O

O

O + 2

OH

H2SO4

heat

O

O

HO

OH –

OH

H

+

O

O

O–

C

C

O–

red dianion phenolphthalein

phthalic anhydride

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.