KCET 2026 Chemistry Solved Paper

Concept:Acidity depends on the stability of the conjugate base formed after losing $H^+$.Greater the resonance stabilisation of the conjugate base, greater is the acidity.Explanation:Carboxylic acids lose a proton to form the carboxylate ion:$\mathrm{RCOOH} \rightarrow \mathrm{RCOO}^- + \mathrm{H}^+$In the carboxylate ion, the negative charge is delocalised equally over two oxygen atoms.The two resonance structures are equivalent:$\mathrm{RCOO}^- \leftrightarrow \mathrm{R}-\mathrm{C}(=O)-O^- \leftrightarrow \mathrm{R}-\mathrm{C}(-O^-)=O$Equal resonance structures make the carboxylate ion highly stable.Phenol also forms a conjugate base, the phenoxide ion:$\mathrm{C}_6\mathrm{H}_5\mathrm{OH} \rightarrow \mathrm{C}_6\mathrm{H}_5\mathrm{O}^- + \mathrm{H}^+$In the phenoxide ion, the negative charge is delocalised into the benzene ring.However, these resonance structures are not equivalent.In some phenoxide resonance forms, the negative charge is on a carbon atom, which is less stable than on oxygen.Thus, the carboxylate ion is more resonance stabilised than the phenoxide ion.A more stable conjugate base means the acid releases $H^+$ more readily.Therefore, carboxylic acids are more acidic than phenols.Answer:Option D: More resonance stabilisation of their conjugate base.