Kinetics, stoicheiometry, and mechanism in the bromination of aromatic heterocycles. I. Aqueous bromination of pyrazole, 1-methylpyrazole, and 3,5-dimethylpyrazole

Abstract

A procedure is described for monitoring the reactivity of a substrate towards bromine in aqueous bromide solutions, as a function of extent of reaction, by following the changes of electrode potential with time in the intervals between successive periods of electrolysis. ��� The title compounds show 1 : 1 stoicheiometry with substitution in the 4-position. The observed rate behaviour may be understood in terms of direct attack by molecular bromine on the neutral substrate molecules, rate coefficients for dilute aqueous solution at 25�C being: ����� k20(Br2+pyrazole) = 3.8x105 dm3 mol-1 s-1, kH/kD = 1.39;����� k20(Br2+1.methylpyrazole) = 8.0x105 dm3 mol-1 s-1;����� k20(Br2+3,5-dimethylpyrazole) = 1.4x109 dm3 mol-1 s-1, kH/kD = 1.08 Values of pKa(pyrazole-H+) = 2.58 and pKa(3,5-dimethylpyrazole-H+) = 4.11 were determined by potentiometric titration methods.