Problem 46 - A Complete Resource Book in Chemistry for JEE Main

Question

Among the following which one does not act as an intermediate in Hoffmann rearrangement? (a) RNCO (b) RCOÑ (c) RCONHBr (d) RNC

Step-by-Step Solution

Answer

Option (d) RNC does not act as an intermediate in Hoffmann rearrangement.

Step 1: Identify the Hoffmann rearrangement process

The Hoffmann rearrangement involves the conversion of primary amides ( RCONH2) into primary amines (RNH2) with one fewer carbon atom. This process typically involves steps such as the formation of an isocyanate intermediate.

Step 2: Recognize intermediates in Hoffmann rearrangement

In the Hoffmann rearrangement, the intermediates often include RCONHBr and RNCO. The RCONHBr is a brominated amide and RNCO is an isocyanate intermediate.

Step 3: Evaluate each option

- Option (a) RNCO is an isocyanate, a known intermediate in Hoffmann rearrangement. - Option (b) RCOÑ is not a standard notation; it could imply a species that typically doesn’t appear in Hoffmann rearrangement. - Option (c) RCONHBr, as noted, is a brominated amide intermediate. - Option (d) RNC is an isocyanide, not typically involved in Hoffmann rearrangement.

Step 4: Determine the non-intermediate

From the evaluations, RNC is the option that does not act as an intermediate in the Hoffmann rearrangement process as it represents isocyanide, not appearing in the typical reaction pathway.

Key Concepts

Intermediate Species in Hoffmann Rearrangement

During the Hoffmann rearrangement, the transformation of a primary amide into a primary amine occurs. This intricate reaction mechanism involves certain intermediate species, which play crucial roles in the progression of the reaction. Among these, the unique intermediates that are crucial for this rearrangement to occur include the brominated amide and the isocyanate. These intermediates are formed sequentially as the reaction progresses. Understanding these intermediate species provides insight into how the rearrangement results in the shortening of the carbon chain by one carbon atom.

• The brominated amide serves as one of the intermediate species. This species results from the initial reaction of a primary amide with a bromine source, leading to the substitution of a hydrogen atom on the nitrogen with bromine.
• The isocyanate intermediate is another critical species that facilitates the final transformation into a primary amine. This compound, known for its reactivity, undergoes further transformation in the reaction pathway.

By identifying and examining these intermediates, the detailed mechanism becomes clearer, aiding in the comprehension of the overall process.

Isocyanate Formation in Hoffmann Rearrangement

Isocyanate formation is a vital step in the Hoffmann rearrangement mechanism. Once the brominated amide is generated, the rearrangement process advances towards the formation of an isocyanate. This intermediate is characterized by the chemical group –N=C=O. The isocyanate formation occurs through a complex sequence of intramolecular reactions.

• Initially, the rearrangement of the brominated amide takes place. This step involves the migration of an alkyl group from the carbonyl carbon to the nitrogen atom.
• This intramolecular shift eliminates the bromine atom, facilitating the creation of the isocyanate.

The formation of the isocyanate is pivotal as it acts as a precursor to the final conversion into a primary amine. Once formed, the isocyanate possesses inherent reactivity that allows it to further undergo hydrolysis and finally yield the desired amine product, completing the Hoffmann rearrangement cycle.

Role of Brominated Amide in Hoffmann Rearrangement

The brominated amide is a critical intermediate in the Hoffmann rearrangement. The reaction begins with the conversion of a primary amide into this brominated species, specifically denoted as RCONHBr. This initial step is often facilitated by the addition of bromine or a bromine-containing reagent. The creation of a brominated amide sets the stage for the subsequent steps in the rearrangement process.

• The bromine atom in the brominated amide plays an essential role in stabilizing the transition state for the rearrangement. It provides a favorable leaving group, assisting the rearrangement to proceed smoothly.
• A key feature of this rearrangement is the fact that the brominated amide is only transiently formed. This ephemeral nature requires the reaction conditions to precisely control the environment, ensuring efficient progression to the next step.

Thorough understanding of the role and behavior of the brominated amide is crucial as it significantly influences the overall yield and efficiency of the Hoffmann rearrangement.