Phenol on treatment with Phthalic anhydride gives Phenolphthalein.

Phenolphthalein is an organic compound that is widely used as an acid-base indicator. It is commonly used in titrations to determine the endpoint of a reaction. Phenolphthalein is colorless in acidic solutions and turns pink or purple in basic solutions.

The reaction between phenol and phthalic anhydride is a condensation reaction, specifically an esterification reaction. During this reaction, a molecule of water is eliminated, and a new compound is formed.

Here is the reaction:

Phenol + Phthalic anhydride → Phenolphthalein + Water

The reaction involves the following steps:

1. Formation of the acyl chloride:
Phthalic anhydride reacts with phenol to form an intermediate compound known as phthalyl chloride.

2. Formation of the phenolphthalein:
The phthalyl chloride reacts with another molecule of phenol, resulting in the elimination of a molecule of hydrogen chloride (HCl) and the formation of phenolphthalein.

3. Formation of water:
The elimination of HCl, which is a strong acid, leads to the formation of water as a byproduct.

Phenolphthalein is a pH indicator that changes color depending on the acidity or alkalinity of the solution. In acidic solutions, it remains colorless, but in basic solutions, it turns pink or purple. This property makes it useful in various laboratory applications, particularly in titrations.

In conclusion, when phenol reacts with phthalic anhydride, it undergoes a condensation reaction to form phenolphthalein. This compound is widely used as an acid-base indicator due to its ability to change color in response to changes in pH.