Amino group acidity in aminopyridines and aminopyrimidines

Abstract

The acidities of the amino group in 18 heterocyclic compounds, nine pyridines, eight pyrimidines, and one triazine, have been measured. The pKHA values (standard state water) were calculated using the Bunnett–Olsen and Marziano–Cimino–Passerini extrapolative techniques from ultraviolet spectral changes accompanying ionization of the amino group in aqueous DMSO containing base. These compounds, together with seven anilines previously studied, constitute a well-behaved Hammett system (σ dependency) with a ρ value of 5.20, provided compounds containing nitro groups ortho or para to the amino group are excluded. The acid-strengthening effect of aza groups is additive and appears to be primarily due to induction rather than resonance, in contrast to the behaviour of the nitro group.