Substituent Effects. 7. Microscopic Dissociation Constants of 4-Amino- and 4-(Dimethylamino)benzoic Acid

Article abstract of DOI:10.1021/jo00317a002

The termodynamic microscopic dissociation constants of the title compounds have determined from the two macroscopic constants K₁ and K₂ and K_A, the carboxylic acid dissociation constant of the cation.The latter were calculated by starting from the experimental value for 4-(trimethylammonio)benzoic acid using our knowledge of the substituent effects of charged substituents.In water, the zwitterion equilibrium contains 10.5percent zwitterion for 4-aminobenzoic acid and 19.4percent for 4-(dimethylamino)benzoic acid.In 50percent and 75percent ethanol the zwitterion content is very low.Extended Hine equations are applied to show the contributions of the several substituent interactions in the individual species, and to predict the percent zwitterion in other acids.Glycine is also discussed.In water, ?-4-NH2 = -0.62 and ?-4-NMe₂ = -0.69; the latter value is rather different from that commonly tabulated.