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, k_H/k_D = 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, k_H/k_D = 1.08 Values of pK_a(pyrazole-H⁺) = 2.58 and pK_a(3,5-dimethylpyrazole-H⁺) = 4.11 were determined by potentiometric titration methods.