摘要:
J.C.S. Perkin I1 Aromatic Substitutions by [3H,]Methyl Decay Ions. A Comparative Study of the Gas- and Liquid-phase Attack on Benzene and Toluene By Fulvio Cacace and Pierluigi Giacomelio, Universitti di Roma, 00100 Rome, Italy A nuclear technique based on the spontaneous decay of tritiated precursors that allows the generation of free carbonium ions of exactly the same nature in different environments, has been exploited in a comparative study of aromatic alkylation by CT3+ ions, both in the gas phase at various pressures and in the liquid phase. The differences between the reactivity pattern of the methyl cation in the two environments can be essentially reduced to the much greater efficiency of collisional stabilization in the condensed phase, allowing a larger fraction of the excited arenium ions, from the highly exoenergetic (AH' <335 kJ mol-l) attack of the CT3+ ions on benzene and toluene, to survive dissociation and/or isomerization.The mechanism of the major competitive processes promoted by CT3+ attack on arenes, i.e. methylation, tritiodemethylation, and methyldemethylation are discussed, and the sub- strate and positional selectivity of the CT,+ ions from the decay of CT, is compared with that of more conventional aI kyI ating reagents . CHARGED reagents obtained from the spontaneous p decay of tritiated precursors have been usefully applied since 1964 to the study of ionic reactions in a variety of organic systems.l So far, the decay technique has been primarily exploited as a unique tool for introducing free, unsolv- ated cations of specified structure into any system of interest, to investigate their reactivity by the classical methods of physical organic chemistry, including in particular the isolation of the neutral end products, thus providing essential information on their chemical identity, isomeric composition, stereochemical con-figuration, etc.2 Little attention has been paid in these studies to another unique, if less conspicuous feature of the decay technique, arising from the nuclear nature of the process responsible for the formation of the daughter ions, which is intrinsically independent of the specific environment where the decay of the tritiated parent occurs.F. Cacace, Proceedings of the Conference on the Methods of Preparation and Storing %larked Molecules, Euratom, Bruxelles, 1964, p.719. Therefore, exactly the same ionic reagent is obtained from the decay, irrespective of the physical state of the system investigated, the pressure or concentration of the substrate(s), the presence of a solvent, etc. Such un- paralleled constancy of the nature of the reagent, over such a broad range of physical and chemical conditions, discloses the intriguing possibility of investigating the reactivity pattern of a given electrophile-substrate pair exclusively as a function of the variables which define the reaction environment. The present paper describes the first comparative study of the reactivity of a decay ion, CT3+ from the decay of [3H4]methane, towards benzene and toluene, carried out in the gas phase at different pressures, and in the liquid phase.The choice of the reaction was suggested in the first place by the potential value of this very simple aromatic substitution as a model for electrophilic alkylation in solution, where the unstable and extremely reactive 2 For a review, see F. Cacace, Adv. Phys. Org. Chem., 1970, 8, 79. methyl cation, in contrast with other more stable car- bonium ions, cannot be conveniently obtained in the free state. Furthermore, benzene and toluene are among the very few substrates whose reactivity towards another free cation, the HeT+ ion from the decay of molecular tritium, had been investigated in the liquid state.3 Finally, the interaction of CT,' decay ions with arenes has been studied by Nefedov and his co-workers in the gas phase at a single press~re.~ While their experimental conditions, in particular the high level of specific activity of the gaseous systems investigated, are far from ideal from a mechanistic stand- point since radiolytic processes cannot be excluded as a parasitic source of the tritiated products, the results of this early radiochemical study were regarded as a useful basis of comparison in an otherwise entirely unexplored area.EXPERIMENTAL Materials.-The preparation, purification, and isotopic analysis of the CT, sample used as a source of the decay ions has been described elsewhere.5~~ The activity distribution, measured by radio g.l.c., corresponded to 60.4 f0.5% CT,, 29.4% CHT,, 8.1% CH,T,, and 2.0% CH,T.The aromatic substrates, the solvents, and the products used as carriers or as standards in the g.1.c. analyses, were research grade chemicals from Merck, and were used without further purification. Growth of Decay Products.-The gaseous samples were prepared with conventional vacuum techniques, introducing CT, (0.6 mCi), diluted with CH, to a specific activity of 0.22 Ci mmol-l, into evacuated and carefully outgassed Pyrex vessels, containing a measured amount of the aromatic substrate(s), together with 0,, used as a radical scavenger. The vessels, closed with a mercury plug and sealed off, were stored in a thermostatic oven at 90 "C for 8-10 months.The liquid samples were prepared by dissolving tritiated methane into the carefully outgassed liquid substrate(s), at a typical specific activity of 0.3 mCi m1-I. The samples were stored in the dark, at 22 OC, for 8-10 months in Pyrex vessels entirely filled with the liquid and equipped with a capillary arm closed by a mercury plug. Analysis of Products.-Two distinct procedures were followed for the analysis of tritiated products from the gaseous systems. Direct injection into the gas chromato- graph of measured portions of the homogeneous gas samples was used essentially for estimating the absolute yields. In most cases, the sample was frozen to -196 "C, n-pentane was added, and the vessel content was thoroughly equilibrated by repeated freeze-thaw cycles.After removal of the un- decayed CT,, by outgassing and washing with excess of CH,, measured portions of the n-pentane solution were subjected to g.1.c. A similar procedure was adopted for the liquid systems, except that no solvents were required in this case. A Hewlett-Packard model 7620 gas chromatograph was used, equipped with a hot-wire detector and a 12 m Bentone 38 (6% w/w)-DC 220 silicone oil (20% w/w) on Chromosorb W column, operated at 140 "C. The activity of the effluents was monitored with a 10 ml internal-flow proportional counter, heated at 150 "C. The tritiated products were F. Cacace and S. Caronna, J.C.S. Pevkin 11,1972, 1604. V. D. Nefedov, E. N. Sinotova, G. P. Akulov, and M.V. Korsakov, Zhur. Org. Khim., 1970, 6, 1214. identified from their retention volumes, compared with those of authentic samples under the same chromatographic conditions. Degradation of Tritiated Toluene.-The tritium content in the methyl group of toluene was determined from the decrease of the molar activity caused by oxidation of C,H, to benzoic acid. This procedure required rigorous purific- ation of the arene, achieved, after addition of pure toluene as a carrier, by preparative g.l.c., using a 2 cm i.d., 3 m long Igepal CO 880 column at 85 "C, repeating the purification step until the molar activity reached a constant value. Purified toluene was converted into methyl benzoate by oxidation to sodium benzoate with alkaline permanganate at 130 "C in a sealed tube, followed by methylation with (CH,),SO, in alkaline solution.Methyl benzoate was isolated and purified by preparative g.l.c., using a 4 m Igepal CO 880 column at 85 "C. The activity of both purified toluene and methyl benzoate were measured with a Tri-Carb scintillation spectrometer from Packard Instrument co. RESULTS The relative yields of the tritiated products from the attack of the CT,+ decay ions on benzene and toluene, and the isomeric composition of the xylenes formed are given in Table 1 for the gaseous systems, and in Table 2 for the liquid ones. The absolute yields, defined by the ratio of the activity of the products to the total activity of the CT,' ions formed within the system can be calculated from the initial activity and the isotopic composition of tritiated methane, the decay rate of tritium, the abundance (82%) of the methyl ions among the decay fragments, and the absolute counting efficiency of the detector employed.These admittedly crude calculations show that the identi- fied products account for ca. 50 & 15% of the activity con- tained in the decay ions, without significant differences among the various gaseous and liquid systems investigated. Furthermore, it has been observed that the total activity of the crude liquid arenes, after removal of the undecayed CT, and of other uncondensable gases, matches almost exactly the combined activity of the products identified by g.l.c., excluding the presence of other significant labelled pro- ducts (e.g.dimers, polyphenyls, etc.), in the liquid, and sug-gesting that the activity balance must, instead, be provided by some gaseous species, most likely HT or partially tritiated methanes which are removed from the liquid, together with CT,, by the outgassing procedure. Finally, oxidation to benzoic acid of different samples of tritiated toluene from the gas-phase attack of CT,+ to C,H, showed consistently, if rather unexpectedly, that the tritium atoms were almost exclusively (ca. 97%) contained in the methyl group, with only a minor fraction in the five ring positions. DISCUSSION Decay versus Radiolysis as tl Source of Tritiated Pro- ducts.-The experimental conditions chosen for the present study, in particular the low level of specific activity, ensure that the formation of the labelled pro- ducts must be traced to the reactions of the tritiated F. Cacace and M.Schuller, J. Labelled Compounds, 1975, 11,313. F. Cacace, G. Ciranni, and M. Schuller, J. Amer. Chem. Soc., 1975,97, 4747. J.C.S. Perkin I1 decay ions, without any appreciable contribution from major decay ions: CH3+ 82.0%,CH2+4.9%,CH+4.0%, the radiolytic processes promoted by the p particles of C+ 4.9y0,H+ 2.4%. These results are consistent with tritium. In fact, even in the most critical cases, i.e. the conclusions of theoretical studies on the molecular TABLE1 Tritiated products from the attack of CT3+decay ions on gaseous arenes Relative yields of products System composition Composition of xylenesy)r--h-7 r (Torr) Ethyl-7 r-(%) C,H, C,H, 36 0, 1.2 Benzene Toluene 55.7 benzene Xylenes 44.3 350 2.4 2.2 39.6 0.7 57.5 36 36 2.4 19.1 53.7 0.2 26.9 120 12 2.4 33.2 61.5 0.2 5.1 3.6 36 2.4 4.8 42.9 0.4 51.9 a All systems contained ca.0.2 Torr CH,, and a tracer concentration of CT,. , ApparentC ortho nzeta para kTlkB 19.5 68.5 12.0 26.1 56.4 17.5 22.0 64.0 14.0 0.80 21.9 63.8 14.3 0.89 21.8 64.4 13.8 0.80 Ratio of the activity of each product to the corn- bined activity of all products identified. For the absolute yields, see the text. Each value is the average of several determinations, with a standard deviation of ca. 5%. Apparent values, not corrected (see text).TABLE2 Tritiated products from the attack of CT,+ decay ions on liquid arenes Systemcomposition (mol%) r--h--7 C,H, C,H, r Benzene Relative yields of products (yo) A Ethyl-Un-Toluene benzene knownb 7 Xylenes Xylene composition (%) Ar -l ortho nzeta para kT/kB 100 13.9 4.2 2.7 79.3 40.4 27.1 32.5 100 13.8 4.5 2.7 79.0 39.9 26.9 33.2 50 50 8.0 36.0 1.5 1.5 53.0 39.1 27.0 33.9 2.19 10 90 2.0 18.4 2.8 2.1 74.7 39.9 26.8 33.3 2.35 90 10 14.1 69.6 0.3 0.6 15.9 38.4 27.0 34.6 2.39 a Reproducibility of the data is illustrated by comparison of the first two lines. The elution volume of this product is close, if slightly lower, than that of ethylbenzene. Apparent values, uncorrected (see text).in gaseous arenes at low pressure, and under the un- realistically unfavourable assumptions that (i) CT, undergoes radiolysis with the enormous G(-bf) value of 100 and, (ii) the activity of the radiolysed CT, molecules is quantitatively incorporated into aromatic end-products, the extremely low molar fraction of CT, makes radiolysis entirely negligible, as a source of tritiated products, in comparison with the reactions of the decay ions ; the relative efficiency of the two labeling channels is calculated as below 1 : lo3. This view is supported by the observation that the combined activity of the tritiated products accounts for a reasonable 50% of the decay ion activity, both in the gaseous and in the liquid systems, despite the substan- tially different (30 : 1) level of specific activity that would undoubtedly cause a measurable change of the yields of tritiated products if radiolytic processes appreciably contributed to their formation.Incidentally, the absence of radiolytic processes as a parasitic source of tritiated products can help explain the considerable discrepancies between the present results and those obtained by Nefedov and his co-workers for gaseous arenes at a single pre~sure,~ at specific activity levels exceeding ours by a factor of at least lo2. The Reagent.-The decay-induced fragmentation pattern of CH,T has been studied at low pressure with a specially built mass spectrometer by Snell and Pleason- ton,' who reported the following abundances for the A.H. Snell and F. Pleasonton, J. Phys. Chem., 1958, 62, 1377. excitation caused by the p decay of a constituent tritium atom,2*sqg showing that most of the primary daughter ions, (CH,"He)+, are formed in their electronic ground state and dissociate immediately into 3He and a methyl cation, while a 20% fraction receives large excitation energies that cause further extensive fragmentation. Given the peculiar distribution of excitation energy, either very large or negligible, the fragmentation pattern measured at Torr can be reasonably assumed to describe the decay of tritiated methane in gases at much higher pressures, or even in liquid systems, since col- lisional deactivation can hardly affect the very fast decomposition of those daughter ions that are formed in very highly (up to 60 eV) excited states.For the same reason, isotopic effects can be reasonably disregarded in the dissociation of the decay ions, and the fragmentation pattern established for CH,T can be reasonably extended to CT,. From these assumptions, the activity distribu- tion of the decay ions can be calculated as follows : CT,+ 82%, other reactive species, i.e. CT,+, CT,, CT+, T+, T, HeT+, <12y0, molecular tritium 6%. These figures, and the experimental yields, show that CT,+ must rep- resent the major source of the tritiated products identi- fied, with only minor contributions from other reactive decay fragments. While the mass spectrometric and the theoretical studies quoted above indicate that CT3+ is formed in its electronic ground state, they provide no evidence con- 13 M.Cantwell, Phys. Rev., 1956, 101, 1747. S. Ikuta, K. Okuno, K. Yoshihara, and T. Shiokawa, Radiochem. Radioanalyt. Letters, 1975, 23, 213. 1978 655 cerning the level of vibrational excitation associated with the relaxation from the tetrahedral structure typical of the CT, parent to the most stable trigonal geometry of the cation following the sudden nuclear event. In T 3He T particular, no information is available concerning the fraction of energy carried away by the receding 3He atom, i.e, the extent to which the relaxation of the organic system can be regarded as an adiabatic process. vibrational energy of the decay-formed CT,+ that could enhance (by up to 134 kJ moll1) the exothermicity of processes (1)-(7), and, more significantly, make proton- ation of arenes by CT3+ an energetically allowed process.Owing to the exoenergetic character of processes (6) and (7), a preliminary question arises, concerning the charge state of the electrophile whose attack on the arenes actually leads to the alkylated products. It is conceivable, in fact, that CT,+ ions undergo charge exchange with the arenes, yielding CT, radicals, whose subsequent attack to the substrate(s) gives the observed alkylated products. Any significant contribution of this reaction pathway is however ruled out by the experi- mental observation that in the gaseous and in the liquid TABLE3 relevant reactions of CT,+ with arenesEnergetics of several Process Nevertheless, an upper limit for the vibrational excit- ation of the decay CT3+ ion can be deduced from theoreti- cal results on the deformation energy of the planar methyl cation.Indeed, in the (extreme) hypothesis that CT,+ is formed in a perfectly tetrahedral structure via an adia- batic process, with no energy released to 3He, the calcul- ation of Burdon et aZ.,1° supporting earlier data of Wil- liams et aZ.,ll set an upper limit of ca. 134 kJ mol-l to the vibrational excitation energy of the decay ion. Energetics of CT,+ Attack to A renes.-Several exo-energetic reaction channels are available to the CT,+ ion in its attack on the aromatic substrates, as illustrated in Table 3, thus providing approximate AHo values for several relevant processes.The enthalpy changes have been calculated from the AHf" value of gaseous CH3+, which is appropriate for the gaseous systems and rep- resents a rough approximation for the liquid-phase reactions, even considering the low degree of solvation of the electrophile. Furthermore, no allowance is made for the excess of lo J. Burdon, D. W. Davies, and G. del Conde, J.C.S.Perkin 11,1976, 1193. 11 J. E. Williams, jun., V. Buss, L. C. Allen, P. v. R. Schlever, W. A. Lathan, W. J. Hehre, and J. A. Pople, J. Amer. Chem. SOC.. 1971, 93, 6867. AH"/kJ mol-l systems as well the yields of aromatic end products are independent of [C,H,], which contrasts with the well documented ability of the methyl radicals to abstract a hydrogen atom from the side chain of toluene, giving quantitative yields of methane and benzyl radicals. This reaction, much exploited in the ' toluene carrier ' method to trap methyl radicals,15 would in fact give higher yields of CT,H, and correspondingly lower yields of alkylated aromatic products when the molar fraction of C,H, is increased in the system.The experimental evidence fails entirely to support such a trend, and provides compelling, if indirect, evidence for the role of the CT,+ ion as an alkylating reagent. AZkylation in the Liquid Phase.-In the discussion of l2 Calculated from the AHf"value for CH,+, 1 079 kJ mol-l, given by (u) F. P. Lossing, ' Mass Spectrometry.' ed.C. A, McDowell, McGraw-Hill, New York, 1963; (a) M. Szwarc, Chem. Rev., 1950, 47, 75, taking into account the AHfovalue of gaseoustoluene from (c) D. R. Stull, E. F. Westrum, jun., and G. C. Sinke, ' The Chemical Thermodynamics of Organic Compounds,' Wiley New York, 1969, and taking the PA of the ips0 positions of toluene and P-xylene equal to that of benzene, given by (d) R. Yamdagni and P. Kebarle, J. Amer. Chem. Soc., 1976, 98, 1320. 13 From the ionization potential of benzene and toluene, cj! K. Watanabe, J. Chem.Phys., 1957,26, 542. l4 From the Hf" of CH,, 384 f4 k J mol-l; W. A. Chupka and C. Lifshitz, J. Chern. Phys., 1969, 48, 1109. l5 J. A. Kerr, Chem. Rev., 1966, 66, 465, cf. also ref. 12b. 656 CT,+ attack on arenes it is convenient to consider first theY liquid-phase reaction, whose features appear to fit into the familiar picture of conventional solution-chemistry alkylation, displaying, in particular, consider- able similarity with the substitution by highly reactive and poorly solvated electrophiles.The evidence from the present study is in fact consis- tent with a straightforward reaction sequence, initiated by the highly exothermic attack of the alkyl cation to the substrate, yielding excited arenium ions, e.g. reaction (2), with CT3+ for CH,+, whose fragmentation and/or isomerization undergo effective competition by the fast collisional deactivation typical of the liquid environment. I (2) J.C.S. Perkin I1 next section, Despite the isomerization processes, a limited, yet well measurable preference if the CT3+ cation for ortho-para attack is apparent from the isomeric composition of the xylenes, characterized by a para : 1/2 meta ratio of ca.2.5. Alkylation in the Gas-$hase.--In contrast with the efficiency of collisional deactivation in the liquid, the gas- phase picture is dominated by extensive isomerization and fragmentation processes affecting the excited arenium ions from CT,+ attack. Such a trend is well documented by the occurrence of tritiodeprotonation of benzene, and of methyldemethylation of toluene. Furthermore, the relative reactivity of the arenes referred to the alkylation channel is inverted with respect to the liquid phase, as shown by an apparent kT : kBratio below unity (0.8--O.9), and the composition of the xylenes is also shifted in favour of the meta-isomer, which becomes by far the most abundant.All such differences can be reasonably traced to the increased rate of fragmentation and isomerization Fragments (X2, CX4, *B (,,)(1) -BH** M* cT3 Deprotonation of the stabilized arenium ions (1)) Or of their isomerlzatlon Products, e.g. (2))by any base con- tajned in the system, including the substrate(s) Provides a direct route to the alkYlated Products, whose yields amount to ca. SO%, the balance being Provided by gaseous fragmentation Products~ most likely HT Or variously tritiated methanes, formed either directly via Processes such as (3)-(5)9 Or from the dissociation (10) of excited arenium ions.The efficiency of collisional stabilization in the liquid is apparent from the relatively low Yields of Products such as C,H~Tfrom benzene and C6H5CT3 from whose f Ormation requires extensive iSOt OpiC scrambling and fragmentation, and which are considerably more abundant in the gas phase. The Occurrence Of fragmentation and iSOmeriZatiOn undoubtedly COmpliCateS the evaluation Of the substrate and positional selectivity of the electrophile. However, restricting the comparison to the alkylation channel, the present indicate that CT3f, quite un-selective, is nevertheless capable of discriminating benzene and ('T: kH 2)* The positional selectivity deduced from the isomeric com- position of the xylenes must be regarded as a lower limit, owing to the occurrence of isomerization of a fraction of the initially formed ortho- and para-substituted inter- mediates to the meta-substituted arenium ion (2),that is thermodynamically the most stable, as discussed in the 16 A number of other mechanisms are conceivable, for instance ring expansion of (3) to a dihydrotropylium structure which would lead to H-T scrambling, cf.(a) n. H. Williams and G. Hvistcndahl, J. Amev. Chem. Soc., 1974; 96, 6753, or the attack of C,H,CT,+ ions from (5) to C,H,, leading to formation of C,H,T by a process analogous to that reported by (b) J. Shen, K. C. Dunbar, and G. A. Olah, zbid.,p. 6228. etc; X= H, TI (10) processes occurring in the gas, where efficiency of the collisional stabilization is necessarily reduced with respect to the liquid environment.This is clearly demon- strated by the effects of the pressure on the products pattern. Thus, a tenfold decrease of the C,H, pressure (from 350 to 36 Torr) raises the C6H,CT3 yield from 40 to 56%, and the extent of meta-alkylation from 56 to 68%. No direct evidence is available concerning the specific nature of the processes responsible for the formation of C6H5T from benzene, and of C,H,CT, from toluene. The most direct route to C,H,T, i.e. the T+ transfer (8) from CT,+ to C,H, does not appear significant, even though it cannot be ruled out on purely energetic grounds, since intervention of vibrationally excited CT,+ ions could overcome the endothermicity of the process (8).How-ever, it must be considered that if T-Ftransfer from CT,S to benzene is postulated as a significant source of C,H5T, the analogous and energetically less unfavourable, Tf transfer to toluene should occur with comparable, if not greater efficiency, contrary to the experimental find- ing that little or no ring-tritiated toluene is formed. A more reasonable explanation 16 involves isotopic scrambl- ing of the hydrogen atoms of the methyl group with those of the ring, e.g. reaction (12) as inferred by Blint 17 from the i.c.r. study of CD~+attack on benzene, followed by triton transfer to another molecule of substrate [reaction (13)]. Formation of C6H5CT, from toluene is likely to represent one of the consequences of the extensive isomerization undergone in the gas phase, especially at low pressure, by the excited primary arenium ions, such as (1).Intramolecular H+, and possibly methyl group shifts, e.g. reaction (141, can yield, interalia, reactive species capable of alkylating, as well as protonating toluene [reaction (15'11. Other conceivable routes to \ IJ C6H,CT3Imay involve fast, reversible dealkylation of the @so-substituted arenium ion from the direct CT3+ attack l7 R. J. Blint, Ph.D. Thesis, California Institute of Technology, 1972, cf. J. L. Beauchamp in ' Interactions between Ions and Molecules,' ed. P. Ausloos, Plenum Press, New York, 1975, p. 441. 1978 657 to the ring carbon bearing the methyl group, or intra- not lead, in the gas as well in the liquid, to kinetically molecular CH3+ transfer from (6) to other substrate predominant ortho/para substitution.On the other molecules. In any case, degradation of the labelled hand, it is known from experimental measurements in r CH~ 1 toluene formed indicates that very little tritium activity is present at the ring positions, suggesting that transfer and migration of the entire methyl group must be fast in comparison with the isotopic scrambling processes, such as (l2),affecting the hydrogen atoms of the methyl group (6) and those of the ring. This result is consistent with the fact that activation energy for intramolecular methyl shifts, measured in acidic solution 18919 and calculated with the CNDO/2-FK method,20 amounts to only a fraction of the excitation energy of the primary arenium ions (l)*,and is also consistent with the results of a study on the gas-phase attack of the strong Brgnsted acid HeT+ on the xylenes, giving tritiated toluene via tritiodemethylation in yields comparable with those of the isomerized xylenes.21 Gas-phase Positional Selectivity and Stability of Isomeric Dimethylbenxenium Ions.-The predominance of meta-substitution in the gaseous systems is to be traced to extensive isomerization of the primary arenium ions, as shown, inter alia, by its pressure dependence.There is no a priori reason why CT,+ attack on toluene should * The time required by a 5.6 keV p particle to reach 100 fi from the decayed molecule is very short, < s.As no C-He bond exists within the primary decay ion [3He-CH3]+,24 dissoci- ation into a free methyl ion can be expected to occur in a time comparable with the period of a bond vibration, roughly s. 18 D. M. Brouwer, E. L. Mackor, and C. MacLean, Rec. Trav. him., 1965, 84, 1564. (4)* C6H 5T 1 ___) (14) CT3 1ex c exc solution 18919 and from theoretical calculations 2os22 that the meta-substituted species (2) is the most stable among isomeric dimethylbenzenium ions. Consequently, the population of primary arenium ions, excited by the exo- thermicity of (2) well above activation energy for intra- molecular proton and methyl shifts, tends towards an equilibrium isomeric composition characterized by a predominance of (2),the extent of conversion depending on the time available for intramolecular isomerization before a deactivating collision takes place.This accounts for the prevalent ortho/para orientation in liquid-phase alkylation, where efficient collisional deactivation helps to preserve, at least in part, the initial, kinetically con- trolled composition of the primary arenium ions, and for the observed pressure dependence of the isomeric corn-, position of xylenes. The present results provide experi- mental support to the theoretical calculations concerning the stability of isomeric, gaseous dimethylbenzenium ions, whose relative energy increases in the order (2) < (1) < (6), with a separation of ca. 84 kJ mol-l between adjacent pairs of isomers according to Heidrich and his co-w~rkers.~~~~~ Conclusions.-The nature of the charged reagent formed by p decay in the site formerly occupied by a CT, molecule in a very short time on the chemical reactivity scale * is essentially the same in the gaseous and in the 39 D.M. Brouwer, Rec. Tvav. ckinz., 1968, 87, 210. 20 D. Heidrich, M. Grimmer, and B. Sommer, Tetrahedron, 1976, 32, 2027. 21 G. Perez, Radiochew. Radioanalyt. Letters, 1975, 20, 383. 22 J. L. Devlin, 111, J. F. Wolf, and R. J. Hehre, J. Amer. Chem. SOC., 1976, 98, 1990. 23 D. Heidrich, personal communication. 24 S. Wexler and D. C. Hess, J. Phys. Chew., 1958, 62, 1382. liquid phase. In both environments the CT3+ cation is absolutely free, lacking a counterion and having its positive charge neutralized by a far removed electron, Moreover, although one could argue that the only un- solvated carbonium ions are to be found in the dilute gas state, the CT3+ion from the decay is as unsolvated as J.C.S. Perkin 11 measured in the solution chemistry experiments where intervention of poorly solvated cationic reagents appears more likely, owing to the specific way of generating the reagent, e.g.diazotization in aprotic media, and/orto the greater stability of the carbonium ion involved, e.g. Pri+, included for comparison purposes in Table 4. On TABLE 4 Substrate and positional selectivity of typical methylating reagents Isomeric composition of products (yo)Reagent r A-h/kB ovtho meta para CH,Br + GaBr, Arenes 7 5 63 11 25 CH,Br + Al,Br, Arenes 3.6 53 17 30 CH,I + Al,Rr, Arenes 4.6 48 12 40 CH,NH, + NOPF, Nitromethane } 1.8-2.0 42 21 37 CH,NH, + NOPF, Acetonitrile 41 21 38 RNH, + HC1 + RONOa Arenes 1.8 40 27 33 N-Nitroso-N-alkylaminefl Arenes 1.8 40 26 34 CT,+ 2.2-2.4 40 27 33 a R = Pri, heterogeneous reaction. 7-Reference 26 27 27 28 28 29 29 Present work conceivable for a charged species in a condensed phase.In fact, owing to the very high collision frequency with the molecules of the liquid substrate, and to the proven ability of the methyl cation to react at a rate approaching collision frequency even with weak nucleophiles, its attack on benzene and toluene is likely to occur within a much shorter time than required for the formation of an organized solvation sphere that requires, inter aZia, rotational relaxation of dipolar molecules in the field of the cation.The unparalleled constancy of the nature of the reagent over an extended range of physical conditions affords the unusual possibility of evaluating its reactivity pattern in different media exclusively as a function of the reaction environment. For the specific case investig- ated, in order to rationalize the observed differences between the liquid and the gaseous systems, no other factors are necessary, except for the greatly enhanced efficiency of collisional stabilization that allows a much larger fraction of the excited arenium ions from (2) to survive isomerization and/or fragmentation.To our knowledge, in the search to substantiate the long postulated 25 role of carbonium ions in Friedel-Crafts alkylation, the present study provides the first data con- cerning a reagent whose +1 net charge, and lack of a counterion, are positively established in the liquid phase. Consequently, a comparison of CT3+ reactivity with that of a few typical representatives of the vast collection of polarized molecules, polarized complexes, ion-pairs, ' incipient ' or ' hot ' carbonium ions (in the solution chemistry sense) employed as reagents in conventional aromatic alkylation is of interest, and is concisely illustr- ated in Table 4. Significantly, the substrate and positional selectivities of the free CT3+cation appear extremely close to those 25 C. C.Price, Chem. Rev., 1941, 29, 37. 26 H. C. Brown and C. R. Smooth, J. Amer. Chem. Soc., 1956, 78, 6255. 27 H. C. Brown and H. Jungk, J. Amer. Chem. SOC.,1955, 77, 5584. the other hand, the reactivity of CT,* is appreciably different from that observed in methyfation by conven-tional Friedel-Crafts reagents, which involves, as shown by Brown and his co-~orkers,~~*~7 displacement by the arenes of the polarized complexes formed by the methyl halides with metallic salts. Finally, it should be noted that the gas-phase methyl- ation by CT3+ ions provides some interesting pieces of evidence concerning the preferred fragmentation pattern of those excited arenium ions that escape collisional stabilization in the pressure range investigated. Thus, in contrast with the unexpected efficiency of the tritiode- met h ylat ion and met h yldeme th ylat ion channels, no evidence could be obtained for the occurrence of the expected dissociation into benzyl ions, e.g. reaction (16), L. Jexc observed at much lower pressures.17 The benzyl ions formed should in fact react with the aromatic substrate, yielding, inter alia, diphenylmethane derivatives ;30 these have not been isolated among the products. The unexpectedly low efficiency of (16)can possibly be traced to its high activation energy which adds to the consider- able endothermicity of the process.16a We express our gratitude to Professor G. Stocklin for his generous help, and to M. Schiiller for technical assistance. F.C. acknowledges financial support from the C.N.R. [7/1670 Received, 21st September, 19771 ** G. A. Olah, N. A. Overchuk, and J. C. Lapierre, J. Amer. Chem. SOC.,1965, 87, 5785. 29 L. Friedman and A. T. Jurewicz, J. Amer. Chem. SOC.,1969,91, 1808. 30 Y. Yamamoto, S. Takamuku, and H. Sakurai, J. Phys.Chem., 1970, 74, 3325.
ISSN:1472-779X
DOI:10.1039/P29780000652
出版商:RSC
年代:1978
数据来源: RSC