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.

Verified step by step guidance

1. Step 1: Protonation of phthalic anhydride - The reaction begins with the acid catalyst (H⁺) protonating one of the carbonyl oxygen atoms in phthalic anhydride. This increases the electrophilicity of the carbonyl carbon, making it more susceptible to nucleophilic attack.
2. Step 2: Nucleophilic attack by phenol - The phenol molecule acts as a nucleophile, attacking the electrophilic carbonyl carbon of the protonated phthalic anhydride. This forms a tetrahedral intermediate.
3. Step 3: Loss of water and formation of an ester bond - The tetrahedral intermediate collapses, expelling a molecule of water and forming an ester bond between the phenol and phthalic anhydride.
4. Step 4: Second nucleophilic attack by phenol - A second molecule of phenol attacks the remaining carbonyl group of the phthalic anhydride, forming another tetrahedral intermediate. This step is similar to the first nucleophilic attack.
5. Step 5: Cyclization and formation of phenolphthalein - The intermediate undergoes cyclization, where the aromatic ring of phenol attacks the carbonyl group, forming a new carbon-carbon bond. This results in the final structure of phenolphthalein, which is stabilized by resonance.

Key Concepts

Acid-Catalyzed Reactions

Acid-catalyzed reactions involve the use of an acid to increase the rate of a chemical reaction. In the synthesis of phenolphthalein, sulfuric acid acts as a catalyst, facilitating the reaction between phthalic anhydride and phenol. This process typically involves protonation of the reactants, making them more reactive and promoting the formation of the desired product.

Phenol and Its Derivatives

Phenol is a simple aromatic compound with a hydroxyl (-OH) group attached to a benzene ring. In the synthesis of phenolphthalein, two equivalents of phenol react with phthalic anhydride to form a complex structure. Understanding the reactivity of phenol and its ability to undergo electrophilic substitution is crucial for grasping how phenolphthalein is formed.

Color Change in Acid-Base Indicators

Phenolphthalein is an acid-base indicator that exhibits a distinct color change depending on the pH of the solution. It is colorless in acidic conditions and turns red in basic conditions due to the formation of a red dianion. This property is essential for its use in titrations and pH testing, highlighting the relationship between molecular structure and colorimetric response.