JEE Main · 2026 · 04 Apr 2026 · Shift 1 · d and f Block Elements

Phenol condenses with phthalic anhydride in concentrated $\mathrm{H}{}_2\mathrm{SO}{}_4$ to give phenolphthalein, which exists as a closed, colourless lactone ring. On adding $\mathrm{NaOH}$ the lactone opens to an extended quinonoid dianion that is pink. Adding excess $\mathrm{NaOH}$ makes hydroxide ion add to the central carbon; the quinonoid conjugation is destroyed and a carbinol carboxylate (compound X) is produced. With the extended conjugation broken, this species absorbs outside the visible region and is therefore colourless.