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

To determine which phenol does not give color when condensed with phthalic anhydride in the presence of concentrated H₂SO₄, we need to understand the chemical reactions involved, specifically the formation of phthaleins. 

1. The reaction of phenols with phthalic anhydride and concentrated sulfuric acid typically leads to the formation of phthaleins, which are colored compounds.
2. Phenol, catechol (1,2-dihydroxybenzene), and resorcinol (1,3-dihydroxybenzene) all have substituents that can react with phthalic anhydride to produce colored phthaleins:
   • Phenol: Forms phenolphthalein, a well-known pH indicator.
   • Catechol: Can form colored products similar to phenolphthalein.
   • Resorcinol: Known to form fluorescein, a brightly colored compound, in this reaction.
3. p-Cresol (4-methylphenol) does not form a colored compound in this reaction due to the methyl group's effect and position. In p-cresol, the methyl group at the para position hinders the typical formation of phthalein-type structures that result in color.

Given the above reactions and the nature of phthalein formation, p-cresol (4-methylphenol) is the phenol that does not produce a color when condensed with phthalic anhydride in the presence of concentrated H₂SO₄.