Which of the following compound gives pink colour on reaction with phthalic anhydride in conc. H2SO4 followed by treatment with NaOH ?
(A)(see structure)
(B)(see structure)✓ Correct
(C)(see structure)
(D)(see structure)
Answer: (B)
Solution
The reaction described is the phenolphthalein test for phenols. When a phenol reacts with phthalic anhydride in concentrated H₂SO₄ followed by treatment with NaOH, it forms phenolphthalein or a phenolphthalein-like compound that gives a pink/magenta color in alkaline medium.
For this reaction to occur, the phenol must have:
A free ortho or para position relative to the -OH group
The -OH group must be directly attached to the benzene ring
Looking at the options:
Option (1): Contains two -OH groups with substituents blocking positions
Option (2): Simple phenol with propyl group - has free ortho and para positions available for reaction
Option (3): Dihydroxy compound with substituents
Option (4): Para-substituted phenol
Option (2), being a monohydric phenol with available ortho/para positions, readily undergoes condensation with phthalic anhydride to form a phenolphthalein derivative that turns pink in alkaline solution.
The reaction mechanism involves electrophilic substitution at the ortho or para position of the phenol ring.
JEE Main · 2021 · 24 Feb 2021 · Shift 1 · Single correct option
