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

Question

In a set of reactions propionic acid yielded a compound D. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{COOH} \stackrel{\mathrm{SOCl}_{2}}{\longrightarrow} \mathrm{B} \stackrel{\mathrm{NH}_{3}}{\longrightarrow} \mathrm{C} \stackrel{\mathrm{KOH}}{\mathrm{Br}_{\mathrm{z}}} \mathrm{D}\) 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) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\) (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CONH}_{2}\)

Step-by-Step Solution

Answer

The structure of D is \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{NH}_{2} \) (option b).

Step 1: Convert Propionic Acid to Acyl Chloride

The initial reaction involves converting propionic acid \( \text{CH}_3\text{CH}_2\text{COOH} \) into an acyl chloride using thionyl chloride \( \text{SOCl}_2 \). The product of this reaction is propionyl chloride \( \text{CH}_3\text{CH}_2\text{COCl} \), which we will call Compound B.

Step 2: Form Amide from Acyl Chloride

Next, react the acyl chloride (Compound B: \( \text{CH}_3\text{CH}_2\text{COCl} \)) with ammonia \( \text{NH}_3 \). This reaction replaces the chlorine atom of the acyl chloride with an \( \text{NH}_2 \) group, forming an amide, specifically propionamide \( \text{CH}_3\text{CH}_2\text{CONH}_2 \). Let's call this Compound C.

Step 3: Perform the Hofmann Bromamide Reaction

Perform the Hofmann bromamide reaction on the amide (Compound C: \( \text{CH}_3\text{CH}_2\text{CONH}_2 \)) using \( \text{KOH} \) and \( \text{Br}_2 \). This reaction results in the removal of one carbon atom from the amide, converting it into a primary amine. The final product \( \text{D} \) is ethylamine \( \text{CH}_3\text{CH}_2\text{NH}_2 \).

Key Concepts

Propionic Acid

Propionic acid is an organic compound with the chemical formula \( \text{CH}_3\text{CH}_2\text{COOH} \). It belongs to the carboxylic acid family, which are known for their characteristic carboxyl group \(-COOH\). This group makes propionic acid capable of engaging in various chemical reactions, especially with reagents that target the \(-OH\) hydroxyl segment of the carboxyl group.

One common transformation involves converting propionic acid to acyl chloride using thionyl chloride (\( \text{SOCl}_2 \)). This reaction is crucial because acyl chlorides are more reactive than carboxylic acids, allowing them to easily participate in further transformations, such as amide synthesis. The formula for propionic acid is simple, yet its reactivity places it as a cornerstone compound in organic synthesis across many scientific and industrial applications.

Acyl Chloride Formation

The conversion of a carboxylic acid, like propionic acid, into an acyl chloride is a foundational transformation in organic chemistry. Using thionyl chloride \( \text{SOCl}_2 \), propionic acid \( \text{CH}_3\text{CH}_2\text{COOH} \) can be transformed into propionyl chloride \( \text{CH}_3\text{CH}_2\text{COCl} \).

This reaction proceeds by the attack of the sulfur atom from thionyl chloride on the hydroxyl group of the carboxylic acid. This forms an intermediate, which subsequently loses a molecule of hydrogen chloride \( \text{HCl} \) and sulfur dioxide \( \text{SO}_2 \), yielding the acyl chloride.

• First, OH functionality is replaced conjugatively with chlorine.
• The reaction preserves the R group, which is the ethyl group in propionyl chloride.

Propionyl chloride formation is a strategic transformation because acyl chlorides are key intermediates in producing various other compounds such as amides, whenever a nucleophilic agent, like ammonia, is introduced.

Amide Synthesis

Amide synthesis from an acyl chloride involves a substitution reaction where the chlorine atom is replaced by an amine group \( \text{NH}_2 \). In the context of propionic acid transformation, when propionyl chloride interacts with ammonia \( \text{NH}_3 \), propionamide \( \text{CH}_3\text{CH}_2\text{CONH}_2 \) is formed.

This reaction is quite efficient because it utilizes the high reactivity of the acyl chloride, making it easier for the ammonia to replace the chlorine atom.

• The product, propionamide, retains the carbon backbone of the original carboxylic acid.
• The reaction is exothermic, which often means it can proceed rapidly with minimum additional energy.

The formation of propionamide is crucial as it sets the stage for further transformations, such as the Hofmann Bromamide Reaction. This particular step is an excellent example of how amide linkages, which are common in biopolymers like proteins, can be synthesized in the lab.