Problem 7 - A Complete Resource Book in Chemistry for JEE Main

Question

The most appropriate reagent to convert \(\mathrm{RCOOEt} \longrightarrow \mathrm{RCH}_{2} \mathrm{OH}\) is (a) \(\mathrm{Li} / \mathrm{NH}_{3}\) (liq) (b) \(\mathrm{NaBH}_{4}\) (c) \(\mathrm{LiAlH}_{4}\) (d) \(\mathrm{H}_{2} / \mathrm{Pd}-\mathrm{C}\)

Step-by-Step Solution

Answer

(c) \( \mathrm{LiAlH}_{4} \) is the most appropriate reagent.

Step 1: Understand the Transformation

We need to convert an ester, \( \mathrm{RCOOEt} \), into a primary alcohol, \( \mathrm{RCH}_{2}\mathrm{OH} \). This involves the reduction of the ester group.

Step 2: Review Reagents Capable of Reducing Esters

Examine the listed reagents. \( \mathrm{Li} / \mathrm{NH}_{3} \) is a birch reduction system, best for alkynes. \( \mathrm{NaBH}_{4} \) is a mild reducing agent that reduces aldehydes and ketones but not esters. \( \mathrm{LiAlH}_{4} \) is a strong reducing agent capable of reducing esters to alcohols. \( \mathrm{H}_{2} / \mathrm{Pd}-\mathrm{C} \) is used for catalytic hydrogenation, typically effective on alkenes and alkynes.

Step 3: Select the Suitable Reagent for Reducing Esters

Among the options, only \( \mathrm{LiAlH}_{4} \) is known to effectively reduce esters to primary alcohols. \( \mathrm{NaBH}_{4} \), \( \mathrm{Li} / \mathrm{NH}_{3} \), and \( \mathrm{H}_{2} / \mathrm{Pd}-\mathrm{C} \) are not effective for this transformation.

Step 4: Confirm the Correct Answer

The most suitable reagent provided to achieve the conversion of \( \mathrm{RCOOEt} \) to \( \mathrm{RCH}_{2}\mathrm{OH} \) is \( \mathrm{LiAlH}_{4} \). This reagent will reduce the ester group to a primary alcohol effectively.

Key Concepts

Ester to Alcohol Conversion

Converting an ester to a primary alcohol is a fundamental reaction in organic chemistry. Esters, characterized by the functional group RCOOR', are derivatives of carboxylic acids. The process of conversion involves reducing the ester to a primary alcohol, in this case specifically converting \( \mathrm{RCOOEt} \) to \( \mathrm{RCH}_{2}\mathrm{OH} \). This reduction involves breaking the ester linkage (the bond between carbon and the alkoxy group) and adding hydrogen atoms to form the alcohol.
Key points to remember during this conversion are:

• Esters contain a stable carbon-oxygen single bond which needs to be cleaved.
• The reduction must add hydrogen to the carbonyl carbon.
• The final product should show the addition of two hydrogen atoms to the carbonyl carbon, transforming it into a hydroxyl group \( \mathrm{OH} \).

Understanding this concept helps in grasping several organic transformations and reactions, supporting both medicinal and synthetic applications.

Reduction Reagents

In organic chemistry, reducing agents are pivotal in transforming various functional groups through the addition of hydrogen or removal of oxygen. These agents vary in terms of strength and specificity, making them suitable for different types of reactions. Let's explore the common types used in the conversion of esters to alcohols:

• \( \mathrm{Li} / \mathrm{NH}_{3} \): Known for birch reduction, primarily effective on alkynes rather than esters or alcohols.
• \( \mathrm{NaBH}_{4} \): A milder reducing agent that effectively reduces aldehydes and ketones but not esters, since esters require a stronger agent to break the robust carbon-oxygen bond.
• \( \mathrm{H}_{2} / \mathrm{Pd}-\mathrm{C} \): Commonly used for hydrogenation reactions involving alkenes and alkynes, not suitable for ester reduction.

The choice of the right reducing agent is critical as it affects the efficiency and outcome of the reaction. Understanding the capabilities of each reagent ensures the correct reagent is selected for each specific conversion.

LiAlH4 as a Reducing Agent

\( \mathrm{LiAlH}_{4} \) (Lithium Aluminum Hydride) is widely recognized as one of the most powerful reducing agents in organic chemistry. It is specifically noted for its ability to reduce esters to alcohols with high efficiency. Here's how it works:

• It donates hydride ions (\( \mathrm{H}^- \)), which are strong reducing moieties, thereby breaking the ester linkage.
• The hydride ion attacks the carbonyl carbon, facilitating the breakage of the \( C-O \) bond in the ester.
• In the subsequent steps, \( \mathrm{LiAlH}_{4} \) continues adding hydrogen atoms, preventing any reformation of the ester and ensuring the full conversion to alcohol.

Another advantage of \( \mathrm{LiAlH}_{4} \) is its ability to work under relatively mild conditions, avoiding the need for high temperatures or pressures. This makes it a preferred choice when ester reduction is required in complex organic synthesis.