The mechanism of the electrophilic substitution of heteroaromatic compounds. Part IX. General form of rate profiles for acid-catalysed hydrogen exchange as illustrated by substituted aminopyridines

Abstract

The general shapes of plots of log rate against the H₀ acidity function (rate profiles) for acid-catalysed hydrogen exchange of heteroaromatic molecules are discussed. The validity of the deductions was tested using 4-aminopyridine, 4-amino-2,6-dichloropyridine, and 2-amino-5-methylpyridine as substrates. Reaction on the monoprotonated species of 2-amino-5-methyl- and 4-amino-pyridine was observed, while 4-amino-2,6-dichloropyridine exchanges as the conjugate acid at higher acidities, changing over to reaction through free base as the acidity is decreased.