Studies on BF₃-Complex Catalysts in the Alkylation Reaction of Benzene Homologues

Part 2. Competitive Alkylation of Benzene and Alkyl Benzenes

Abstract

Competitive experiments in the alkylation of benzene and its homologues with olefins and alcohols were carried out in the presence of boron trifluoride and its complex catalysts.
Considering the results obtained, the substance attacking the benzene ring directly was in the form of a carbonium ion (R⁺), an ion-pair (R⁺…Cat.^-) or a polarized intermediate (R^δ+…Cat.^δ-), according to the type of alkylating reagents and the nature of the catalysts.
Generally speaking, the alkylating reagents producing a stable carbonium ion such as tertiary butyl cation or the complex catalysts making a homogeneous reaction phase had the tendency to give a carbonium ion as the attacking substance. While, in the presence of the alkylating reagents (which produced an unstable carbonium ion and formed a stable ester with acid or the catalysts, giving a heterogeneous reaction phase), the attacking substance seemed to be in the form of an ion-pair or a polarized intermediate.
The form of the attacking substance exerted a great influence on the relative reactivity of benzene homologues.
In the case of the attack by a carbonium ion, the inductive effect of the alkyl groups on the benzene ring was predominant. Hence, toluene and alkylbenzenes were more reactive than benzene in the tertiary butylation or in the propylation, with the catalysts giving a homogeneous reaction phase.
In the case of the attack by an ion-pair, or a polarized intermediate having a less positive charge than a carbonium ion, the inductive effect was surpassed by the stereochemical and probability effects. Hence, benzene showed a higher reactivity than toluene and other alkyl benzenes in the ethylation and the propylation, with the catalysts giving a highly heterogeneous reaction phase. The reactivity of benzene decreased with the decrease of the heterogeneity of the reaction phase.