Which one of the following phenols does not give colour when condensed with phthalic anhydride in presence of conc. H₂SO₄ ?

Solution

Chemistry ma’am · 3 steps

Look at the position para to the OH: if it is blocked, the phenol cannot build a coloured phthalein.

1. How the colour forms. Conc. H₂SO₄ makes a carbonyl carbon of phthalic anhydride an electrophile. It attacks the phenol ring at the carbon para to the OH. Two phenol units join this way and give a phthalein or a related dye, which is coloured, especially in alkali.
2. What it needs. The phenol must have a free para position. If a group already sits there, the condensation cannot happen and no dye forms.

   Option	Phenol	Para to OH	Result
   (a)	Phenol	free	phenolphthalein, pink in alkali
   (b)	p-Cresol	blocked by CH₃	no phthalein, no colour
   (c)	Resorcinol	free	fluorescein, green fluorescence
   (d)	Catechol	free	coloured anthraquinone dye (alizarin)

3. So. In p-cresol the CH₃ group occupies the para position, so it cannot condense with phthalic anhydride. The answer is (b).

The idea to keep. The phthalein test needs a free position para to the phenolic OH. A para-substituted phenol like p-cresol gives no colour.

Answer from the NTA final answer key. Solution by PrepWiser.