Question

Which one of the following phenols does not give colour when condensed with phthalic anhydride in presence of conc. H\(_2\)SO\(_4\) ?

• A. Phenol
• B. Catechol (1,2-dihydroxybenzene)
• C. Resorcinol (1,3-dihydroxybenzene)
• D. p-Cresol (4-methylphenol)

Hint:

The phthalein dye test is a characteristic test for phenols where the para position relative to the hydroxyl group is vacant. If the para position is blocked by any group other than hydrogen, the test will be negative. This is a common question to test the understanding of regioselectivity in electrophilic aromatic substitution.

Answer 1

The Correct Option is D

Step 1: Understanding the Reaction
The reaction of a phenol with phthalic anhydride in the presence of concentrated sulfuric acid is the phthalein dye test. This reaction forms a phthalein dye, which is typically colored. The reaction is an electrophilic aromatic substitution where two molecules of phenol react with one molecule of phthalic anhydride.
Step 2: Mechanism of the Phthalein Dye Test
In this reaction, the electrophile (a carbocation generated from phthalic anhydride and conc. H\(_2\)SO\(_4\)) attacks the phenol ring. For the reaction to proceed and form the characteristic dye structure, the \textit{para} position (with respect to the -OH group) on the phenol ring must be unsubstituted (i.e., must have a hydrogen atom). The substitution occurs at this para position.
Step 3: Analyzing the Options
(A) Phenol: The para position is free. It will react to form phenolphthalein, which is a well-known indicator.
(B) Catechol (1,2-dihydroxybenzene): The para position with respect to one -OH group (C4) is free. It will give a positive test.
(C) Resorcinol (1,3-dihydroxybenzene): The para position with respect to one -OH group (C4) is free and highly activated. It will react readily to form fluorescein.
(D) p-Cresol (4-methylphenol): In this molecule, the para position with respect to the -OH group is already occupied by a methyl (-CH\(_3\)) group. Since there is no hydrogen atom at the para position, the electrophilic substitution required to form the phthalein dye cannot occur.
Step 4: Final Answer
Therefore, p-cresol does not give colour with phthalic anhydride because its para position is blocked.