferentially in the latest crystals of a potash felspar. (2) The deputy is the right size but has a higher valency. Such a deputy e.g. Sc"' for Mg** will be held more firmly in the lattice and will accordingly be accepted preferentially to the major constituent. In fact magnesium ortho- and meta-silicate minerals arc richer than lime felspars in scandium.Deputies of the right size but of a lower valency will obviously be accommodated in the later fractions of a mineral. (3) The deputy has the right valency and the right size. Here two subsidiary possibilities exist. ( a ) The major constituent or the deputy has a slight tendency to exhibit a higher or lower valency than normal. ( b ) The valency of both elements is constant. In the latter case we have the very interesting situation of an element present to the extent of say O.Ol-O-OOl% in igneous rocks ubiquitous in distribution not by any means negligible in aggregate amount but everywhere swamped (and camouflaged) by the major partner from which no natural process of crystallisation solution, GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 2 75 or precipitation can separate it.Thus is explained the amazing fact that an element as abundant as hafnium remained unsuspected till 1923 in zirconium minerals. The swamping of gallium by aluminium is not quite so perfect. It exemplifies the possibility (a). The deputy has a slight tendency to exhibit a lower valency. Accordingly whilst the main bulk of gallium is swamped by aluminium in felspars and their products of weathering a very minor amount not so incorporated manifests the latent thiophilic character of gallium by entering into certain zinc ores. As is well known it was in such form that the element was first discovered. Even more interesting is the biological concentration referred to later. Similar considerations account for the occasional juxtaposition of such unexpected pairs as lead and strontium (cf.Goldschmidt s). Moreover it often happens that a rock contains two sets of crystals of the same mineral some large even several centimetres long others barely visible. Evidently the magma had cooled slowly forming the large crystals and then before the crystallisation was complete the rate of cooling had greatly increased perhaps by the eruption of the magma. Two things are certain the large crystals must have been formed earlier than the small and the outside layers of the large crystals must be later than the inside layers. Consequently we expect enrichment of preferred deputies in the inside of the large crystals and enrichment of deferred deputies in the mass of small crystals. The very interesting work of C. W. Bunn 41 on two-dimensional isomorphism suggests still another possibility for which as yet no mineralogical data are available.If an element E"' or E' deputises for the normal lattice tenant E** some adjustment of the anionic charges is demanded. If deputising is only to the extent of say 5% there may actually be vacant cationic positions e.g. Yo** replacing Ca" in CaF, or vacant anionic positions e.g. K' replacing Mg" in MgCl,. If however the increment in cationic charge is balanced by a corresponding increment in anionic charge there need be no limit to the extent of the replacement e.g. ferrous tungstate for scandium niobate. It must be emphasised that although such enrichments are often of the order of a hundred- to a thousand-fold over the average concentration of the element in the lithosphere they are nevertheless of no immediate importance.Their true significance is appreciated when the parent rock is weathered. Then these minimal concentrations supply the trace elements indispensable to agriculture. Moreover E. B. Sandell and S. S. Goldich,42 who have by an independent method confirmed many of Goldschmidt's data and conclusions point out that a deputy accepted in a lattice may a t ZL later stage in the crystallisation be re-absorbed into the liquid phase. The Final Crystallisation.-The products of the final crystallisation are called pegmatites. In the aggregate they amount to only a minute fraction of the total magma but they have a much greater economic and scientific value because they are more accessible and more differentiated than the products of the first and the main crystallisations.The reason for the greater differentiation of minerals in pegmatites is that the viscosity of the 4l Proc. Roy. Soc. 1933 A 141 567. 4 2 J . Geol. 1943 51 115 167. 276 QUARTERLY REVIEWS solution has decreased so much. Not only has water assumed the role of solvent but thiophilic elements which were too large or too small to be accommodated in any of the main lattices have now (by disappearance of the major constituents) become sufficiently abundant to form lattices suited to their own dimensions in combination with anions which were likewise rejected by the silicate lattice e.g. borate phosphate sulphide niobate and molybdate. The solution is moving and cooling so that the tempera- ture proper to the crystallisation of each metal corresponds to a spatial separation.In the ideal case there is a zonal distribution of minerals about the igneous rock. \ \ \ \ ,* regmatitic 1 / / / i I \ ’ (High-temperature hydrothermal) (Low-temperature hydrothermal) In the high-temperature hydrothermal zones the temperature is still high enough for the solvent to react with the rock walls through which it is passing ; in the low-temperature hydrothermal zone the country rock is merely an inert container of the solution. The high-temperature phase though it deposits directly less than 1 yo of the material of the original intrusion exerts a quite disproportionate effect on the country rock. In earlier stages of the intrusion the conduc- tivity of the rock is too poor and the viscosity of the magma is too great for the effect to be widespread and in the very last stage the solutions are too cool to effect drastic change ; but in the high-temperature phase corresponding to the pressure maximum deposition of minerals is accom- panied and even facilitated by the absorption of acid from the supercritical solution.SnF + 2H,O + SnO + 4HF 4HF + 2CctC0 -+ 2CaF2 + 2H20 + 2C0 In this way tin and tungsten are deposited as oxides The oxides of these metals are frequently accompanied by such accessory minerals as fluorite fluorapatite and tourmaline. At the very end of the crystallisation the deposition of mercury may in favourable cases be actually observed.43 It is present in solution as a complex sulphide influenced by the equilibrium Na,S + II,O + CO + Na,CO + H2S Mercuric sulphide is not soluble unless sodium sulphide is present; hence since ([Na,S][CO,])/( [Na,CO,][H,S]) is a constant the higher the concentration of carbon dioxide thc lower is that of sodium sulphide.Mercuric sulphide therefore tends to be deposited under impermeable shelves of rock where carbon dioxide accumulates. Just because it is a peripheral deposit it is the first to be lost on subsequent erosion. 43 G. Berg ref. 3 p. 266. GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 277 In addition to the thiophilic elements which are thus often far more accessible than their overall abundance in the lithosphere would suggest many very small or very large lithophilic cations are not deposited until the final crystallisation. They are often found combined in phosphates borates titanates and tantalates-acidic oxides likewise unacceptable in the silicate lattices.One of the most interesting of the rejected cations is uranium. The geochemistry of this element has recently been described by S. T0mkeieff,4~ who showed that it is deposited entirely during the final crystallisation. In the earlier phase it separates before tin as a dioxide along with ceria and thoria all of which form similar lattices of the fluorite type. Again at the end of the hydrothermal phase it appears as trioxide in complicated carbonaceous minerals. In addition to metallic minerals pegmatites contain quartz fluorspar and barytes. Barytes results from interaction between the ascending sul- phide solutions and descending aerated rain-water. The sulphuric acid formed is immobilised by the small amount of barium in the ascending solution. Consequently a vein will carry quartz all the time fluorspar at deeper levels and barytes towards the surface.It now remains to consider the results of weathering on the igneous rock. This is particularly important for whereas 95% of the Earth’s crust consists of igneous rock 90% of our mineral resources are derived from the remaining 5% which has undergone weathering. The Atmosphere and the Ocean The disintegration of igneous rocks is intimately bound up with the atmosphere which ‘‘ weathers ” them and with the ocean which ultimately receives all the soluble constituents and much of the sediments. The origin of these masses of gas and water is therefore of great interest to the geo- chemist and also to the biologist for the development of air and sea is a clue to the origin of life it~elf.4~> 46 Two fundamentally distinct views would account for the atmosphere.The simpler supposed that all the oxygen of our planet would be locked up in combination with reactive metals. The fact that iron was known to exist uncombined although in the siderosphere indicated that there was insufficient oxygen. The original atmosphere would consist of the less reactive gases including nitrogen water and carbon dioxide. According to this view photosynthesis has ever since been progressively replacing the carbon dioxide by an equal volume of oxygen co + H,O -+ (CH,O) + 0 Consequently allowing for the carbonate locked up in limestones and the oxygen withdrawn from the atmosphere to convert igneous ferrous into sedimentary ferric iron there should be one gram-molecule of oxygen in the atmosphere for every 12 grams of sedimentary organic carbon-coal oil shale living matter.It has however been objected that to suppose 4 6 R. Wildt Rev. Mod. Physics 1942 14 151. 4 4 Sci. Progress 1946 34 696. 46 G A. Riley Arner. Scientist 1944 32 132. 278 QUARTERLY REVIEWS all the present organic carbon once formed the primaeval atmosphere would be to postulate an atmosphere so rich in carbon dioxide that no known form of life present or extinct could have existed in it. J. H. J. Poole *' goes further and suggests that the great " excess of fossil carbon over atmospheric oxygen " indicates that the original oxygen must have been consumed by some such reaction as CH + 20 -+ CO + 2H,O A different origin for atmospheric oxygen was suggested by G. Tam- mann 49 who pointed out that a t the temperature of molten magma all terrestrial water would be vaporised and slightly dissociated 2H,O + ZH + 0 In the Earth's gravitational field hydrogen is not stable and consequently the equilibrium will be displaced more and more to the right by the escape of hydrogen.In fact there would be a race between total disappearance of all water and the retention of hydrogen as water resulting from the falling temperature-the dissociation of water below 2000" being negligible. Any theory of the atmosphere should be able to indicate how the three planets Venus Earth and Mars which obviously had a similar origin and are of approximately equal size nevertheless have respectively a dense carbon dioxide atmosphere a cloudy aqueous atmosphere and scarcely any atmosphere a t all.32 Confining our attention to the Earth it is obvious that the two extreme views sketched above are not mutually exclusive for whilst much of our oxygen may have been derived from dissociated water much may have come photosynthetically not from a huge original carbon dioxide atmo- sphere but from carbon dioxide continually fed into the atmosphere by volcanic discharge.Incidentally Tammann's view does not account for the very great rarity of neon if it did not escape from the Earth's gravita- tional field. If it did escape why should water a molecule slightly lighter than neon not likewise have escaped even without dissociation ? I n one of the early calculations of the Age of the Earth J. Joly 5O assumed the original Ocean to have been fresh water and estimated the annual contribution of salt by the rivers of the world.His value was widely accepted at the time but many factors are now recognised as com- plicating the original simple concept. E. J. ConwayYB1 in a careful analysis of the problem shows that not only has the rate of supply of sulphate and chloride to the sea not been constant but that these two anions and pos- sibly others participate in cyclic processes going down to the sea returning to the atmosphere and participating once more in the attack on igneous rock. Obviously if the chloride anion takes part in cyclic processes some process must withdraw its cation. This Conway considers is effected by the formation of glauconite potassium being withdrawn from solution and forming an insoluble potash iron silicate on the ocean floor. Conway con- 47 Proc. Roy. Dublin SOC.1941 222 345. 48 V. M. Goldschmidt Geologiska Peren. Stockholm 1934 400. 4g 2. physikal. Chem. 1924 110 17. 61 Proc. Roy. Irish Acad. 1942-43 B 48 119 161. PhiE. Mag. 1911 22 357. GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 279 siders that the Ocean is being continually replenished with both chloride and water by volcanic action. The primzeval ocean was not substantially less saline than it is to-day ; a conception which casts doubt on the fre- quently suggested idea that the salinity of body fluids corresponds to that of the Ocean a t the period when life began on the planet. The Third Major Parting of the Elements. Weathering Weathering is a physical and mechanical process as well as a chemical one ; the former predominantly in a dry climate the latter in a moist climate but seldom exclusively either.I n general terms weathering in so f a as it is chemical effects a selective extraction of the constituents of igneous rock leaving some as residue (TiO, Fe203 SiO, clay) and trans- porting others away in solution (Na' Ca" Fe.' Mg" ions). These elements which have passed into solution are then selectively removed by adsorp- tion ( e . g . K) or hydrolysis (e.g. Fe) or interaction with anions yielding insoluble precipitates (e.g. Ca) or by biological intervention ( e . g . Ca) until oiily sodium magnesium chloride and sulphate and mere traces of other ions remain.52 These constitute sea water. Further under favourably arid conditlions a fractional crystallisation may result in separate beds of common salt and of magnesium and occasionally even of potassium salts being formed.A convenient summary of Van't Hoff's classical work on the formation of the Stassfurt salt deposits has been given by A. W. Stewart.53 Since these various methods of abstracting ions from solution are not simultaneous and since the solution is moving (seawards) the various ions may be dcposited in different localities constituting important commercial deposits. The aqueous medium being far more mobile than magma weathering is often a very efficient type of economic segregation. Such in general terms is the fate of all igneous rock. Its stability is relative to its environmcnt and so with change of temperature pH and oxygen a.vailability quite different types of molecule are favoured. This is not to say that it matters little what type of rock is undergoing weathering.The ultimate fate will be the same but the rate of disintegration and the rela- tive importance of the physical and the chemical types of weathering will be very different. Therefore tho effect of weathering on the silicate containing the oxides which are common to all but the most extreme types of rock may be summarised Igneous K' Rock -.--+ ME" ci; solution I NaCl -1 CaCO I l i Hydrolysate 1 Oxidate I Carbonate Evaporate Fe(OH) I MnO, H,O I I Residue Sic) A1203 I 1 I + Adsorbate K' + 6 H. Wattenberg 2. anorg. Chem. 1938 236 339. 5 3 " Some Physico-Chemical Thomos " Longmans London 1922. 280 QUARTERLY REVIEWS Before discussing the mechanism of weathering in detail two cognate problems must be considered (i) the atomic characteristic which controls these separations and (ii) the fate of minor constituents of the rock.It was shown that in the first parting the type of bond which the element formed determined its siderophilic thiophilic or lithophilic character. I n the second parting the formation of igneous rocks the governing factor is ionic radius. Ionic potential expressed by the quof,ient ionic charge/ionic radius is a measure of the density of the charge a t the surface of an ion. If this is low (< 4) electrons are easily lost and the ion goes readily into solution and remains in solution. If the ionic potential is high (> 12) electrons are more readily shared. The element accordingly appears as an oxygenated anion e.g. SOa” PO4”’. Elements with intermediate ionic-potential values go into solution in reducing media and suffer reprecipitation in oxidising media.The conversion of an ion into one of higher valency results in an increase in the ionic potential because the numerator (valency) increases while the denominator (radius) decreases since the ion has fewer electrons. The fate of the minor constituents is controlled by these considerations also; but since we have to do with a quotient in sedimentary deposits elements may be associated which have neither the same valency nor ionic size but for which the quotient is similar e.g. scandium and thorium. It will be realised that in the crystallising magma such elements will show little tendency to foregather for a t that stage similarity in radius is required to bring about juxtaposition. When however similar valency and similar ionic radius combine to give similar ionic potential the elements concerned are separ- ated neither in the second nor in the third parting as e.g.AI/Ga or Zr/Hf which are mentioned above. The latter case is interesting for it was only the clamant demand of an important theoretical inquiry which led to the detection of hafnium ; and it was only by a physical separation of similar covalent derivatives that a complete purification was effected. It has been suggested that given suitable commercial or scientific stimulus a modifica- tion of the purification of bauxite would make available considerable quantities of gallium. The process of chemical weathering may be divided into three sections. (a) Weathering of the main igneous rock in so far as it concerns oxides of major importance silica and alumina ; iron and manganese oxides ; lime and magnesia; soda and potash.(b) Weathering as it affects the anions ; the nitrogen carbon iodine etc. cycles in Nature. ( c ) Weathering of the constituents of pegmatites. This is of the greatest mineralogical interest and economic importance but in view of the pre- ponderance of highly insoluble derivatives formed by heavy metals it is much more restricted in scope and might even be regarded as a kind of outdoor analytical chemistry. The study of the resulting secondary-ore deposits finds a key in the electrochemical series for many of the reactions In weathering both valency and size are significant.48 GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 281 involve alternate oxidation and reduction. It will suffice to consider the mechanism of the formation of “ zones of secondary enrichment ’,.The Main Oxides.-Aluminium and Xilicon Oxides.-As mentioned above the various silicates formed at high temperatures in the absence of oxygen fall ready victims to cold oxygenated water containing carbon dioxide. Dissolution of the rock proceeds even more rapidly if the rain has per- colated through peat and become charged with humic acids or attacked pyrites and carried away dilute sulphuric acid. Moreover it has long been recognised that volcanic hydrogen chloride and sulphur dioxide would locally and spasmodically supplement the action of these weaker acids. It is now evident from Conway’s calculations that a far more substantial supply of these ions participates in a natural cycle being returned by the sea to the atmosphere and so in rain attacking a fresh quota of rock and assisting its transport seawards.The geochemistry of the most abundant metal of the lithosphere has been exhaustively treated by G. E. Hut~hinson.~~ Vernadsky states that (‘ alumino-silicates form the skeleton of the biosphere ” natural zeolites providing a ‘‘ bureau de change ” where biologically active fluids can collect cations most suited to their natural reactions. This is not to say that aluminium compounds never participate per se in organic development the well-known mordanting action of lichens being due to the alumina they contain whilst in certain soils pink hydrangeas are turned blue. Never- theless compared with iron magnesium potassium and calcium aluminium plays we believe,’a quite insignificant r81e. There is however one very interesting possibility suggested by Hutchinson i.e.that aluminium may be an important factor in minimising silicosis for in mines and quarries where it is a severe hazard intentional admixture of aluminiferous dust has been found to reduce the incidence of this disease. The weathering of micas and felspars is in effect an exhaustive extraction in which all the soluble constituents are elutriated leaving a residue of alumina and silica (clay) with variable lesser amounts of iron oxide. It is however not merely a matter of removing the solublc from the insoluble. Interesting and important as are the reactions of the ions which go into solution the nature of the clay which results depends naturally on the severity of the conditions under which it has been formed and the nature of the clay is of paramount importance to the fertility of the soil.In temperate climates weathering takes place with rain-water containing sulphate and chloride ions carbon dioxide and humic acids the colloidal products of plant decay. It is believed that initially both silica and alumina go into solution. Sub- sequently the former develops into a negative the latter into a positive colloid. In this form a certain amount of migration is possibIe especially if the alumina is protected by humic acid. So long as either oxide is in great excess no precipitation occurs but eventually a gel forms by mutual precipitation and this gel contains varying amounts of iron. Such a gel has the very important property of adsorbing cations from solutions per- colating the soil whilst allowing anions to pass on.The essential condition 6 4 Quart. Rev. Biol. 1943 18 passim. 282 QUARTERLY REVIEWS is that the soil should possess a fine granular structure so that it may have the maximum poro’sity. If it should be drenched with water it becomes peptised and finally non-por~us.~ the main direct source of carbon dioxide for weathering is decaying humus which plays the triple r6le of supplying carbon dioxide being a protective colloid and acting as a reducing agent whereby much iron is removed. If there should be a deficit of electrolyte to precipitate the humus sol the whole products of weathering may escape downwards to the water table where an overpowering concentration of electrolyte will bring about immediate precipitation as a thin hard impervious layer bonding all the rubble that happens to be at that level a disastrous situation agriculturally.The significance of normal temperate weathering is that it does not separate silica from alumina but recombines them into a porous framework wherein a varioty of cations the essential nutrients of plant life are held in a form available for use. I Under tropical conditions the two oxides are extensively separated. It has been suggested that this is owing to the absence of humus and/or the presence of nitric acid ,(tropical thunder-storms) but neither of these would indicate why silica should be removed. ,It would seem more probable that under arid conditions the alkali leached from the rock would remain nearby in undiluted solution and so would remove the more acidic oxide i.e. silica leaving laterite.This mixture of alumina ferric oxide and titania has no base-exchange properties but provides the raw material for the manufacture of aluminium. The importance of trace elements in fertile soils is now well known6 These trace elements are ultimately derived from the igneous rocks from which by weathering the soils wcre produced.$ An early example was provided by the prevention of grass sickness in New Zealand by dressing the pasture with “ iron ore ”. Only when the local supply was exhausted and a neighbouring supply proved ineffective was the real prevention traced to cobalt. Again it is well known that V Ni Co Mo Ge Ga and As are concentrated in certain coals. This may well be the reason why some coals are superior for hydrogenation ; but it is certainly a serious factor in atmospheric pollution and is one lcause of the difficulty in obtaining carbon pure for spectro~copy.~~ The non-metals also are essential in minimal amounts in fertile soils.The indispmsability of boron has been well publiciscd whilst on the other hand vast areas occur where selenium in the soil makes agriculture impo~sible.~~ Iron and Manganese Oxides.-Owing to its abundance in the lithosphere and to its ready change of valency iron plays a very important part in weathering. These are heavy refractory minerals which sink to the bottom of the magma reservoir and are there shielded from weathering till very extensive erosion has occurred. From the resistant residue of such rocks titaniferous sands and chromite form the main sources of these two metals. The iron-nickel is of a different 6 6 V.M. Goldschmidt Ind. Eng. Chem. 1036 37 1100. 66 W. V. Searight et at. Soil Sci. 19iG 61 455. Iron occurs in two main forms (a) I n the first crystallisation with Ti Ni Cr and S. GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 283 character for it is produced by the segregation of that part of the sulphide zone (chalcosphere) which is soluble a t high temperature in the lithosphere. This iron-nickel sulphide separates. in the first crystallisation and contains too much sulphur to be a desirable ore of iron but frequently contains important concentrations of thiophilic and even siderophilic elements e.g. Sudbury platinum. Here ferrous iron is associated with magnesium calcium and manganese ferric iron with aluminium. The first effect of weathering is to remove lime and magnesia as hydrogen carbonates and to immobilise iron and manganese by oxidation.Subsequent action by reducing media e.g. peaty water results in reduction of the last two elements to the bivalent state again and their removal in solution as hydrogen carbonates or as organic complexes. In the absence of decaying organic matter iron and manganese do not long remain in solution for the predominant surface environment is an oxidising one. Ferric hydroxide is a positive colloid and therefore tends to adsorb anions in particular phosphate e.g. Lorraine ores whereas manganese hydroxide (a negative colloid) tends to adsorb cations e.g. Cu and Ba providing a valu- able natural mechanism for removing poisonous heavy metals from sea water. The precipitation of iron is not always a purely inorganic process but one in which bacteria may play a part feeding on the humic acid which has brought about the solution of the iron.Vernadsky 4 has paid special attention to the r6le of manganese in the biosphere where it resembles iron in being a transporter of oxygen particularly in making it available a t depth in the Ocean. In igneous rock manganese is always bivalent and as such it goes into solution. When the pH of the solution rises manganese is more readily oxidised. As the colloidal hydrated dioxide carries a charge opposite to that of hydrated ferric oxide the two oxides mutually precipitate and as “ wad ” are accumulating in enormous quan- tities on the Ocean floor. There it serves to oxidise decaying organic sediments. It is thus itself reduced and so the cycle begins once mom.Lime and Magnesia.-Although approximately equally abundant in the lithosphere lime is much more familiar and available than magnesia. At least three factors are responsible. First although they are indeed approxi- mately equally abundant in the layer of the earth’s crust for which analyses are available nevertheless since magnesium minerals separate early in the main crystallisation along with iron they sink. Consequently they are less exposed to weathering than the felspars which contain most of the calcium. Further both enter into biological processes calcium t o form bone and shell magnesium to form chlorophyll. Both elements are thus in steady circulation and demand in the biosphere but whereas magnesium is widely used in low concentration and in ephemeral form calciferous “disjecta membra ” having in life served to sustain and protect the organism are themselves durable after its death and accumulate as bone beds and coral islands.Ultimately so far from being dissipated by ( b ) In the main crystallisation along with Mg Ca Mn and Al. Of the two iron is the more readily oxidised. 284 QUARTERLY REVIEWS geological action they are consolidated forming limestones and even under pressure marble. Another reason for the apparent scarcity of magnesium is that much that is commonly called limestone and regarded as CaCO contains appre- ciable amounts of MgCO without actually reaching equimolarity as in dolomite. This does not mean that magnesium carbonate has partly gone to the make-up of the shell for the organism uses aragonite the form of calcium carbonate with which magnesium carbonate is not isomorphous.The replacement has taken place a t a later stage. Consolidation of coral to limestone results in the change aragonite to calcite. Thereafter if the limestone is percolated by solutions containing magnesium partial or even complete replacement will occur with the formation of dolomite or (more rarely) magnesite. Strontium carbonate is however capable of entering into the composition of shells. Its solubility product is greater than either of the others consequently magnesian limestone will probably be poor in strontium as it will be the first to be replaced. The solutions containing lime and magnesia as hydrogen carbonates produced in weathering are deprived of these elements by adsorption on clay. Another less important mechanism sometimes removes calcium hydrogen carbonate from solution.It has been known for many years that aquatic plants flourish on dissolved carbon dioxide. In so doing they disturb the equilibrium which maintains calcium in solution Thence via plants they pass into animals. removal of CO by plants excess of CO ’ CaCO + H20 + C 0 2 Ca(HCOd2 \ A particularly interesting example of this is described by F. Darling 57 as causing seasonal turbidity in a lochan on Lismore island. The most drastic separation of calcium from magnesium is purely physical in operation. Joseph Black who first clearly discriminated between these two elements emphasised the difference in solubility of the sulphates. An important consequence of this difference is that when land- locked seas are subjected to severe evaporation the first salt to separate is calcium sulphate.On the other hand magnesium salts are intensely soluble and therefore accumulate as an invisible asset in the Ocenn-one of the few major raw materials which are in no danger of being cornered. Nickel which has an ionic radius nearly equal to that of magnesium,s accompanies it in igneous rocks. Being a much weaker base it is separated on weathering magnesium passing into solution leaving in favourable cases nickel silicate as a residual concentrate as in the well-known ores of New Caledonia. Xoda and Potash.-These resemble the previous pair in being approxi- mately equally abundant but most unequally available. The sodium ion being the smaller is according to Fajans’s rule the more heavily hydrated in fact the effective radius of a hydrated sodium ion is greater than that “ A Naturalist in the Highlands ” Nelson London 1948.GIBSON TERRESTRIAL DISTRIBUTION O F THE ELEMENTS 285 of the non-hydrated potassium ion as may be seen from the respective ionic mobilities. Goldschmidt calculates that the radius of K+ is approxi- mately equal to that of [OH,]+. An important consequence of this is that clays are flocculated by K+ rather than by hydrated Naf ion because the water envelope round the sodium ions would tend to prevent the clay micelles from coalescing. Hence potassium ions are preferentially adsorbed on clays as was shown in Way’s classical experiments. Further D’Arcy Thompson 57a mentions that in a growing cell cations are attracted to the site of growth. Potassium is therefore a t a double advantage over sodium being more abundant in thc clay and therefore in the sap and being more mobile in tohe solution.E. J. Conway 58 has more- over illustrated the significance of ionic size in fermentation ; a yeast cell will develop if given glucose and potassium phosphate ; glucose combined with phosphate and potassium can penetrate the cell. Fermentation breaks down the glucose releasing phosphate which leavcs the cell to combine with more glucose and so on. If sodium phosphate is used the hydrated sodium ion is too large to enter the cell. Therefore the phosphate ion to preserve electrical neutrality must also stay out and fermentation cannot proceed. A consequence of these and other biological processes is that whereas the sodium potassium ratio in igneous rocks is approximately 1 1 the solution which reaches the sea contains about thirty times as much sodium as potassium.Although potassium salts are in general the more soluble it is actually they which are being steadily removed from the sea as mentioned above to form glauconite permitting the release of a corresponding amount of chloride to the weathering cycle. Only under exceptionally long continued arid conditions e.g. the Dead Sea can evaporation bring about the separation of solid sodium chloride. This has often happened in the past but only with advanced concentration can potassium and magnesium salts crystallise out. Even if they do they will form the uppermost layer of the deposit. If the arid conditions con- tinue it will be potassium salts which will be blown away. If rainy conditions ensue they will be washed away.Weathering of Non-metals.-Whilst all the major constituents of igneous rocks with the possible exception of aluminium play an essential part in the economy of living organisms it is really only of calcium that it may be said that biological concentration is characteristic and extensive indecd nearly exclusive. The situation with the important non-metals other than silicon is quite opposite. They are not lithophilic and since they have high ionic potentials they appear as acidic radicals in pegmatites or as vapours escape into the atmosphere from centres of igneous activity. Initially the acids to which they give rise supplement the action of carbon dioxide in the weathering of the lithosphere. Thus some anions e . g . chlorine find a life partner with which to accumulate in the Ocean.As 67a “ On Growth and Forin ” Cambridge 1942 p 459. Nature 1912 150 461. T 286 QUARTERLY REVIEWS such sodium chloride plays an extremely important r6le in biology lower- ing the freezing point of plant fluids providing a source of strong acid in the digestive tract and regulating the osmotic pressure in other media. Nevertheless in its many forms of usefulness it may be regarded as a pas- sive electrolyte returning to its dead level concentration of n 374 solution. No biological process concentrates any appreciable amount of chloride or concentrates it permanently. The purely physical process of evaporation to dryness as in the Dead Sea area is the only form of concentration] Nothing emphasises so forcibly the difference between our small-scale rapid laboratory technique and the slow cosmic processes than the differing geo- chemical behaviour of elements which we are used to regard as closely allied.The differing r6les of sodium and potassium have already been discussed. No less significant are the fate of fluorine chlorine and iodine. Thanks to the extreme insolubility of calcium fluoride and to its stability over a wide range of temperature fluoride escaping from magmatic solution is rapidly and permanently immobilised by reaction with limestone. For this reason fluorine is not in the true etymological sense a halide for none can exist as an ion in sea water./ Here is the classical case of biological concentration but of the mechanism whereby the thyroid gland and certain seaweeds preferentially extract iodine from solutions containing less than 10-50/, very little is known.The remaining halogen bromine is also susceptible to biological concentration as in the mollusc from which Tyrian purple was obtained. Nowadays it is being extracted much more prosaically from sea water by careful pH and redox control for industrial purposes. 59 Another acid which supplements carbon dioxide in weathering processes is sulphuric acid locally produced by the weathering of pyrites and the hydrolysis of the resulting ferric sulphate or more generally and in wide- spread effect by the dissolution of volcanic sulphur dioxide in rain.) Such sulphate in arid regions may (rarely) produce deposits of natural Glauber’s aalt or more commonly react with clay to produce alum shale or simply contribute to the sulphate content of the Ocean.In one important respect SO,” differs from SiO,”” it is susceptible to biological reduction. It is not too much to say that whereas the geologist looks on life as a process of reduction the biologist knows that all life involves oxidation. Thus in putrefaction sulphate supplies the bacteria with oxygen being itself reduced to sulphide. E. J. Conway 6o has recently shown that it thus returns to the atmosphere where it is again oxidised and attacks the rocks de novo. ‘ The contrast between metal and non-metal stands out most markedly in consideration of the carbon cycle./ It was mentioned earlier that there is reason to believe that in earlier stages of this planet’s history the atmo- sphere contained more carbon dioxide than now. There is in fact a pro- gressive replacement of silica in the lithosphere by carbon dioxide.This The position with iodine is the opposite. 6Q L. C. Stewart Ann. Repts. Smithsonian Inst. 1934 153. 6o Quart. Rev. Biol. 1943 18 337. GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 287 is however not the only process whereby carbon dioxide is abstracted from the atmosphere. Of far more immediate importance to us is the photosynthetic reduction of carbon dioxide resulting in the production of carbohydrates nCO2 + nH,O + (CH,O)n + nO2 Such carbohydrate then has two routes to comparative permanence. More familiarly via peat brown coal and coal it may under certain con- ditions form anthracite. Less familiarly carbon may be locked up in oil derived from the decomposition of fish protein. This may have a bearing on the frequent association of brine with oil-fields.I These two mechanisms for abstracting carbon dioxide from the atmo- sphere are most effectively controlled by the hydrogen carbonate content of the Ocean Ca(HCO,) f CaCO + H,O + CO The amount of calcium hydrogen carbonate held in solution is regulated by the partial pressure of carbon dioxide in the atmosphere. Should this decrease dissolved carbon dioxide will escape to replenish the atmospheric store and the equilibrium will move to the right. The resulting precipita- tion is effected not in the ordinary sense of a chemical precipitation but as increased biological activity the enhanced growth of coral-building organ- isms or to a lesser extent by aquatic plants as mentioned above. On the other hand the increased pressure of atmospheric carbon dioxide will be relieved by increased dissolution of the gas in the Ocean.Equilibrium will move to the left and an equivalent amount of coral will pass into solution. These and other well-known aspects of the carbon cycle in Nature are mmmarised in Table IV.61 Whilst the numerical data are admittedly tentative they illustrate the sort of balance sheet which will one day be available for all the elements.‘ The chloride cycle has already been considered in connexion with the origin of the Ocean and the nitrogen cycle is too well-known to require recapitulation but the phosphorus cycle is worth discussion. It was considered from the geological point of view by J. W. Gregory.62 The ultimate source of all our phosphate is the apatite of igneous rock. This mineral has been synthesised in the laboratory by the action of phos- phorus chloride vapour on red-hot lime.There seems little doubt that the natural mineral is similarly produced from phosphorus trichloride or tri- fluoride vapour and lime-silicates. It is thus widespread in occurrence but as the average phosphorus pentoxide content of rocks is only 0-25% and as the mineral is very resistant to weathering we can only admire the excellence of the natural device for ensuring a constant supply of an indis- pensable element however prodigal any generation might incline to be. The geochemistry of phosphorus is the story of its concentration from this wide diffusion into rich beds and veins. Appropriate to an element bi V-. M. Gofdschmidt Greoiogz’sh Foren. Stock&lin 1~x4 416. ii2 Trans. Geol.SOC. G’lasgolu 1917 16 115. 288 QUARTERLY REVIEWS of which it has been said " ohne Phosphor kein Gedenken " the mechanism involves not merely biological intervention but very intelligent intervention. TABLE IV - -1 L I 2kTMOSPHERE -r--- ' 0.4 g I A 4- - Breathing and decay 40,000 y 0" Photo- synthesis 2 1 Humiis 2 g . I .3 * 2 I F u rr I - - - * - - - - I I BIOSPHERE 1 + U .3 .3 I. - - - - - - -1 x 0 a a Coal 1 0 ?. 0 0 0 i 2 Bitumen Limestone-doloinit e I LITHOSPHERE 670-940 6. 6600 g. I I I li5 I I 1 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -1 Annual balance eheet per sq. cm. of the Earth's surface. The richest sources of phosphate lie in tropical or in semi-tropical zones. Their origin may be summarised thus Weathered apatite . . . phosphate ion in solution .. . marine organisms . . . larger organisms . . . fish. . . . Thus far the concentration is merely biological. Where small arid islands lie off a fertile coast myriads of fish-eating birds resort there to nest. The climatic conditions protect them from molestation a t the nesting season and also preserve the valuable highly soluble mixture of ammonium salts including phosphate of their excreta. The ammonium salts are eventually leached away and in a less arid climate the phosphate too is gradually washed down into the crevices of the underlying rock. On coral islands the product is rock phosphate which may contain up to 95% of calcium phosphate. On volcanic islands the phosphate is naturally immobilised by the bases which form the least soluble phosphates e.g. ferric and aluminium oxides.It wonld not be surprising if the mineral was also found to be substantially enriched in zirconium. Not all the phosphate is thus concentrated. Very important amounts react directly with the shelly detritus of the sea floor partly forming cal- cium phosphate and partly acting as a colloid in cementing together par- ticles of inert minerals on the sea floor. Phosphate of such origin is of the greatest historical interest for it was from beds of such origin in Cam- bridgeshire that phosphate for the earliest experiments at Rothamsted was obtained. It formed the basis for the English superphosphate (and indirectly sulphuric acid) industry. Phosphate in the animal body has two disfinct rales as a capital stlore GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 289 of which the mechanical structure of the body is constructed and as a transport agent carrying the fat and carbohydrate to the site of combus- tion and carrying off wastes as phosphoric esters.Recent experiments with radio-phosphorus 63 have shown that bone is continually being renewed so that there must be a steady flow of phosphorus through the system and a continuous enrichment of the surface of the soil with phosphate in excreta ; much of the richest arable land in Australia was infertile until this was realised there had been no small mammals so that the phosphate had to be supplied artificially. Phosphate so interestingly concentrated is as yet largely dissipated in sewage. Owing to the fact that it is a tribasic anion it is more extensively adsorbcd on the sludge and therefore not so completely lost as the equally essential potassium.The frequent occurrence of vanadium in the soot of oil burners indicates that it also is probably of marine origin. In fact certain marine organisms do effect the oxidation-reduction cycle in their blood through the changing valency of a vanadium complex. As will be seen from the table of abun- dance of the elements vanadium is by no means as scarce as might b= supposed from the rarity of its ores and in analysis it is probably often included with phosphate. In one important respect i t differs markedly from phosphorus; it does not form a volatile hydride. Consequently in reducing media such as decaying organic matter and particularly protein phosphorus may be removed as a gaseous hydride whilst vanadium combines with simultaneously-formed sulphide to give ores of the patronite type.Among the rare amphoteric elements Goldschmidt 21 55 has discovered a special type of physical concentration in which plant growth plays an important part. l ( a ) The roots of trees extract a variety of elements from a large bulk of rock; ( 6 ) the sap transports them in solution to the leaves; ( c ) the leaves fall and the resulting humus contains (i) Na Fe Ca Mg etc. (ii) As Ga Ge etc. ; ( d ) the common elements are leached out by rain whilst the rarer ones are retained in the humus possibly as complexes with tannin; and (e) the humus forms peat and finally coal. This explains why the ash of many coals is enriched many-fold even as much as a thousand-fold in the rarer elements particularly in gallium and germanium.Weathering of Pegmatites.-The weathering of the products of the crystallisatioii of the final liquors (pegmatites) cannot occupy such a con- spicuous part in the economy of Nature since these form merely 1% of the products of the solidification of the magma. Moreover the heavy metals which constitute such a significant part of pegmatites are char- acterised by the variety of insoluble salts which they form. Consequently if any of the heavy metals are weathered the material is seldom trans- ported far in solution before it encounters an anion capable of immobilising it once more. The phenomena are therefore similar to those encountered in weathering iron pyrites leading to secondary enrichment. In Fig. 3 the His findings may be summarised 63 0. von Hevesy Nature 1937 139 149.290 QUARTERLY REVIEWS dotted zone represents a vein of pyrites reaching the surface 88. Aerated rain-water will convert iron sulphide into soluble ferric sulphate and wash it downwards till at the level AB the oxygen supply is exhausted. Further oxidation of the iron sulphide is brought about by reduction of ferric to ferrous sulphate. At a slightly lower level A’B’ the ferric sulphate too will be exhausted. Now the aerated rain-water will extract not only iron but also copper silver and gold and will carry them down in solution so long as an excess of oxygen is present. Consequently the whole precious-metal content of the upper part of the vein will be reprecipitated as soon as the oxygen supply is exhausted L e . between level AB and A’B’. Further erosion by lowering the level of the hill surface to X‘S‘ will obviously lower the reduction level by a corresponding amount.In course of millions of years the surface’may fall hundreds of feet. The precious-metal content of FIG. 3 [After Berg.] all this pyrites will be concentrated in a shelf 5-10 feet thick. Once below the zone of secondary enrichment the precious-metal content is low ; it has not yet been concentrated. Such a deposit has often an iron cap a t the surface formed by ferric sulphate ascending in solution by capillarity and then undergoing hydrolysis. Quite a different manner of selective concentration of elements is dis- cussed by R. A. Mackay.64 Relatively impermeable barriers such as exist a t the contact of shale with limestone can so impound solutions as to cause some metals to be deposited within the structure and allow the ore-carrying fluid to pass on through the barrier.If the metal is in colloidal solution this is called dialysis ; if in true solution it is exosmosis removal of solvent by application of pressure at a semipermeable barrier. Mackay is concerned with the order of deposition of heavy-metal ores from aqueous e 4 Econ. Geol. 1946 41 13. GIBSON TERRESTRIAL DISTRIBUTION OF THE ELEMENTS 291 solution. Consequently in the series Hg Pb Zn Cu Sn the larger ions will be least impeded by an " entourage '' of water molecules and will penetrate furthest i.e. Hg will penetrate further than Sn. On the other hand where migration takes place in the vapour phase or in anhydrous melts the largest ions will penetrate least as was pointed out by Lapadu-Hargues l8 in his studies on metamorphic rocks.It is therefore not surprising that mineral sequences are observed which conform neither to the order of increasing nor to the order of diminishing ionic radius for it is quite conceivable that deposition may have occurred at an intermediate temperature so that the two effects are superimposed. W. Q . Kennedy a5 suggests that the original ore concentrations may have been formed in the granite layer. The r81e of igneous rock is a secondary one involving transport and reconcentration of ores previously localised within specific sectors of the crust. These and other schemes whereby the separation of metallic ions from solution and from each other may be envisaged are not mutually exclusive. It is for the chemist to call attention to such as are possibEe and for the geologist to decide which if any are exemplified in Nature. The larger an ion the less it will be hydrated. The author is greatly indebted to Dr. G. W. Tyrrell for aucess to his unique collection of reprints and to Dr. Walther Bierther for having called attention to the work of Kuhn and Rittmann. 65 Schweiz. Mineralogist 1948 28 1 .

ISSN:0009-2681    

DOI:10.1039/QR9490300263

出版商:RSC    

年代:1949

数据来源: RSC