Structures of benzoic acids with substituted pyridines and quinolines: salt versus co-crystal formation

Abstract

A series of five substituted benzoic acids with 10 substituted pyridines and quinolines have been crystallized so that their ΔpK_a, defined as pK_{a base}–pK_{a acid}, varied from −1.14 to +4.16. This spans the ‘uncertainty’ region for the formation of salt versus co-crystals. Although most of our results confirmed that structure formation of co-crystal versus salt parts at ΔpK_a ≈ 2, we report here a structure that does not follow the general rule and serves as a cautionary tale.