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

The correct reagent is (c) LiAlH₄.

Step 1: Identify the Type of Reaction

We need to convert an ester ( RCOOEt ) to a primary alcohol ( RCHOH ). This is a reduction process where the ester functional group is reduced to an alcohol.

Step 2: Determine the Suitable Reducing Agent

Lithium aluminium hydride ( LiAlH ) is known for its ability to reduce esters to primary alcohols. Other options like NaBH and H/ Pd-C are not strong enough for this process, and Li/NH is not suitable for this conversion.

Step 3: Choose the Correct Answer

The most suitable reagent of the options given to convert an ester to a primary alcohol is LiAlH .

Key Concepts

Lithium Aluminium Hydride (LiAlH4)

Lithium Aluminium Hydride, commonly abbreviated as \( \text{LiAlH}_4 \), is a powerful reducing agent used extensively in organic chemistry. It is particularly known for its ability to convert esters, amides, carboxylic acids, and other carboxy derivatives into primary alcohols.
Due to its high reactivity, \( \text{LiAlH}_4 \) must be handled with caution and is typically used in dry ether or tetrahydrofuran (THF). When introduced to water or other protic solvents, it reacts violently, making it crucial to maintain anhydrous conditions during reactions.
\( \text{LiAlH}_4 \) reduces esters directly to alcohols by breaking the \( C=O \) bond and replacing it with an \( OH \) group, alongside hydrogens bound to the carbon. This attribute makes it a prime candidate for reactions requiring significant reductions, such as ester to alcohol conversions.

Reducing Agents in Organic Chemistry

Reducing agents play a critical role in organic synthesis, as they enable the transformation of molecules by reducing certain functional groups. The choice of a reducing agent depends on several factors, such as the substrate, the reaction conditions, and the desired product. Let's explore some reducing agents commonly seen in organic chemistry and their uses.

• Lithium Aluminium Hydride (LiAlH4): Extremely reactive and potent, used for converting esters, amides, and carboxylic acids to alcohols.
• Sodium Borohydride (NaBH4): Weaker than LiAlH4 and typically used for milder reductions, such as converting ketones and aldehydes to alcohols, but not strong enough for esters.
• Hydrogen with Palladium on Carbon (H2/Pd-C): Mainly used for hydrogenation reactions, this option isn't suitable for ester reduction but effective in other transformations like alkene reductions.
• Lithium in Ammonia (Li/NH3): Used for the Birch reduction, which reduces aromatic rings, and isn't applicable for ester-to-alcohol transformations.

The selection of the appropriate reducing agent is key to obtaining the desired reduction and maximizing yield. Understanding their reactivity and specificity can significantly influence the success of organic synthesis.

Conversion of Esters to Alcohols

The process of converting esters into alcohols involves breaking down the ester functional group, which is characterized by the presence of a carbonyl group (\( C=O \)) bonded to an oxygen atom (\( O \)). In organic synthesis, achieving this transformation efficiently can be crucial to developing specific kinds of alcohols.
Using \( \text{LiAlH}_4 \), the reduction of esters proceeds through hydride transfer. This occurs when the hydride ion (H\(^-\)) from \( \text{LiAlH}_4 \) attacks the electrophilic carbon in the ester, leading to the cleavage of the \( C-O \) bond.

• The mechanism begins with hydride ion attacking the ester carbonyl carbon, breaking the \( C=O \) bond first.
• A tetrahedral intermediate forms, quickly collapsing to release an alkoxide, which then captures another hydride ion.
• The final product is a primary alcohol after acidic work-up, where any remaining aluminium species are removed, leaving a clean alcohol.

This conversion is valuable in creating various primary alcohols from esters and is a demonstration of how reducing agents facilitate complex organic transformations.