2018 H2 Chem P1 Q24 — Hydroxyl Compounds Solution

The answer is D (6).
Hot excess NaOH(aq) reacts with every acidic group and hydrolyses every ester group. Rosmarinic acid has:
• 4 phenolic OH groups (two on each benzene ring): 4 mol NaOH, \(\text{ArOH} + \text{NaOH} \rightarrow \text{ArO}^-\text{Na}^+ + \text{H}_2\text{O}\)
• 1 carboxylic acid group: 1 mol NaOH, \(\text{RCO}_2\text{H} + \text{NaOH} \rightarrow \text{RCO}_2^-\text{Na}^+ + \text{H}_2\text{O}\)
• 1 ester group, hydrolysed on heating: 1 mol NaOH, \(\text{RCO}_2\text{R'} + \text{NaOH} \rightarrow \text{RCO}_2^-\text{Na}^+ + \text{R'OH}\)
Total = 4 + 1 + 1 = 6 mol NaOH per mole of rosmarinic acid. The alcohol R'OH released by the hydrolysis is not acidic enough to react with NaOH.
• A (1) is wrong: it counts only the carboxylic acid group.
• B (2) is wrong: it counts the carboxylic acid and the ester but misses the four phenol groups.
• C (5) is wrong: it counts the carboxylic acid and the four phenol groups but misses the ester hydrolysis.