Question

How will you convert phenol to phenolphthalein?

Hint:

To remember the reagents: - Phenol + Phthalic anhydride = Phenolphthalein. The name literally combines the names of the two starting materials!

Answer 1

Concept:
Phenolphthalein is a popular acid-base indicator synthesized through a condensation reaction. It involves the reaction of a cyclic anhydride with a substituted aromatic compound (phenol) in the presence of a dehydrating agent.

Phenol reacts with phthalic anhydride in a 2:1 molar ratio. This reaction is carried out by heating the mixture in the presence of a few drops of concentrated sulphuric acid (or anhydrous zinc chloride), which acts as a catalyst and a dehydrating agent.

During the reaction, one molecule of water is eliminated as the carbonyl carbon of phthalic anhydride links with the para-positions of two phenol molecules.

Phthalic Anhydride + 2 × Phenol → Phenolphthalein + H₂O (Conc. H₂SO₄, heat)

The resulting product, phenolphthalein, is a white crystalline solid. It is colorless in acidic and neutral solutions but turns a deep pink or magenta in basic solutions (pH > 8.3).