BASICITY (PKBH+) AND ACIDITY CONSTANTS (PKA-ASTERISK) OF SOME 3-X-BENZOIC, 4-X-BENZOIC, AND 2,6-DIMETHYL-4-X-BENZOIC ACIDS

The behaviour of some benzoic acids both as bases (pK(BH+)) and acids (pK(a)*) has been compared with that of their 2,6-dimethyl derivatives. In particular the protonation equilibria, while assigning (through the value of the m* parameter of the excess-acidity method) a significant role to solvation as a stabilising effect on the cations, seem to further assess a major contribution of pi-polarisation to the resonance effect of para-substituents, although some through-conjugation with electron-donating groups cannot be excluded, possibly enforced by the strong requirement for stabilisation of the positive charge in the protonated forms.