Which of the following compound gives pink colour on reaction with phthalic anhydride in conc. H₂SO₄ followed by treatment with NaOH ?

Solution

Chemistry ma’am · 3 steps

Pink in NaOH means a phenolphthalein-type dye, and that needs a phenol whose para position is free.

1. What the test makes. In conc. H₂SO₄, a carbonyl carbon of phthalic anhydride is attacked by two phenol rings. The bond forms at the carbon para to each OH, giving a phenolphthalein-type product.
2. Why it turns pink. In NaOH the product opens into a coloured quinonoid dianion, which is pink, just like phenolphthalein. With the para position blocked, a phenol cannot form this dye.

   Option	Type of phenol	Para to OH	Result
   (a)	catechol (1,2-diol)	both blocked (by CH₃ and propyl)	no phenolphthalein; catechols give alizarin-type dyes
   (b)	resorcinol (1,3-diol)	both blocked (by CH₃ and propyl)	no phenolphthalein; resorcinols give fluorescein-type dyes (green fluorescence)
   (c)	2-propylphenol	free	pink phthalein dye
   (d)	4-propylphenol	blocked by propyl	no phthalein, no pink

3. So. Only 2-propylphenol, option (c), has a free para position, so it forms the phthalein and turns pink in NaOH.

The idea to keep. Phthalein test: a simple phenol with a free para position gives pink in NaOH. Resorcinols give fluorescein (green fluorescence), and catechols give alizarin-type dyes.

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