JEE Main 2024 · 27 January · evening · Single correct · +4 / −1
Phenolic group can be identified by a positive:
- (a) Tollen's test
- (b) Phthalein dye test
- (c) Carbylamine test
- (d) Lucas test
Answer and solution
Answer (b) Phthalein dye test
Solution
Chemistry ma’am · 3 steps
Phenol condenses with phthalic anhydride to make a dye, so the phthalein test is built for phenols.
- The phthalein test. Heat the compound with phthalic anhydride and a few drops of concentrated H₂SO₄, then pour the mixture into dilute NaOH. Two phenol rings condense with the anhydride to form phenolphthalein, which turns pink to red in alkali.
- Check the others. Each of the other three tests needs a different functional group, and phenol does not have it.
Test Reagent Detects Positive sign Tollens' test Ammoniacal AgNO₃ Aldehydes Silver mirror Phthalein dye test Phthalic anhydride, conc. H₂SO₄, then NaOH Phenols Pink or red colour Carbylamine test CHCl₃ and alcoholic KOH Primary amines Foul smell of isocyanide Lucas test Conc. HCl with anhydrous ZnCl₂ Alcohols (1°, 2°, 3°) Turbidity - So. The phenolic group is identified by a positive phthalein dye test, option (b).
The idea to keep. Match each named test to its functional group: Tollens for aldehydes, carbylamine for primary amines, Lucas for alcohols, and phthalein dye (or neutral FeCl₃) for phenols.
Answer from the NTA final answer key. Solution by PrepWiser.