JEE Main · 2021 · 17 Mar 2021 · Shift 1 · General Organic Chemistry

A ring is aromatic when it is cyclic, planar, fully conjugated and carries $(4n+2)$ $\pi$ electrons. In the cyclopentadienyl anion the carbanion centre is $s{p}^2$ hybridised and its lone pair occupies a $p$ orbital, so it joins the two $\mathrm{C}=\mathrm{C}$ bonds to give six $\pi$ electrons delocalised over all five carbons, which satisfies $4n+2$ with $n=1$. A cyclopentadiene ring that still keeps an $s{p}^3$ carbon is not fully conjugated and so is only non-aromatic, and the cyclopentadienyl cation has four $\pi$ electrons in a fully conjugated ring and is antiaromatic. The aromatic species is the cyclopentadienyl anion.