(a) Explain why we speak of acidic hydrolysis of an ester as acid-catalyzed, but of basic hydrolysis as base-promoted.

Verified step by step guidance

1. Understand the mechanism of acidic hydrolysis of an ester: In acidic conditions, the ester is protonated, making it more susceptible to nucleophilic attack by water, which is a weak nucleophile. The acid is not consumed in the reaction, acting as a catalyst.
2. Recognize the role of the acid: The acid provides protons (H+) that facilitate the formation of a more reactive intermediate, but it is regenerated at the end of the reaction, hence the term 'acid-catalyzed'.
3. Examine the mechanism of basic hydrolysis of an ester: In basic conditions, a strong nucleophile, typically hydroxide ion (OH-), attacks the ester directly, leading to the formation of a carboxylate ion and an alcohol.
4. Identify the role of the base: The base is consumed in the reaction as it provides the nucleophile (OH-) that initiates the reaction, and it is not regenerated, hence the term 'base-promoted'.
5. Compare the two processes: Acidic hydrolysis involves a catalytic cycle where the acid is regenerated, while basic hydrolysis involves the consumption of the base, which drives the reaction forward.

Key Concepts

Acidic Hydrolysis

Acidic hydrolysis of an ester involves the addition of water in the presence of an acid catalyst, typically a strong acid like hydrochloric acid. The acid donates protons (H+) to the ester, enhancing the electrophilicity of the carbonyl carbon, which facilitates the nucleophilic attack by water. This process leads to the formation of a carboxylic acid and an alcohol.

Basic Hydrolysis

Basic hydrolysis, often referred to as saponification, occurs when an ester reacts with a strong base, such as sodium hydroxide. In this reaction, the hydroxide ion (OH-) acts as a nucleophile, attacking the carbonyl carbon of the ester. This process is termed 'base-promoted' because the base is not merely a catalyst; it is consumed in the reaction, resulting in the formation of a carboxylate ion and an alcohol.

Catalysis vs. Promotion

The distinction between 'catalyzed' and 'promoted' reactions lies in the role of the reactants. In acid-catalyzed reactions, the acid is not consumed and can be reused, acting solely to speed up the reaction. In contrast, in base-promoted reactions, the base is consumed in the reaction, leading to the formation of products, which is why we refer to it as 'promoted' rather than 'catalyzed.'