Which of the following molecules, in pure form, is (are) unstable at room temperature ?
(A) (see structure) (B) (see structure) (C) (see structure) (D) (see structure)
(A)(see structure)
(B)(see structure)✓ Correct
(C)(see structure)✓ Correct
(D)(see structure)
Answer: (B), (C)
Solution
Aromatic stability is determined by Hückel's rule: planar, cyclic, fully conjugated systems with (4n+2) π electrons are aromatic and stable; those with 4n π electrons are antiaromatic and unstable.
(A) Cyclohexadiene — not fully conjugated, non-aromatic, stable. (B) Cyclobutadiene — 4 π electrons (4n with n=1), antiaromatic, highly unstable. (C) Cyclopentadienone — 4 π electrons in the ring (excluding the exocyclic C=O), antiaromatic, unstable. (D) Tropone (cycloheptatrienone) — 6 π electrons (aromatic), stable.
Options (B) and (C) are antiaromatic and therefore unstable at room temperature.
JEE Advanced · 2012 · 08 Apr 2012 · Paper 1 · One or more correct options
