Problem 76 - A Complete Resource Book in Chemistry for JEE Main

Question

In a set of reactions propionic acid yielded a compound D. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH} \stackrel{\mathrm{SOCl}_{2}}{\longrightarrow} \mathrm{B} \stackrel{\mathrm{NH}_{3}}{\longrightarrow} \mathrm{C} \stackrel{\mathrm{KOH}}{\mathrm{Br}_{2}} \mathrm{D} \mathrm{D}\) The structure of \(D\) would be (a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NHCH}_{3}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{2} \mathrm{CH}_{2} \mathrm{NH}_{2}^{3}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2}^{2} \mathrm{CONH}_{2}\)

Step-by-Step Solution

Answer

The structure of compound D is (b) \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2 \).

Step 1: Identify Compound B

Begin by converting propionic acid \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COOH} \) with \( \mathrm{SOCl}_2 \). This reaction converts the carboxylic acid into an acyl chloride. Hence, compound B is \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COCl} \).

Step 2: Convert B to Compound C

The next reaction involves treating compound B with \( \mathrm{NH}_3 \). An acyl chloride treated with ammonia forms an amide. Thus, compound C is \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CONH}_2 \).

Step 3: Convert C to Compound D

The final reaction is the Hofmann bromamide reaction where the amide \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CONH}_2 \) reacts with bromine in the presence of \( \mathrm{KOH} \). This reaction converts the amide to a primary amine with one less carbon atom. Therefore, compound D is \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2 \).

Step 4: Determine the Structure of D

With the transformation steps complete, the structure of compound D, \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2 \), matches choice (b) in the options provided.

Key Concepts

Acyl Chloride Formation

When propionic acid, a carboxylic acid \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COOH} \), reacts with thionyl chloride \( \mathrm{SOCl}_2 \), it undergoes a transformation to form acyl chloride. This reaction is known as Acyl Chloride Formation and is a fundamental process in organic chemistry. Acyl chlorides, like \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COCl} \), are highly reactive and are often intermediates in various synthesis pathways.
Propionic acid reacts with thionyl chloride in a nucleophilic substitution reaction where the hydroxyl group of the carboxylic acid is replaced by a chlorine atom, creating an acyl chloride. Although acyl chlorides are highly reactive, the reaction conditions are usually mild, making the synthesis process efficient. This transformation is useful in further chemical reactions, especially those involving the formation of amides and esters.

• The reaction is specific to the use of thionyl chloride, though other reagents can also facilitate this conversion.
• Acyl chlorides are versatile and can participate in numerous subsequent reactions.

Amide Synthesis

Amide synthesis is a process that involves the conversion of acyl chlorides into amides. This is achieved by reacting the acyl chloride with ammonia \( \mathrm{NH}_3 \), leading to the formation of an amide and the release of hydrochloric acid as a byproduct. In the solution provided, the acyl chloride \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{COCl} \) was treated with ammonia to form the amide \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CONH}_2 \).
This transformation is a crucial step in organic synthesis because amides are important compounds in many chemical and pharmaceutical applications. The reaction involves a nucleophilic attack by the ammonia nitrogen on the carbonyl carbon of the acyl chloride, displacing the chloride ion.

• Amide linkages are significant in the formation of peptides and proteins.
• Controlling the reaction conditions, such as temperature and concentration, is key to ensuring a successful amide synthesis.

Primary Amine Synthesis

The Hofmann Bromamide Reaction is an essential transformation in organic chemistry. It allows the conversion of amides into primary amines through a reaction with bromine and a strong base such as potassium hydroxide (KOH). In the context of the problem, the amide \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{CONH}_2 \) is converted to the primary amine \( \mathrm{CH}_3 \mathrm{CH}_2 \mathrm{NH}_2 \).
This reaction is unique because it results in a primary amine that contains one less carbon atom than the original amide. The mechanism involves the formation of an isocyanate intermediate, which then hydrolyzes to form the primary amine.

• This reaction can effectively remove a carbonyl group, simplifying the molecular structure.
• The process is vital for synthesizing smaller organic compounds and has applications in the pharmaceutical industry.
• The reaction conditions typically require a precise balance of reagents and temperature control.