PROTONATION OF SOME 4-SUBSTITUTED BENZAMIDES AND 2,6-DIMETHYLBENZAMIDES

The protonation of the title compounds has been studied in aqueous sulphuric acid at 25-degrees-C. Relevant pK(BH+) values have been calculated according to both modified Hammett and excess-acidity methods. The data obtained show that the para substituents exert practically the same electronic effects in the two series of amides in spite of the much larger torsion of the carbamoyl group out of the aromatic plane in the 2,6-dimethyl derivatives. The observed behaviour can be related to the internal conjugation of the carbamoyl group which strongly reduces that between the same group and the ring.