Problem 40 - 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) \(\mathrm{RCO} \mathrm{O}\) (c) \(" \mathrm{R} \mathrm{CONHBr}\) (d) \(\mathrm{RNC}\)

Step-by-Step Solution

Answer

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

Step 1: Understanding Hoffmann Rearrangement

Hoffmann rearrangement is a reaction where an amide is converted to an amine with the loss of one carbon. It typically involves the formation of isocyanate as an intermediate.

Step 2: Identifying Possible Intermediates

In Hoffmann rearrangement, the intermediates usually include RNCO (isocyanate), but not RCOO- or RNC. The molecule forms from RCONH2 by reacting with bromine and base to form RNCO.

Step 3: Finding the Odd One Out

By examining the options, RNCO is a known intermediate, RCONHBr can be an intermediate stage, and RCOO- is not directly involved, nor is RNC (isocyanide) a part of Hoffmann rearrangement.

Step 4: Conclusion

RNC is the compound that does not act as an intermediate in Hoffmann rearrangement because it is not formed or needed during the rearrangement process.

Key Concepts

Reaction Intermediates

In the Hoffmann rearrangement, reaction intermediates play a crucial role in the transformation process from amide to amine. Essentially, intermediates are temporary structures formed during the reaction steps that lead to the final product.
As these reactions do not occur in a single step, recognizing these intermediates helps in understanding the entire reaction pathway.
Within the Hoffmann rearrangement, the most prominent intermediate is the isocyanate structure (RNCO).

• Intermediates act as a bridge between reactants and products.
• The identification of intermediates helps predict reaction mechanisms.

Therefore, knowing which intermediates are involved can clarify why only certain pathways are taken during the reaction process. This understanding is particularly useful when analyzing why certain compounds do not fit as intermediates, such as RCOO- or RNC.

Isocyanate Formation

Isocyanate formation is a pivotal part of the Hoffmann rearrangement. It begins with the amide reacting with bromine and a base, forming the needed intermediates, and eventually leading to isocyanate.
The key step is the reaction between the amide (RCONH2) and bromine, which halogenates the nitrogen, followed by rearrangement under the influence of the base, yielding the isocyanate (RNCO).

• This process includes nitrogen atom migration, crucial for the reaction.
• RNCO is a significant intermediate that then proceeds to further transformation.

Understanding the formation of isocyanate is important as it is a transient yet essential step that allows the subsequent conversion of the molecule into the desired amine.

Amide to Amine Conversion

The ultimate goal of the Hoffmann rearrangement is to convert an amide into an amine, reducing the carbon chain by one carbon atom. This conversion is achieved through the rearrangement steps involving isocyanate intermediates.
Upon forming the isocyanate, further reaction typically involves hydrolysis or an analogous step, adding water to break down the isocyanate into an amine.

• The carbonyl group in the amide is removed, leading to the formation of an amine.
• The newly formed amine often possesses different physical and chemical properties.

This transformation is significant because it allows chemists to strategically modify the molecular structure of compounds, making amines with varying functionalities that are vital in numerous applications.