Zwitterion ratios of aminobenzoic acids

Abstract

Relations have been established between the pK_a values for the non- zwitterionic forms of meta- and para-aminobenzoic acids, pK_B, and the pK_a values of the corresponding methyl and ethyl esters, pK_E. For methyl m-aminobenzoate, the expression is pK_B = pK_E + 0.09, whilst for the ethyl ester it is pK_B = pK_E + 0.03. The corresponding relations for p- aminobenzoic acid derivatives are pK_B = pK_E + 0.06 for the methyl ester, and pK_B = pK_E + 0.03 for the ethyl ester. The mioroscopic constants pertaining to the zwitterionic equilibria are given, and are used to establish the Hammett substituent constants, σ_m, for the following meta substituent groups: NH₂, 0.00; NH₃+, 0.98; NHCH₂COOEt, -0.10; COOH, 0.30; COOMe, 0.33; COOEt, 0.31; COO-, 0.01.