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

Step 1: Recall the overall purpose of the reaction.
Hoffmann bromamide degradation converts an amide into a primary amine that has one carbon less than the starting amide.
Step 2: Check which reagents this conversion actually needs.
The starting material has to be a primary amide, $\text{R-CONH}_2$, so reagent (i) is required. Bromine, reagent (iii), is needed to brominate the nitrogen first. A base, sodium hydroxide, reagent (ii), is needed both to form the N-bromoamide and to hydrolyse the isocyanate formed later.
Step 3: Rule out the reagent that does not belong.
Chloroform, reagent (iv), is used in a completely different reaction, the carbylamine test for detecting primary amines, and plays no part in the Hoffmann degradation. \[ \text{R-CONH}_2 + \text{Br}_2 + 4\text{NaOH} \rightarrow \text{R-NH}_2 + \text{Na}_2\text{CO}_3 + 2\text{NaBr} + 2\text{H}_2\text{O} \]
Step 4: State the final set of reagents.
Only (i), (ii) and (iii) are used.
\[ \boxed{(i), (ii) \text{ and } (iii)} \]