The decarboxylation of trichloroacetic acid and the reactions of the trichloromethyl anion with 1,3,5-trinitrobenzene and with hydrogen ions: kinetic measurements in dimethyl sulphoxide solution

Abstract

Rate measurements are reported for the formation of a Meisenheimer adduct in solutions of 1,3,5-trinitrobenzene and trichloroacetic acid or sodium trichloroacetate at very low concentrations (<10^–4M). Study of the rate constants and final absorbances of the solutions shows that the rate-limiting step is in all cases the unusually rapid unimolecular breakdown of the trichloroacetate anion (E_a83 kJ mol^–1; ΔS^‡ 11 J mol^–1 K^–1). The rate constants of the reactions of the trichloromethyl anion thus formed with trinitrobenzene or hydrogen ions are nearly equal. From the effect of added hydrochloric acid on the reaction rate a value of the hiterto unreported dissociation constant of trichloroacetic in DMSO (2.9 × 10^–4 mol l^–1) has been deduced. Approximate deuterium isotope effects are derived for the dissociation constant and for some of the rate constants. Preliminary measurements show that the analogous breakdown of tribromoacetic acid in DMSO is a considerably faster reaction.