Methyl p-nitrobenzoate has been found to undergo saponification faster than methyl benzoate.
(a) Consider the mechanism of saponification, and explain the reasons for this rate enhancement.

Verified step by step guidance

1. Understand the saponification mechanism: Saponification is the base-catalyzed hydrolysis of an ester. The reaction involves nucleophilic attack by hydroxide (OH⁻) on the carbonyl carbon of the ester, forming a tetrahedral intermediate, which then collapses to release the alcohol and form the carboxylate ion.
2. Analyze the structure of methyl p-nitrobenzoate: The ester group is attached to a benzene ring that has a nitro group (-NO₂) in the para position. The nitro group is an electron-withdrawing group (EWG) due to its strong -I (inductive) and -M (mesomeric) effects.
3. Explain the role of the nitro group: The electron-withdrawing nature of the nitro group stabilizes the partial positive charge on the carbonyl carbon of the ester. This makes the carbonyl carbon more electrophilic and more susceptible to nucleophilic attack by hydroxide ions.
4. Compare with methyl benzoate: Methyl benzoate lacks the nitro group, so its carbonyl carbon is less electrophilic. As a result, the nucleophilic attack by hydroxide ions occurs more slowly in methyl benzoate compared to methyl p-nitrobenzoate.
5. Conclude the rate enhancement: The presence of the nitro group in methyl p-nitrobenzoate significantly increases the rate of saponification by making the carbonyl carbon more reactive toward nucleophilic attack, thereby lowering the activation energy of the reaction.

Key Concepts

Saponification Mechanism

Saponification is a nucleophilic substitution reaction where an ester reacts with a base, typically hydroxide ions, to form an alcohol and a carboxylate salt. The mechanism involves the nucleophile attacking the carbonyl carbon of the ester, leading to the formation of a tetrahedral intermediate. Understanding this mechanism is crucial to analyze how substituents on the aromatic ring can influence the reaction rate.

Electronic Effects of Substituents

The presence of electron-withdrawing groups, such as nitro groups, can significantly affect the reactivity of aromatic compounds. In methyl p-nitrobenzoate, the nitro group stabilizes the negative charge that develops during the transition state of the saponification reaction, thereby facilitating the nucleophilic attack. This concept is essential for understanding why methyl p-nitrobenzoate reacts faster than methyl benzoate.

Steric Hindrance

Steric hindrance refers to the repulsion between bulky groups that can impede the approach of nucleophiles to the electrophilic center. In the case of methyl benzoate, the lack of electron-withdrawing groups means that steric factors may play a more significant role in slowing down the reaction. Recognizing the balance between electronic effects and steric hindrance is vital for explaining the observed differences in reaction rates.