Reactivities of 4-Substituted 2,6-Dimethylbenzoic Acids

In an earlier investigation, it was found that the substituent effects in 4- and 5-substituted 2-methylbenzoic acids paralleled closely those obtained with benzoic acids without the 2-methyl group. One possible interpretation of the results is that in either series of acids, the reactivity of a carboxyl group is not importantly determined by resonance-coupling of the carboxyl group with various 4-substituents. The argument is not strong in view of the certainly limited ability of a single ortho-methyl group to reduce coupling by sterically forcing the carboxyl out of the plane of the aromatic ring. To gain further evidence on this point, we have determined the apparent ionization constants in 50% water-50% ethanol (by volume) at 25.0º and the reactivities toward diphenyldiazomethane in absolute ethanol at 30.0º of a series of 4-substituted 2,6-dimethylbenzoic acids (I).