Indian Forest Service (Mains), Chemistry optional · 2024 · Paper II

Q1(c). Five aromaticity decisions

Question

Classify the five structures I–V as aromatic, non-aromatic or antiaromatic, with reasons. [structures I–V given in the paper]

I: non-aromatic; II: aromatic; III: non-aromatic as drawn; IV: antiaromatic in its conjugated planar form; V: aromatic.

I, fulvene, is cross-conjugated: the exocyclic double bond is not a third double bond within the five-membered circuit.
II is a mesoionic five-membered heterocycle; delocalization of the formal charges and a heteroatom lone pair produces a cyclic six-π system.
III contains a saturated CH₂ that interrupts the cyclic p-orbital array; classify the displayed diketo tautomer.
IV, pentalene, has an eight-π conjugated perimeter, a 4n count, accounting for its antiaromatic destabilization and high reactivity.
V, tropolone, has charge-separated contributors with a six-π tropylium-like ring and negative charge on oxygen, giving aromatic stabilization.

The full model answer

As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.

Coversfulvenesydnone mesoionicpentalene antiaromatictropolonearomatic non-aromatic antiaromatic