EFFECTOF METHYL IN TOLUENE-^^ EXCHANGE
Jan. 20, 1962 [CONTRIBUTION FRO35
THE
249
DEPARTMENT OF CHEXISTRY, UNIVERSITY O F CALIFORNIA, BERKELEY, CALIF.]
Acidity of Hydrocarbons. 11. Effect of Methyl Substituents in the Exchange Reaction of Toluene-a-d with Lithium Cyclohexylamide BY A. STREITWIESER, J R . , ~AND D. E. VAN SICKLE^ RECEIVED JUNE 21, 1961 The rates of exchange of several deuterated hydrocarbons with lithium cyclohexylarnide in cyclohexylamine a t 49.9' give the following relative rates: toluene-a-d, 1.00; m-xylene-a-d, 0.60; p-xylene-a-d, 0.29; ethylbenzene-a-d, 0.116; cumenea-d, 0.0079. Because of the electron-donating character of methyl substituents, these results indicate a highly carbanionic transition state and provide evidence that such exchange rates are valid measures of the relative acidities of hydrocarbons.
In the first paper of this ~ e r i e swe , ~ presented data on the kinetics of the proton exchange reaction of toluene-a-d with lithium cyclohexylamide in cyclohexylamine and showed that the e x p e r i m e n t a l techniques developed would give reproducible and meaningful rate constants. This system was studied with the hope that the relative rates of the exchange reaction for different deuterated hydrocarbons would reflect the relative acidities of these compounds. In this paper we test this presumption by an examination of the effect of methyl groups on the rate of exchange. Methyl groups are known to be electron-donating in general and aniondestabilizing in particular; for example, m- and p cresol are 0.81 and 0.54, respectively, as acidic as phenoL5 If our exchange rates are measures of a c i d i t y , we expect methyl substitution to result in lower rates; if this general pattern is followed the details may then contribute to our understanding of carbanions. Experimental Deuterated Hydrocarbons.-m-Xylene-ad and p-xylenea-d were prepared from the Grignard reagents of the corresponding halides and Dj0. In such preparations we have found that the use of a small amount of EtBr prior to the addition of the desired halide removes any residual water and gives product of high deuterium content. For example, the m-xylene-a-d prepared in this manner showed 0.96 3~ 0.12 methyl deuterium in the n.m.r. (determination by Mr. R . Lawler using a Varian A60 n.m.r. instrument). mMethylbenzyl chloride was prepared from m-xylene and sulfuryl chloride using the procedure of Kharasch and Brown.6 p-Methylbenzyl bromide was Eastman Kodak Co. white label recrystallized from pentane. 7%-Xylene-a-d had b. 138", ?2".'D 1.4941 and a C-D stretching band a t 4.66 p . p-Xylene-a-d had b. 137-135", n 2 6 , 51.4932, ~ and a C-D stretching band a t 4.53 p . Ethylbenzene-a-d was prepared by the reduction of a phenethyl chloride with lithium aluminum deutetide and lithium deuteride as described by Eliel,7.8 b. 133-1313', ?zZb1.4939, C-D stretching band a t 4.57 p with small shoulders a t 4.45 and 4.65 p . 2-Methoxy-2-phenylpropane was converted to cumylpotassium according to Brown, Mighton and Senkus.9 (1) This work was supported b y T h e United Slates Air Force through t h e Air Force Office of Scientific Research of the Air Research and Development Command. under contract S o . A F 49(638)105. Reproduction in whole or in part is permitted for any purpose of T h e United States Government. This paper was presented in part at t h e Sixteenth National Organic Symposium of t h e American Chemical Society, Seattle, Wash.. June, 1089. (2) Alfred P. Sloan Fellow. (3) Shell Development Co. Fellow, 1937-1938. (4) A. Streitaieser, Jr., D. E. Van Sickle and W. C . Langworthy, J . A m . Chem. Sac., 84, 214 (1962). ( 5 ) E. F. G. Herinston and W. Knyaston, Trans. F a r a d a y Sac., 53, 138 (1957). ( 6 ) M. S. Kharnsch and H. C. Brown, J. A m . Chem. Sac., 61, 2142 (1939). (7) E. L. Eliel, i b i d . , 71. 3970 (1949). ( 8 ) We are indebted to Mr. Gilbert Burtou for this preparation.
Treatment with DzO gave cumene-ad, b. 150-152', n 2 6 . 61.4882, ~ C-D stretching band a t 4.74 p with strong shoulders a t 4.60 and 4.86 p . Extensive deuteration was established in the infrared by the virtual absence of intense bands a t 7.8, 7.9,9.6 and 10.9 p present in cumene. The infrared spectra were taken as carbon tetrachloride solutions on a Baird double-beam infrared spectropbotometer. In all cases, known mixtures were made up with the corresponding undeuterated hydrocarbons; Beer's law was followed for the C-D stretching intensities. Exchange Rates.-The kinetics of the proton exchange reactions of the deuterated hydrocarbons with lithium cyclohexylamide in cyclohexylamine were conducted in the manner of procedure ( A ) of the preceding paper.4 SemiD - D,)generally gave excellent straight lines. log plots of 70( An example for ethylbenzene-a-d is given in Fig. 1. The experimental pseudo-first order-rate constants, koxp, derived from such plots are summarized in Table I.
TABLE I PROTON EXCHANGE RATESOF DEUTERATED HYDROCARBOXS WITH LITHIUM CYCLOHEXYLAMIDE IN CYCLOHEXYLAMINE AT 49.9O
Hydrocarbon
Formal Concn. of concn hydroof lithium carbon, cyclohexyl- 105 a, amide, c, kex., rnole/l. mole/l. sec. -1
m-Xylene-a-d 0.48 m-Xylene-a-d .48 p-Xylene-a-d .49 .48 p-Xylene-a-d Ethylbenzene-a-d .47 Ethylbenzene-a-d .48 Ethylbenzene-a-d .69 Ethylbenzene-a-d .71 Cumene-a-d .47
0.044 .056 .057 .042 .062 ,067 ,055 .049 ,061
6.7 7.4 3.35 3.2 1.21 1.29 1.26 1.20 0.080
106 k , sec. - 1
103 k i , I./m.sec.
5.2 4.0 5.7 4.0 2.57 1.79 2.46 1.90 1.17 0.80 1.18 .78 1.12 .79 1.07 .79 0.078 .053
Discussion The kinetics of each of the present exchanges is pseudo-first order. As in the comparable r e a c t i o n with toluene-a-d studied in the preceding paper, the rate constants obtained from plots of the experimental data, k,,,, must be corrected for the back reaction and for the increasing probability during reaction that a reacting system can enc o u n t e r a deuterium in solvent and end with no net reaction. These complications were treated in t h e same manner as with toluene-a-d by a correction factor (eq. 8 in ref. 4). The k's so derived are the true pseudo-first-order rate constants for the forward reaction and are summarized in Table I. The kinetics of the exchange of toluene-a-d with lithium cyclohexylamide in cyclohexylamine was i n t e r p r e t e d with a scheme in which the reactive c a t a l y s t , m o n o m e r i c lithium cyclohexylamide, is in equilibrium with relatively inert dimer, trimer and (9) W.G. Brown, C. J. RZighton and X. Senkus, J . Ovg. Chem.. 3 , 62 (1039).
250
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STREITWIESER, JR., AND
90( 80
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indicates further the carbanionic character of the transition state. The closer proximity of a methyl group a t the reacting center would be expected to exert a considerably greater electronic effect than at the p position. Yet, ethylbenzene-a-d is hardly less than half as reactive as p-xylene-a-d. Certainly, most, if not all, of this diminished reactivity must be associated with the increased effectiveness of the electron-donating power of the attached methyl group in ethylbenzene-a-d; we are left, then, with rather little rate-retardation to be ascribed to any steric hindrance to approach of the attacking catalyst a t a secondary rather than a primary hydrogen. We must conclude t h a t methyl groups TABLE I1
30
v
8 RD. R
20
C6HsCH2 m-CHsCaHdCH2 P-CHaCaH4CH2 Ce"CH( CH3) CsH5C(CH3)2 a Reference 4.
103 k2,
i./m.-sec.
Relative rate
6.i' 4.0 1.85 0.79 0,053
1.00 0.60 .29 ,116 ,0079
Rel. r a t e R H f KNDz 100 (ref. 11)
1.0
0.1 0.03
attached to the reactive center provide no effective steric hindrance in the reaction ; the relatively low reaction rate of cumene-a-d with two methyls at the reacting center must then also be largely electronic in origin. Similarly, the small magnitude I I 10 0 1000 2000 3C"S of any remaining effect means t h a t steric inhibition of resonance cannot be a matter of large imporMinutes. tance. Fig. 1.-Typical kinetic run with [C6HjCHDCH3]= 0.5 X, It is of value to compare these results with those c = 0.063 M . of Shatenshtein and Izraelevich" who examined higher aggregates. The kinetic development gives the rate of introduction of deuterium into several hydrocarbons with potassium deuterioamide in the actual concentration of monomeric lithium liquid ND3. Their relative rates for toluene, cyclohexylamide from the formal concentration, ethylbenzene and cumene are summarized in c, and allows the calculation of second-order rate Table 11; these values have rather large probable constants, kz (eq. 16 in ref. 4). We assume the errors but compare qualitatively with our results. correctness of this treatment and summarize the This comparison gains added significance from corresponding &values in Table I for the present Shatenshtein's investigation of comparable exset of compounds. The discussion that follows is change reactions of far more acidic hydrocarbons actually not dependent on this assumption since all (e.g., fluorene, triphenylmethane, etc.) in liquid of the present kinetics were done a t similar catalyst ND3 without added catalyst.12 The rates of such concentrations. Nevertheless, we find that the kz- exchanges were found to correlate well with the PK values so derived are highly reproducible. values assigned by McEwen13 to these hydrocarThe relative rates of the present set of hydrocarbons. bons are compared with toluene-a-d in Table 11. l y e conclude that the exchange rates with lithThe effect of methyl substituents is clearly to retard the reaction, as expected if negative charge is de- ium cyclohexylamide in cyclohexylamine provide veloped a t the benzylic position a t the transition meaningful measures of hydrocarbon acidities. state. A methyl group is more rate-retarding in the bTeare currently applying the method to a variety p-position than in the m-position, indicating sub- of additional compounds and hope to report on stantial transmission of negative charge in the ring. these studies in subsequent publications. The three points, H, m-methyl and p-methyl, (11) A. I. Shatenshtein and E. A. Izraelevich, Z h u i . F i z . K h i m . , 32, give an excellent Hammett up-line with p = 3.2, a 2711 (1958). (12) A. I. Shatenshtein, Doklady A k a d . X a u k S.S.S.R., 7 0 , 1029 rather large value substantially in excess of t h a t (1950); cf. also A. I. Shatenshtein and E. A. Yakovleva, Zhur. Obsch. given by phenol dissociations in water, 2.11,''and K h i m . , 28, 1713 (1958). (10) H H JaffE, Chein. Rem., 53, 191 (1953)
(13) W , K. LlcEwen, J . A m Chem. SOL.,68, 1124 (1936).
