The immense acidifying effect of the supersubstituent NSO2CF3 on the acidity of amides and amidines of benzoic acids in acetonitrile

Abstract

The pK_a values of acidic dissociation of the conjugate acids of derivatives of benzoate anions, where one or two oxygen atoms are replaced by an NSO₂CF₃ group, N-aroyltrifluoromethanesulfonamides 1a–f and previously unreported N,N′-bis(trifluoromethylsulfonyl)benzamidines 4a–f, were measured in acetonitrile. In the case of the parent compound, the incorporation of the first NSO₂CF₃ group instead of the oxygen atom leads to a sharp (by 9.6 pK_a units) increase in the acidity, whereas the replacement of the second oxygen atom results in a further huge increase in the acidity by 4.9 powers of ten. It was found that the sensitivity of the reaction series under consideration towards substituent effects (in the benzene ring) decreases in the following order: benzoic acids > benzamides (1a–f) > benzamidines (4a–f). The results of this work carry potentially important implications for the design of new types of superacids and catalytic materials.