The mechanism of the electrophilic substitution of heteroaromatic compounds. Part VIII. The α-, β-, and γ-nitration of pyridine 1-oxides

Abstract

Kinetic evidence confirms that the 4-nitrations of pyridine 1-oxide and several of its substituted derivatives occur on the free-base species. However, 2,6-dimethyl-4-methoxy-, 2,6-dimethoxy-, and 2,4,6-trimethoxy-pyridine 1-oxides are all shown to undergo β-nitration, which, in each case, occurs on the conjugate acid. 3,5-Dimethoxypyridine 1-oxide undergoes successive 2-mono- and 2,6-di-nitration; at least the first stage is a reaction of the conjugate acid. Reaction rates for the nitrations of the pyridine oxides are compared where possible with analogous benzenoid compounds and the results discussed in terms of partial rate factors.