Problem

The relative acidities of carboxylic acids (and, by inference, the stabilities of their...

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 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?

(b) The phenyl group is a mild ortho, para-director in electrophilic aromatic substitution. Is the phenyl group electrondonating or electron-withdrawing in electrophilic aromatic substitution? 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-dimethylbenzoic acid is a stronger acid than benzoic acid, but 2,6-dimethylphenol is a weaker acid than phenol. Explain these confusing experimental results.

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