The mechanism of the electrophilic substitution of heteroaromatic compounds. Part X. Acid-catalysed hydrogen exchange at the α-, β-, and γ-positions of substituted pyridine 1-oxides

Abstract

Hydrogen exchange reactions on 2,4,6-trimethyl- and 2,6-dimethyl-pyridine 1-oxide in the 3-position proceed on the conjugate acids. Hydrogen exchange of 3,5-dimethylpyridine 1-oxide by contrast proceeds on the free base in the 2-, 4-, and 6-positions, while 3-hydroxypyridine 1-oxide also reacts on the free base, in the 2-position. For 3,5-dimethoxypyridine 1-oxide, changeover from 2- and 6-position exchange on the free base to exchange at the same positions on the conjugate acid is observed with increasing acidity. The orientations of acid-catalysed hydrogen exchange and nitration in pyridine 1-oxides are compared and discussed.