Q1(a). Annulenes and aromaticity

Answer in brief

III, 1,6-methano[10]annulene, is aromatic. I is [8]annulene and II is [10]annulene; their usual ground-state structures are non-aromatic.

Key points

• I is cyclooctatetraene, [8]annulene. Its eight π electrons would be antiaromatic in a planar ring, but the molecule adopts a tub geometry and avoids that arrangement.
• II has ten π electrons, satisfying 4n + 2, but internal H–H crowding and bond-angle constraints prevent a suitably planar continuous π circuit.
• III has a methylene bridge joining C1 and C6. This replaces the offending inward hydrogens and supports effective overlap around the ten-electron perimeter.
• The bridge carbon is saturated and does not supply extra π electrons. Aromaticity requires cyclic conjugation and suitable geometry as well as the electron count.