JEE Main · 2025 · 23 Jan 2025 · Shift 1 · General Organic Chemistry

Species p is the cyclopropenyl anion, carrying 4 $\pi$ electrons in a three-membered ring. By Hückel's $4n+2$ rule this is a $4n$-electron system and is antiaromatic, which is strongly destabilising.

Species q is the cyclopentadienyl anion, with 6 $\pi$ electrons delocalised over five equivalent carbons, satisfying Hückel's rule ($4n+2$, $n=1$) and making it fully aromatic and highly stabilised.

Species r is cyclooctatetraene, an eight-membered ring that adopts a non-planar, tub-shaped geometry, so its $\pi$ system is not continuously conjugated; it is simply non-aromatic, neither stabilised nor destabilised by aromaticity effects.

Aromatic stabilisation exceeds a non-aromatic system, which in turn is more stable than an antiaromatic one, giving the order $q>r>p$.