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 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.
The two phenolic oxygen atoms are equivalent in dianion of phenolpthalein.