Question

Which of the following reagents are used to prepare primary amines by Hoffmann bromamide degradation reaction ?
(i) \( \text{R}-\text{C}(=\text{O})-\text{NH}_2 \)
(ii) NaOH
(iii) \( \text{Br}_2 \)
(iv) \( \text{CHCl}_3 \)

• A. (i), (ii) and (iv)
• B. (i) and (iii)
• C. (i), (ii) and (iii)
• D. (i), (iii) and (iv)

Hint:

This reaction is a "descending" reaction as it decreases the carbon chain length by one.
The intermediate of this reaction is an alkyl isocyanate \( (\text{R-N=C=O}) \).
Yields are usually high, and it is a preferred industrial method for certain amines.

Answer 1

The Correct Option is C

Concept:

• Hoffmann bromamide degradation is a unique method for synthesizing primary amines.

• It involves the "degradation" of an amide, resulting in an amine with one fewer carbon atom than the starting material.

• The reaction mechanism involves an intramolecular rearrangement.
Step 1: Identifying required reagents
The reaction requires a primary amide \( (\text{R}-\text{CONH}_2) \), which matches reagent (i).
The reaction uses bromine \( (\text{Br}_2) \) as the halogen, which is reagent (iii).
An aqueous or alcoholic alkali solution is needed, typically Sodium Hydroxide \( (\text{NaOH}) \), matching reagent (ii).

Step 2: Analyzing the chemical transformation
The amide reacts with bromine and base to form an N-bromoamide.
This is followed by the migration of the alkyl group and the formation of an isocyanate.
Hydrolysis of the isocyanate yields the final primary amine and carbonate.
\[ \text{R-CONH}_2 + \text{Br}_2 + 4\text{NaOH} \rightarrow \text{R-NH}_2 + \text{Na}_2\text{CO}_3 + 2\text{NaBr} + 2\text{H}_2\text{O} \]

Step 3: Reagent verification
Reagent (iv) \( \text{CHCl}_3 \) is used in the Carbylamine reaction for testing amines, not for this synthesis.
Therefore, reagents (i), (ii), and (iii) are essential.