The relative acidities of carboxylic acids (and, by inference, the stabilities of their carboxylate ions) have been used to compare the electron-donating and electron-withdrawing properties of substituents. These studies are particularly valuable to distinguish between inductive and resonance effects on the stabilities of compounds and ions. Some examples:
a. The phenyl group is a mild ortho, para director in electrophilic aromatic substitution. Is the phenyl group electron-donating or electron-withdrawing in EAS?
b. The pKa of phenylacetic acid is 4.31, showing that phenylacetic acid is a stronger acid than acetic acid. Is the phenyl group electron-donating or electron-withdrawing in the ionization of phenylacetic acid? How can you resolve the apparent contradiction?
c. 4-methoxybenzoic acid is a weaker acid than benzoic acid, but methoxyacetic acid is a stronger acid than acetic acid. Explain this apparent contradiction.
d. Methyl groups are usually electron-donating and propanoic acid is a weaker acid than acetic acid. Yet 2,6-dimethylbenzoinc acid is a stronger acid than benzoic acid, but 2,6-dimethylphenol is a weaker acid than phenol. Explain these confusing experimental results.