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 (pKa of the pyrrolium ion ≈ -4). The conjugate bases of the others are ordinary amines : pyridine (conjugate acid pKa 5.2) from (C), 4-dimethylaminopyridine, whose basicity is boosted by the NMe₂ donor (pKa ≈ 9.7), from (D), and pyrrolidine (pKa ≈ 11.3) from (A). The most stable conjugate base means the strongest acid, so (B) is the most acidic.