JEE Advanced Chemistry: Aromatic Compounds questions with solutions

Q1. Which of the following is an aromatic ion according to Huckel's rule (4n+2 pi electrons in a planar cyclic conjugated system)?

1. Cyclopropenyl cation (with OH substituent)
2. Cyclopentadienyl cation
3. Cyclohexadienyl cation
4. Cyclohexadienyl anion

Answer: Cyclopropenyl cation (with OH substituent)

Huckel's rule: a monocyclic planar fully conjugated system with (4n+2) pi electrons (n = 0,1,2,...) is aromatic. Cyclopropenyl cation (C3H3+): three sp2 carbons, one empty orbital, 2 pi electrons — this satisfies 4(0)+2 = 2. The OH substituent on the ring does not disrupt ring aromaticity if it's on a sp2 carbon. This is the smallest aromatic cation. Cyclopentadienyl CATION: 4 pi electrons (4n, n=1) — antiaromatic. Cyclohexadienyl cation/anion: the sp3 carbon in the ring breaks conjugation. Answer: cyclopropenyl cation.