Reactions of N-heteroaromatic bases with nitrous acid. Part 4. Kinetics of the diazotisation of 2- and 4-aminopyridine 1-oxide

Abstract

The kinetics of the diazotisation of 2- and 4-aminopyridine 1 -oxide have been studied in 0.0025–5.0M-perchloric acid. The reaction is of the first order in both the amine and nitrous acid. The rate coefficient of the reaction increases with an increase in the concentration of perchloric acid and of sodium perchlorate. In perchloric acid solutions whose ionic strength is maintained constant by the addition of sodium perchlorate the rate constant for the reaction does not show a linear dependence on the h0 parameter for the medium, in contrast to the reported linear relationship between the rate coefficient for the diazotisation of 2- and of 4-aminopyridine and the h0 function. The results suggest that diazotisation of 2- and 4-aminopyridine 1 -oxide proceeds by two simultaneous reaction paths both of which contribute significantly to the overall reaction rate. The first involves attack of the nitrous acidium ion on the free amine whilst the second involves attack of the same nitrosating agent on the protonated amine. The amines react faster in the free than in the protonated form. The free and protonated forms of 4-aminopyridine 1 - oxide are diazotised at a greater rate than the corresponding forms of 2-aminopyridine 1-oxide.