A series of five substituted benzoic acids with 10 substituted pyridines and quinolines have been crystallized so that their ΔpKa, defined as pKa base–pKa 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 ΔpKa ≈ 2, we report here a structure that does not follow the general rule and serves as a cautionary tale.
