Kinetics and mechanism of the thalliation of benzene and toluene by thallium(III) acetate catalysed by perchloric and sulphuric acid in aqueous acetic acid

Abstract

The thalliation of benzene and toluene by thallium(III) acetate in 4 : 1 (w/w) acetic acid–water is catalysed by both perchloric and sulphuric acid. The catalysis can be explained by the acid-base interaction with thallium(III) acetate to produce Tl(OAc)₂X, Tl(OAc)X₂, and TlX₃(X = ClO₄ or HSO₄) which on ionization yield more reactive electrophiles than does Tl(OAc)₃. Equilibrium constants have been determined for the formation of Tl(OAc)₂X. Isomer distributions, and the relative reactivity of toluene to benzene indicate that thalliation is a typical electrophilic aromatic substitution. The reaction shows a primary hydrogen isotope effect, k_H/k_D= 2·4. The products from toluene isomerise under the reaction conditions. Thalliation is mechanistically similar to mercuration by mercury(II) salts.