Q1(a). Pyridine versus imidazole

Answer in brief

Imidazole B is more basic; its conjugate acid is resonance-stabilized over two nitrogen atoms.

Key points

• Protonation occurs at the pyridine-like nitrogen of imidazole; its lone pair lies outside the aromatic π system, so protonation preserves the six-π-electron ring.
• The positive charge in imidazolium can be distributed between the two nitrogens; the pyrrole-like N supplies electron density through the conjugated ring.
• Approximate aqueous conjugate-acid pKa values are 7 for imidazolium and 5.2 for pyridinium; the higher pKa corresponds to the stronger base.
• Do not protonate the pyrrole-like N lone pair as the preferred basic site; that pair contributes to aromaticity.