
%0 Journal Article
%A Boulton, BE 
%A Coller, BAW 
%T Kinetics, stoicheiometry, and mechanism in the bromination of aromatic heterocycles. I. Aqueous bromination of pyrazole, 1-methylpyrazole, and 3,5-dimethylpyrazole
%J Australian Journal of Chemistry
%D 1971/07/01
%V 24
%N 7
%P 1413-1423
%K 
%R https://doi.org/10.1071/CH9711413
%U https://www.publish.csiro.au/paper/CH9711413 
%X 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. 
