JEE Main 2025 Apr 3 Shift 1, Chemistry Q19: Resonance & Mesomeric Effect

The acidity of each species is judged by the stability of its conjugate base.

(B) Sulphonic acid: The conjugate base ${\text{Ar-SO}}_3^{-}$ is heavily resonance-stabilised over three equivalent O atoms. Strongly acidic ($p{K}_a\approx -2$).

(C) Carboxylic acid: The carboxylate is resonance-stabilised over two O atoms. $p{K}_a\approx 5$.

(A) Phenol: The phenoxide is resonance-stabilised by the aromatic ring. $p{K}_a\approx 10$.

(D) Terminal alkyne ${\text{EtO}}_2\text{C-C}\equiv \text{C-H}$: Removing the terminal alkynyl proton gives an acetylide anion in which the negative charge sits on an $sp$ carbon. It is localised and not delocalised onto the ester (geometry-wise the $sp$ lone pair lies perpendicular to the ester $\pi$ system). $p{K}_a$ is around 20–25.

$\therefore$ Compound (D) is the least acidic.