Phenolphthalein, a common nonprescription laxative, is also an acid-base indicator that is colorless in acid and red in base. Phenolphthalein is synthesized by the acid-catalyzed reaction of phthalic anhydride with 2 equivalents of phenol.

(a) Propose a mechanism for the synthesis of phenolphthalein.

(b) Propose a mechanism for the conversion of phenolphthalein to its red dianion in the base.

(c) Use resonance structures to show that the two phenolic oxygen atoms are equivalent (each with half a negative charge) in the red phenolphthalein dianion.

Phthalic anhydride Phenolphthalein red dianion

Phenolphthalein is a chemical compound generally used as an indicator in acid-base titration which turns colorless in acidic solution and pink in basic solution.

(a) The mechanism for the synthesis of phenolphthalein is represented as follows:

Mechanism for the synthesis of phenolphthalein

(b) The phenolphthalein molecule is converted to red dianion ( two negative charge ions) in the presence of a base. This dianion form exists in numerous resonance structures by the delocalization of negative charge on ring and oxygen atom.

Mechanism for the conversion of phenolphthalein to its red dianion

(c) The resonance structure describes the delocalization of electrons or charge on an ion or a molecule. This structure helps in determining the reason for the stability of a molecule.

Resonance structures

Hence two phenolic oxygen atoms are equivalent in the dianion of phenolphthalein.