AIIMS 2005 Chemistry Rearrangement Reactions MCQ Question

Explanation of Hofmann Rearrangement:

The Hofmann rearrangement is a well-known reaction in organic chemistry that involves the conversion of primary amides into primary amines with one fewer carbon atom. This reaction is typically initiated by the treatment of an amide with halogens (like bromine) and a strong base (like sodium hydroxide). The overall reaction can be summarized by the following transformation:

$\text{RCONH}_2 \xrightarrow{\text{Br}_2, \text{NaOH}} \text{RNH}_2 + \text{RCOOH}$

In this mechanism, the amide undergoes several key steps, including the formation of an isocyanate intermediate, followed by rearrangement and hydrolysis to yield the final products.

Analyzing the Options:

A) RNCO (Isocyanate)
This compound is a key intermediate in the Hofmann rearrangement. It forms from the reaction of the amide with halogen and base. The amide is first converted to an isocyanate, which is crucial for the rearrangement process.

B) RCOCl (Acyl Chloride)
Acyl chlorides can also participate in reactions similar to the Hofmann rearrangement. Although they are not the direct intermediates in the traditional sense, they can yield isocyanates upon reaction with amines and can undergo rearrangement. However, in the context of the direct mechanism of Hofmann rearrangement, they do not play a primary role.

C) RCO₂NHBr (Bromoamide)
This compound acts as an intermediate in the Hofmann rearrangement. The bromoamide is formed when the primary amide is treated with bromine and base. It then undergoes further transformations to yield isocyanate, which leads to the final products.

D) RNC (Tertiary Amine)
This is the correct answer. Tertiary amines do not participate in the Hofmann rearrangement as intermediates. The rearrangement specifically starts with primary amides, and tertiary amines do not have the necessary structure (lacking the carbonyl group required for conversion) to form isocyanate or participate in the rearrangement. Therefore, RNC does not act as an intermediate in this reaction.

Summary of Why D is Correct:

• RNC (Tertiary Amine): It does not act as an intermediate because it is not a primary amide and lacks the carbonyl functionality necessary for the reaction. Tertiary amines cannot undergo the Hofmann rearrangement as they do not form isocyanates.

Why Other Options are Incorrect:

• RNCO: Directly involved as an intermediate.
• RCOCl: Although not a direct intermediate, it can lead to isocyanate formation under certain conditions.
• RCO₂NHBr: This compound is indeed an intermediate in the rearrangement mechanism.

In conclusion, the Hofmann rearrangement specifically requires primary amides and their corresponding intermediates for the reaction to occur, while tertiary amines do not fit into this pathway. Thus, the correct answer is D) RNC.