The kinetics and mechanism of the electrophilic substitution of heteroaromatic compounds. Part XXVII. The nitration and hydrogen exchange of 1,3,5-trimethylpyrazole, 3,5-dimethylisoxazole, and 3,5-dimethylisothiazole

Abstract

The nitration and hydrogen exchange in the 4-position of the title compounds and corresponding N-methyl cations are studied kinetically. Criteria previously developed to distinguish between reaction on the free-base or conjugate-acid forms of six-membered heterocyclic aromatic compounds are applied. Both hydrogen-exchange and nitration occur on the free-base form of 3,5-dimethylisoxazole. For 1,3,5-trimethylpyrazole and 3,5-dimethylisothiazole, nitration occurs on the conjugate acids, while hydrogen-exchange undergoes a changeover from free-base to conjugate-acid reaction as the acidity is increased.

The kinetic rates are compared with those for related compounds and the influence of N, O, S, and NMe atomic groupings discussed.