JEE Main · 2021 · 31 Aug 2021 · Shift 2 · Aldehydes Ketones and Carboxylic Acids

The substrate carries an aromatic $-\mathrm{CHO}$ and a $-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ para to each other. The aldehyde has no $\alpha$-hydrogen, so with concentrated $\mathrm{NaOH}$ two molecules undergo the Cannizzaro reaction: one aldehyde is reduced to $-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ and the other is oxidised to $-\mathrm{CO}{\mathrm{O}}^{-}$, which gives $-\mathrm{COOH}$ on acidification. The reduced molecule becomes benzene-1,4-dimethanol, a diol, and the oxidised molecule becomes 4-(hydroxymethyl)benzoic acid, which carries both an alcohol group and a monocarboxylic acid group. The $-\mathrm{C}{\mathrm{H}}_2\mathrm{OH}$ groups are not oxidised, so a dicarboxylic acid is not formed.