JEE Main · 2023 · 30 Jan 2023 · Shift 1 · Aldehydes Ketones and Carboxylic Acids

The acidity of each proton is judged by how stable the anion is once that proton is removed. The carboxylic acid proton ${\mathrm{H}}_{\mathrm{C}}$ leaves a resonance-stabilised carboxylate and is by far the most acidic. The proton ${\mathrm{H}}_{\mathrm{D}}$ sits on the carbon $\alpha$ to the carboxyl group and is also benzylic, so its loss gives a carbanion delocalised onto the carbonyl oxygen and the ring, making it next most acidic. The terminal alkyne proton ${\mathrm{H}}_{\mathrm{A}}$ leaves an acetylide on an $sp$ carbon, whose high $s$-character holds the charge well and places it above the last proton. The benzylic-propargylic proton ${\mathrm{H}}_{\mathrm{B}}$ sits on an $s{p}^3$ carbon and, despite some delocalisation, gives the least stabilised anion. The order of decreasing acidity is ${\mathrm{H}}_{\mathrm{C}}>{\mathrm{H}}_{\mathrm{D}}>{\mathrm{H}}_{\mathrm{A}}>{\mathrm{H}}_{\mathrm{B}}$.