Carboxylic Acids and Derivatives: Question 6

Worked solution

Comparing the acidity of ethanoic acid and phenol

A carbonate is a weak enough base that it is only converted into $\text{CO}_2$ and water by a sufficiently strong acid. This makes it a useful practical test for distinguishing acids of different strength:

• Ethanoic acid (a carboxylic acid, $\text{p}K_a \approx 4.8$) is a strong enough acid to react with sodium carbonate: $2\text{CH}_3\text{COOH(aq)} + \text{Na}_2\text{CO}_3\text{(s)} \rightarrow 2\text{CH}_3\text{COONa(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$ Brisk effervescence is seen as the solid dissolves and colourless carbon dioxide gas escapes.

• Phenol ($\text{p}K_a \approx 10$) is far too weak an acid to protonate the carbonate ion and release $\text{CO}_2$. Although phenol is acidic enough to react with the stronger base $\text{NaOH(aq)}$ (forming sodium phenoxide and water), it shows no visible reaction with solid sodium carbonate.

Why this difference exists

The underlying reason phenol is a weaker acid than ethanoic acid is how well each conjugate base stabilises its negative charge. The carboxylate ion formed from ethanoic acid delocalises its charge equally over two oxygen atoms, which is very effective stabilisation. The phenoxide ion formed from phenol can delocalise its charge only partly, into the benzene ring, which is a less effective form of stabilisation. This makes the carboxylate ion much more stable (and so ethanoic acid much more willing to lose its proton) than the phenoxide ion, placing ethanoic acid’s acidity comfortably above phenol’s. High enough to react with a carbonate, whereas phenol’s is not.

Why the other options are wrong

• B reverses the correct behaviour of the two compounds.
• C is wrong because phenol is not acidic enough to react with a carbonate at all.
• D is wrong because ethanoic acid, unlike phenol, is a strong enough acid to react with sodium carbonate.

Final answer

A. Ethanoic acid is acidic enough to react with sodium carbonate, releasing $\text{CO}_2$ with effervescence, whereas phenol is too weak an acid to do so and shows no visible reaction.