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

Question

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.

Explanation / Answer

A)electron withdrawing

B) phenyl group's electron withdrawing effect stabilises the acetate ion. hence it is more stable than acetic acid

C) at meta position the resonance effect of ----OCH3 dimnishes while at ortho position its effect is dominant

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