SOLVENT EFFECTS IN BROMINATION OF NEOPENTYLBENZENE T-BUTYLBENZENE TOLUENE AND BENZENE IN AQUEOUS TRIFLUOROACETIC ACID

It is concluded that the step ArH + Br2 → ArHBr+ + Br- is rate controlling in the molecular bromination of neopentylbenzene, t-butylbenzene, and toluene in 78.3, 86.1, and 93.3% trifluoroacetic acid, and benzene in 93.3% trifluoroacetic acid. By contrast, reversal of this step is significant in the bromination of benzene in 78.3 and 86.1 % trifluoroacetic acid. In the former instances, the reaction is cleanly first order in bromine ([Br2]0 ∼ 10-3 M, [ArH] ∼ 10-1 M) and is practically unchanged by added sodium bromide (10-2 M). In the latter instances, the kinetic behavior in the absence of added excess bromide ion is complex (apparent contribution of kinetic terms higher than first order in bromine); added excess sodium bromide reduces the dependence on bromine to solely first order, and suppresses the rate without significantly reducing the concentration of molecular bromine. The value of pfMe reached a maximum, in 93.3% trifluoroacetic acid, of 42,400, which is by far the largest rate-enhancing effect of a p-methyl substituent yet encountered. The extreme selectivity of the bromination also was reflected in the failure to detect any ortho bromination of t-butylbenzene or meta bromination of any of the alkylbenzenes. Between 78.3 and 93.3% trifluoroacetic acid, Kp-t-bu/kp-me, increased from 1.06 to 1.40, kp-Neop/Kp-Me from 0.82 to 1.07, and ko-Neop/ko-Me from 0.13 to 0.21. The behavior of the relative rates is in contradiction to the Baker-Nathan hypothesis, and serves to illustrate once again the important role of solvation in governing the relative kinetic parameters of alkyl-substituted compounds. In the Discussion, the alkyl-substituent effects on ground- and transition-state solvation are considered explicitly and in some detail. The conclusion reached is that the Schubert-Sweeney hypothesis of steric hindrance to specific nucleophilic solvation near bulky alkyl substituents provides a reasonable rationalization of the results, but is not unique in this respect. © 1969, American Chemical Society. All rights reserved.