2023 H2 Chem P1 Q26 — Carboxylic Acid Solution

The answer is B (2).
Only acidic hydrogen atoms, those bonded to O in the phenol \(-\text{OH}\) and the carboxylic acid \(-\text{CO}_2\text{H}\), can be exchanged. For example:
\(\text{RCOOH} + \text{D}_2\text{O} \rightleftharpoons \text{RCOO}^- + \text{HD}_2\text{O}^+\)
In the reverse reaction, \(\text{RCOO}^-\) can take back a D instead of an H, forming RCOOD. With a large excess of \(\text{D}_2\text{O}\), the H is replaced by D. The phenol O-H exchanges in the same way. So a maximum of 2 H atoms can be replaced.
The C-H hydrogens (3 on the ring and 2 on the CH=CH) are not acidic and do not ionise, so they are not replaced.
• A is wrong: it counts only the \(-\text{CO}_2\text{H}\) hydrogen, but the phenol \(-\text{OH}\) is also acidic and exchanges with \(\text{D}_2\text{O}\).
• C is wrong: 4 would add the 2 alkene C-H hydrogens to the 2 O-H hydrogens, but C-H hydrogens do not ionise.
• D is wrong: 5 is the number of C-H hydrogens (3 ring + 2 alkene), none of which is acidic.