Q1(a). Aromaticity of small rings

Answer in brief

I: antiaromatic idealised oxirene; II: aromatic; III: aromatic; IV: antiaromatic if planar and conjugated; V: non-aromatic.

Key points

• I contains two alkene π electrons and an oxygen lone pair in a three-membered conjugated circuit: four π electrons.
• II is a borirene derivative; trigonal boron supplies an empty p orbital, so the circuit contains only two π electrons: 4n + 2 with n = 0.
• III is a substituted cyclopropenyl cation; its empty carbon p orbital completes a two-π aromatic circuit.
• IV has an alkene, a carbocation p orbital and an oxygen lone pair around the four-membered ring: four π electrons if planar and conjugated.
• V has a saturated CH₂ ring atom interrupting the cyclic p array, so the whole ring is non-aromatic.