JEE Main · 2021 · 24 Feb 2021 · Shift 2 · General Organic Chemistry

An aromatic ring must be cyclic, planar and fully conjugated, with every ring atom $s{p}^2$ hybridised so that an unbroken loop of $4n+2$ $\pi$ electrons can form over the ring. Cyclohepta-1,3,5-triene carries one $s{p}^3$ $\mathrm{C}{\mathrm{H}}_2$ group in the ring, and this saturated centre interrupts the cycle of $p$ orbitals, so no closed $\pi$ loop exists and the delocalisation energy is lost. The remaining three species are planar rings with uninterrupted conjugation and a Hückel count of $4n+2$ $\pi$ electrons, so they are aromatic. The non-aromatic compound is cyclohepta-1,3,5-triene.