Kinetics and mechanism of thalliation of aromatic compounds in trifluoroacetic acid

Abstract

A detailed kinetic study of aromatic thalliation in trifluoroacetic acid is reported. Owing to impurities present in the commonly used reagent, thallium(III) trifluoroacetate, which cause irreproducible reaction rates, thallium(III) triacetate sesquihydrate was used as the thalliating agent. The results obtained from u.v. data were analysed in terms of a rapidly formed π-complex followed by its rate-determining conversion into products. The overall secondorder rate constant showed a better Hammett correlation with σ⁺ than σ values yielding a ρ⁺ value of –8.3 which indicates greater charge development during the reaction than the corresponding mercuriation process. The stability of the intermediate π-complex however shows a greater sensitivity towards substituent effects than that of the mercury complexes. The overall free energies of activation of thalliation and mercuriation are very similar but significant variations occur in their component parameters, notably the entropy of formation of the π-complex is more negative for thallium than for mercury. The overall primary kinetic isotope effect of 4.3 is lower than that for mercuriation but the reverse is true for the conversion of the π-complex into products.