Question

The gas evolved when methylamine reacts with HNO₂ is :

• A. NO
• B. N₂
• C. NH₃
• D. NO₂

Hint:

Primary aliphatic amines react with HNO2 to give alcohol and nitrogen gas.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Nitrous acid is made in the flask from \( \mathrm{NaNO_2} \) and HCl at low temperature. It reacts differently with different classes of amines.

Step 2: Key Formula or Approach:
Aliphatic primary amines give an unstable diazonium salt. It breaks down at once and releases nitrogen gas.

Step 3: Detailed Explanation:
Methylamine is an aliphatic primary amine. It reacts as:
\[ \mathrm{CH_3NH_2 + HNO_2 \rightarrow [CH_3N_2^+] \rightarrow CH_3OH + N_2 + H^+} \]
Overall, methanol, water and nitrogen are formed, and the gas given off is \( \mathrm{N_2} \). This is option (B).
NO and \( \mathrm{NO_2} \) are oxides of nitrogen. They are not produced in this reaction.
\( \mathrm{NH_3} \) is not formed either, because no step removes the nitrogen as ammonia.

Step 4: Final Answer:
The gas evolved is nitrogen, option (B). \[ \boxed{\text{(B)}\ \mathrm{N_2}} \]