JEE Main · 2026 · 04 Apr 2026 · Shift 1 · General Organic Chemistry

Electron-withdrawing strength reflects the combined $-I$ and $-R$ effects. Iodine acts only through a weak $-I$ effect, so $-\mathrm{I}$ is weakest. The carboxyl group withdraws more, the cyano group still more, and the nitro group most of all because of its strong $-I$ and $-R$ withdrawal. The increasing order is therefore $-\mathrm{I}<-\mathrm{COOH}<-\mathrm{CN}<-NO2$, that is $d<b<a<c$.