Problem 78 - A Complete Resource Book in Chemistry for JEE Main

Question

In the mechanism of Hoffimann reaction, which intermediate rearranges to alkyl isocyanate? (a) nitrene (b) amide (c) nitroso (d) bromamide

Step-by-Step Solution

Answer

The intermediate that rearranges to alkyl isocyanate is bromamide (d).

Step 1: Understanding the Hoffmann Reaction

The Hoffmann rearrangement is a chemical reaction in which a primary amide is converted to a primary amine with the loss of one carbon atom. This reaction typically involves stages including the formation of a bromamide intermediate.

Step 2: Identifying the Key Intermediate

In the Hoffmann reaction, a primary amide reacts with bromine in an alkaline solution to initially form a bromamide. This bromamide then rearranges.

Step 3: Recognizing the Rearrangement Process

The bromamide intermediate undergoes a rearrangement where the carbonyl group excides from the nitrogen, leading to the formation of an alkyl isocyanate intermediate before hydrolysis to a primary amine.

Step 4: Conclusion on Rearranging Intermediate

Based on the mechanism of the Hoffmann rearrangement, the bromamide is the key intermediate that rearranges to form the alkyl isocyanate.

Key Concepts

Understanding Primary Amines

Primary amines are organic compounds that contain a nitrogen atom bonded to one hydrocarbon group and two hydrogen atoms. They are essential in various biological processes and synthetic chemistry.
Some basic characteristics of primary amines include:

• The nitrogen atom in primary amines has a lone pair of electrons, making them nucleophilic and reactive.
• They are generally basic, meaning they can accept a proton to form an ammonium ion.
• Primary amines have a distinctive fishy odor and are typically polar due to the N-H bond.

An example of a primary amine is methylamine, where the nitrogen is attached to a single methane group. The Hoffmann rearrangement, a notable reaction in synthetic chemistry, often targets primary amides to produce primary amines after rearrangement and hydrolysis processes.

Formation and Role of Bromamide Intermediate

The bromamide intermediate plays a critical role in the Hoffmann rearrangement mechanism. It is a pivotal step that involves significant transformation in the conversion process from a primary amide to primary amine.
In the initial step of the Hoffmann reaction, a primary amide reacts with bromine in an alkaline solution, typically sodium or potassium hydroxide, to form a bromamide. This reaction introduces the bromine atom to the amide nitrogen, forming a haloamide structure.

• Bromamide intermediate is prone to rearrangement due to its reactive structure.
• The carbonyl group excides from the nitrogen, paving the way for subsequent steps.
• This transformation is crucial as it sets the stage for the formation of an alkyl isocyanate.

Understanding the role and formation of bromamide intermediate is key in mastering the Hoffmann rearrangement as it directly leads to the creation of the alkyl isocyanate, leading to primary amine synthesis.

Alkyl Isocyanate: A Transitional Stage

During the Hoffmann rearrangement, one intermediary stage involves forming an alkyl isocyanate from the bromamide intermediate. Alkyl isocyanates are highly reactive compounds characterized by an -N=C=O group.
This formation is a crucial rearrangement process where the carbonyl group is eliminated, allowing the remaining structure to reorganize into an isocyanate.

• Alkyl isocyanates are reactive and generally short-lived, swiftly proceeding to further reactions.
• In the Hoffmann sequence, isocyanates hydrolyze to yield a primary amine and carbon dioxide as by-products.
• The transition through an alkyl isocyanate is pivotal as it symbolizes the conversion stage that enables carbon elimination.

Thus, the generation of alkyl isocyanate is a critical step in the pathway from a bromamide to a primary amine, completing the rearrangement sequence in the Hoffmann mechanism.