Problem 118 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Hydrolysis of esters in presence of an acid is a reversible reaction. What is true about ester hydrolysis in presence of a base? (a) It is irreversible because salts of carboxylic acids are insoluble. (b) It is irreversible because salts of carboxylic acids have high melting points. (c) It is irreversible because carboxylate ion is resonance stabilized. (d) It is a reversible reaction.

Step-by-Step Solution

Answer

Ester hydrolysis in the presence of a base is irreversible because carboxylate ions are resonance stabilized (option c).

Step 1: Understanding Ester Hydrolysis

Ester hydrolysis in an acidic medium is a reversible reaction where esters react with water to form an alcohol and an acid, which can recombine to form the ester again.

Step 2: Hydrolysis in Basic Conditions

In the presence of a base, ester hydrolysis leads to the formation of an alcohol and a carboxylate ion. This process is commonly known as saponification.

Step 3: Irreversibility of Ester Hydrolysis in Base

The reaction of an ester with a base like NaOH results in the formation of a carboxylate ion, which is not easily reversed to form the ester again.

Step 4: Reason for Irreversibility

The irreversibility is primarily due to the fact that carboxylate ions are stabilized by resonance, making them less reactive to reform the ester.

Step 5: Identifying the Correct Option

The reason for irreversibility corresponds to option (c), which states that carboxylate ions are resonance stabilized, preventing the reverse reaction.

Key Concepts

Saponification

Saponification is a chemical reaction that involves the hydrolysis of an ester in the presence of a base to form an alcohol and a salt of the carboxylic acid, commonly referred to as a carboxylate ion. This process is a cornerstone in soap-making. When fats or oils (which are types of esters) react with a strong base, they break down into glycerol and soap, which is primarily the sodium or potassium salt of fatty acids.

• The base most commonly used in saponification is sodium hydroxide (NaOH).
• This reaction results in a stable product, which is irreversible under normal conditions.
• Saponification not only produces soaps but also demonstrates the practical application of ester hydrolysis in real-world chemistry.

The irreversibility of this reaction is notably due to the formation of stable carboxylate ions, which do not readily recombine to form the original ester.

Carboxylate Ion

The carboxylate ion is a key product in the hydrolysis of esters when a strong base is present. It is formed when the base reacts with the ester, resulting in the breakdown of the ester into an alcohol and a carboxylate ion, often stabilized by the metal cation from the base.

• This ion carries a negative charge on the oxygen, resulting from the loss of a proton from the carboxylic acid group.
• Carboxylate ions act as good leaving groups in chemical reactions due to their stability.
• Their stability arises from resonance, which helps to delocalize the negative charge across the molecule.

This stability through resonance makes it difficult for carboxylate ions to react backward to form the ester, contributing to the irreversibility of the saponification process.

Resonance Stabilization

Resonance stabilization is a powerful concept that explains the distribution and delocalization of electrons in molecules. For carboxylate ions, resonance stabilization plays a crucial role in their stability. When an ester undergoes saponification, one of the products is a carboxylate ion, which experiences this stabilization.

• Resonance allows the negative charge to be spread over different atoms, reducing the energy and increasing the stability of the ion.
• As a consequence, the carboxylate ion is less reactive and does not easily revert to the ester, ensuring the irreversibility of the reaction.
• Understanding resonance stabilization not only clarifies why saponification is irreversible but also offers insight into predicting the behavior of other chemical species.

The logic behind resonance stabilization helps us appreciate why reactions involving resonance-stabilized species tend to be one-directional under standard conditions.