The Electronic Theory of Organic Chemistry

Abstract

The kinetics of the reaction of methyl and ethyl bromide with benzene and toluene have been determined. The alkyla-tions were found to proceed homogeneously in 1,2,4-trichlorobenzene with aluminum bromide as the catalyst. However, the rate could be followed only over the early stages of the reaction. In the later stages, the instantaneous rate decreased much more rapidly than any reasonable order would predict. The large decrease in rate is attributed to the removal of the aluminum bromide catalyst through formation of a stable-complex. By restricting the measurements to the early portion of the reaction, the alkylation reaction was observed to be third order, first order in each component: aluminum bromide, alkyl halide, aromatic. The rate expression for the reaction is d[HBr] /dl =-d[RBr]/dí = kz [RBr] [AlBr3]· [ArH]. The kinetic data demonstrate that toluene reacts faster than benzene-the relative rates are 5.10 in methylation and 2.89 in ethylation. Furthermore, ethylation of benzene proceeds at a rate approximately 57 times more rapid than methylation. The energies of activation for methylation and ethylation of benzene are 14.6 and 10.7 kcal. mole-1, respectively , with entropies of activation of-20.4 and-25.1 e.u. Isopropylation of benzene proved too fast to measure, even at very low concentrations. The relative rates of these reactions with benzene are: methylation (1), ethylation (57), isopropylation (>2500). It is proposed that methylation proceeds essentially by a displacement mechanism involving a nucleophilic attack by the aromatic nucleus on the polarized alkyl bromide-aluminum bromide addition compound. As the alkyl group becomes better able to accommodate a positive charge, i.e., methyl < ethyl < isopropyl