Basicity (pKBH+) and Acidity Constants (pKa*) of some 3-X-, 4-X and 2,6-Dimethyl-4-X-benzoic Acids

The behaviour of some benzoic acids both as bases (pK(BH+)) and acids (pKa*) 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.