Chemistry - GOC Question with Solution | TestHub

Compare the stability of the conjugate bases. Losing a proton from (B) gives pyrrole, a fully aromatic, very stable neutral molecule ($\mathrm{p}{K}_{\mathrm{a}}$ of the pyrrolium ion ≈ -4). The conjugate bases of the others are ordinary amines : pyridine (conjugate acid $\mathrm{p}{K}_{\mathrm{a}}$ 5.2) from (C), 4-methylpyridine ($\mathrm{p}{K}_{\mathrm{a}}$ ≈ 6) from (D), and piperidine ($\mathrm{p}{K}_{\mathrm{a}}$ ≈ 11.1) from (A). The most stable conjugate base means the strongest acid, so (B) is the most acidic.