Reactions of N-heteroaromatic bases with nitrous acid. Part 5. Kinetics of the diazotisation of substituted 2-aminopyridine and 2-aminopyridine 1-oxide

Abstract

In perchloric acid (up to 3.0M) kept at constant ionic strength the diazotisation of 2-amino-5-methylpyridine and 2-amino-5-chloropyridine.1-oxide takes place mainly by the interaction of the nitrous acidium ion with the protonated form of the former and the free form of the latter. However, the diazotisation of 2-amino-5-chloropyridine and 2-amino-5-methylpyridine 1-oxide takes place by the simultaneous interaction of the nitrous acidium ion with the protonated and the free form of both amines. The nitrous acidium ion shows a distinct discrimination in its reaction with the free amines and this is manifested in a rectilinear relationship between the rate constant for the diazotisation and the K_a values of the amines. Unlike the diazotisation of the free aromatic amines, the diazotisation of the 2- and 4-aminopyridines seems to involve an initial interaction between the nitrosating agent and most probably that part of the heteroaromatic ring system (including the 1 -oxido group when appropriate) which is richer in electrons than the amino-group. The effects of substituents on the rate coefficient for the diazotisation of the protonated 2- and 4-aminopyridines support the view that the diazotisation of these amines involves an initial association of the nitrosating agent with the heteroaromatic ring. There is evidence that the hydroxy-group of the protonated amine 1 -oxide molecules, when in the para-position with respect to the amino-group and therefore not involved in hydrogen bonding with it, accelerates the reaction by providing an additional site for the initial association with the nitrosating agent. pK_a Values of 2-amino-5-methyl- and 2-amino-5-chloro-pyridine 1-oxide are recorded.