Electrophilic aromatic substitution. Part 20. Protiodetritiation and protiodesilylation of tri- and tetra-phenylmethane. The curious effect of the triphenylmethyl substituent

Abstract

Rates of protiodetritiation of benzene and of tri- and tetra-phenylmethane in anhydrous trifluoroacetic acid have been measured at temperatures between 70 and 180°C, and lead to the following partial rate factors at 70 °C: f₂(CHPh₂), 14; f₃(CHPh₂), 2.3; f₄(CHPh₂), 42; f₂(CPh₃), 4.1; f₃(CPh₃), 3.9; f₄(CPh₃), 66. The markedly low reactivity of the ortho-position of tetraphenylmethane is consistent with steric hindrance to exchange, but the high reactivities of the meta- and para-positions relative to those of the corresponding positions in triphenylmethane is anomalous. Protiodesilylation by a mixture of 72% aqueous perchloric acid and methanol (2 : 5 v/v) at 50 °C gives the following partial rate factors: f₄(CHPh₂), 3.35; f₄(CPh₃), 2.83, and these data, taken along with those for toluene and diphenylmethane also show that tetraphenylmethane is more reactive than expected. The anomalous reactivity may derive either from desolvation of the ground state for reaction oftetraphenylmethane, or, more probably, from steric enhancement of neighbouring group participation and/or carbon–carbon hyperconjugation.

The large temperature range involved in the exchange studies reveals that the Arrhenius plots decrease in slope with increasing temperature, and this is true also for exchange of toluene and t-butylbenzene over a 40 °C range at lower temperatures in media composed either mainly of trifluoroacetic acid or of sulphuric acid. This is consistent with a decrease in the acidity function with increasing temperature, which itself may be due to an increase in the mean free path of the medium as it expands.