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

Question

Reaction of 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 are 1 and 3, so the correct answer is (c) 1,3.

Step 1: Identify the Reaction Type

This reaction is a Hofmann bromamide degradation reaction, where a primary amide (RCONH2) reacts with Br2 in the presence of KOH to produce a primary amine (RNH2) with one less carbon atom.

Step 2: Consider Initial Bromination

In the first stage, the amide undergoes bromination to form the intermediate bromamide RCONHBr (option 1).

Step 3: Evaluate Formation of Isocyanate Intermediate

Under basic conditions, the bromamide loses an acyl group to form an isocyanate intermediate (R-N=C=O, option 3).

Step 4: Formation of Amine

The isocyanate intermediate then hydrolyzes in the presence of water, afforded by the hydroxide, yielding the primary amine RNH2 as the final product.

Key Concepts

Bromination of Amides

Bromination of amides is a critical step in the Hofmann bromamide degradation, where primary amides ( - RCONH2 ) undergo a transformation. Initially, amides react with bromine ( - Br2 ) in the presence of a strong base such as - KOH . This process leads to the formation of bromamide (RCONHBr). The bromination alters the amide functional group into a bromamide, setting the stage for subsequent reactions. This step is crucial as it sets up the molecule for further transformations. Understanding this step involves grasping the role of bromine as the halogenating agent that modifies the structure by replacing a portion of the amide, facilitating the eventual formation of the desired product.

Isocyanate Intermediate

The isocyanate intermediate is a pivotal part of the Hofmann degradation pathway, formed after initial bromination. Once the bromamide (RCONHBr) has been introduced, the next shift involves the migration and rearrangement under basic conditions. - The acyl group is removed in basic conditions, which results in the formation of the isocyanate ( - R-N=C=O ). This intermediate is highly reactive and serves as a crucial transitional compound. Its formation is a testament to the rearrangement capabilities under specific conditions. Upon formation, this isocyanate intermediate opens up the avenue for subsequent hydrolysis, which will push the reaction towards its final desired product. This step demonstrates a unique pathway to manipulate carbon-nitrogen bonds through an acyl migration process.

Amine Synthesis

Amine synthesis via Hofmann bromamide degradation culminates in the transformation of the isocyanate intermediate to a primary amine. Upon formation, the isocyanate is prone to hydrolysis, especially in the presence of water provided by the base (KOH) used in the reaction. - Water facilitates the cleavage of bonds within the isocyanate, ultimately leading to the release of - RNH2. This transformation effectively results in a primary amine with one less carbon atom than the original amide. The elegance of this process lies in how it rescues carbons and forms amines efficiently, showcasing a reduction in molecular complexity while achieving a valuable functional group transformation. This reaction exemplifies a smart synthetic pathway, widely appreciated for its application in organic synthesis, particularly when working with complex nitrogen compounds.