Problem 71 - A Complete Resource Book in Chemistry for JEE Main

Question

Reaction of \(\mathrm{RCONH}_{2}\) with a mixture of \(\mathrm{Br}_{2}\) and \(\mathrm{KOH}\) gives \(\mathrm{RNH}_{2}\) as the main product. The intermediate involved in the reaction are 1\. RCONHBr 2\. \(\mathrm{R}-\mathrm{NHBr}\) 3\. \(\mathrm{R}-\mathrm{N}=\mathrm{C}=\mathrm{O}\) 4\. \(\mathrm{RCONBr}_{2}\) (a) \(1,2,3\) (b) \(1,3,4\) (c) 1,3 (d) 1,2

Step-by-Step Solution

Answer

The intermediates in the reaction are (1) and (3), so the answer is (c) 1,3.

Step 1: Understand the Reaction

The given problem involves the reaction of an amide (\(\mathrm{RCONH}_{2}\)) with bromine (\(\mathrm{Br}_{2}\)) and potassium hydroxide (\(\mathrm{KOH}\)), which leads to the formation of a primary amine (\(\mathrm{RNH}_{2}\)). This is a classic example of the Hofmann bromamide reaction.

Step 2: Recall the Mechanism

In the Hofmann bromamide reaction, an amide reacts with \(\mathrm{Br}_{2}\) and \(\mathrm{KOH}\). The mechanism involves the formation of an intermediary brominated compound \(\mathrm{RCONHBr}\), which undergoes rearrangement to eventually form the isocyanate \(\mathrm{R}-\mathrm{N}=\mathrm{C}=\mathrm{O}\). The isocyanate is then hydrolyzed to the primary amine \(\mathrm{RNH}_{2}\).

Step 3: Identify the Intermediates

Based on the mechanism, the intermediates formed are \(\mathrm{RCONHBr}\) and \(\mathrm{R}-\mathrm{N}=\mathrm{C}=\mathrm{O}\). \(\mathrm{R}-\mathrm{NHBr}\) and \(\mathrm{RCONBr}_{2}\) are not intermediates in this reaction.

Step 4: Choose Correct Options

From the intermediates we've identified (\(\mathrm{RCONHBr}\) and \(\mathrm{R}-\mathrm{N}=\mathrm{C}=\mathrm{O}\)), the correct options would be (1) and (3). Therefore, the correct answer is option (c), which includes intermediates 1 and 3.

Key Concepts

Reaction Mechanism

The Hofmann Bromamide Reaction is a fascinating process that transforms an amide into a primary amine. This occurs through a series of steps involving the action of bromine (\(\mathrm{Br}_2\)) and potassium hydroxide (\(\mathrm{KOH}\)). Initially, the amide structure \(\mathrm{RCONH}_2\) reacts with bromine. This interaction facilitates the formation of a brominated intermediary compound \(\mathrm{RCONHBr}\).
The reaction doesn’t stop there, however. Subsequent steps involve a rearrangement, where the brominated compound shifts its atomic positions to form an isocyanate intermediate \(\mathrm{R}-\mathrm{N}=\mathrm{C}=\mathrm{O}\). Eventually, this isocyanate is hydrolyzed. Hydrolysis refers to a reaction involving the breaking of a bond in the presence of water, leading to the production of a primary amine \(\mathrm{RNH}_2\).
This transformation underscores the importance of understanding chemical reaction mechanisms, as each step provides insights into how raw materials are converted into significant end products, in this case, the primary amine.

Amide Conversion

In the realm of organic chemistry, the conversion of amides to other functional groups is essential, especially for synthesizing various compounds. The Hofmann Bromamide Reaction helps tackle this conversion efficiently. Specifically, it converts an amide (\(\mathrm{RCONH}_2\)) into a primary amine (\(\mathrm{RNH}_2\)).
Amides generally exhibit high stability due to their resonance structures, where electrons are delocalized. This stability makes them less reactive under normal conditions. However, using bromine and strong base like \(\mathrm{KOH}\) allows the disruption of these resonance structures. Through the reaction, the amide group loses its carbonyl component, and the nitrogen acquires a proton, forming the primary amine.
The transformation from a neutral amide to a primary amine offers chemists a reliable pathway to create amines, fundamental building blocks in synthesis and drug design.

Intermediate Compounds

During the process of the Hofmann Bromamide Reaction, several key intermediates are formed. Understanding these intermediates can enhance grasp on how the reaction proceeds and highlights the intricate nature of chemical reactions.
The first intermediate encountered is \(\mathrm{RCONHBr}\) which is formed as soon as the \(\mathrm{Br}_2\) interacts with the amide group. This compound sets the stage for a rearrangement to occur, demonstrating the ability of reactions to shift molecular structures temporarily.
Following this, the isocyanate \(\mathrm{R}-\mathrm{N}=\mathrm{C}=\mathrm{O}\) intermediate takes over. This is a pivotal transition state in the reaction mechanism, facilitating the final conversion to a primary amine. It's important to note that these intermediates don't remain stable but are crucial steps towards the synthesis of the final product.
Understanding these intermediates not only illustrates the transitional nature of molecules during reactions but also aids in predicting the behavior of compounds under similar conditions.