Chemistry - GOC Question with Solution | TestHub

(a) is the carboxylic acid $\mathrm{O}-\mathrm{H}$. Its anion is stabilised by two equivalent resonance forms on oxygen ($\mathrm{p}{K}_{\mathrm{a}}$ about 3).

(c) is the enolic $\mathrm{O}-\mathrm{H}$ on the ring. Its anion is stabilised by conjugation with the ring $\mathrm{C}=\mathrm{O}$ (a vinylogous acid) and is far more acidic than an alcohol ($\mathrm{p}{K}_{\mathrm{a}}$ about 8-9).

(b) is the alcoholic $\mathrm{O}-\mathrm{H}$ of $-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$. Its anion has no resonance stabilisation ($\mathrm{p}{K}_{\mathrm{a}}$ about 16).

Order: a $>$ c $>$ b.

Answer: (D).