Chemistry - GOC Question with Solution | TestHub

The $\mathrm{sp}$ carbon of the ethynyl group has greater s character and withdraws electron density more strongly than an ${\mathrm{sp}}^2$ or ${\mathrm{sp}}^3$ carbon group. It stabilizes the adjacent carboxylate, giving $\mathrm{HC}\equiv \mathrm{C}-\mathrm{COOH}$ the strongest acidity and lowest ${\mathrm{pK}}_{\mathrm{a}}$ among these options.