JEE Main · 2021 · 26 Aug 2021 · Shift 2 · Alcohols Phenols and Ethers

Condensation with phthalic anhydride is an electrophilic attack by the anhydride carbonyl carbon on the ring position para to the phenolic hydroxyl, and two such attacks on two phenol molecules build the extended conjugated phthalein or fluorescein dye that gives the colour. Phenol, benzene-1,3-diol and benzene-1,2-diol all leave that para position free and condense readily. In 4-methylphenol the position para to the hydroxyl group already carries the methyl group, so the second ring cannot be attached there and no dye is formed. The phenol that gives no colour is 4-methylphenol.