Question

Statement I: Primary aliphatic amines react with HNO₂ to give unstable diazonium salts.
Statement II: Primary aromatic amines react with HNO₂ to form diazonium salts which are stable even above 300 K.
Choose the most appropriate answer from the options given below:

• A. Both Statement I and Statement II are correct.
• B. Both Statement I and Statement II are incorrect
• C. Statement I is correct but Statement II is incorrect.
• D. Statement I is incorrect but Statement II is correct.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The reaction of primary amines with nitrous acid (\(HNO_2\)) leads to the formation of diazonium salts. The stability of these salts is determined by the resonance stabilization provided by the substituent group.
Step 2: Detailed Explanation:
Analysis of Statement I:
Primary aliphatic amines (\(R-NH_2\)) react with nitrous acid to form alkyl diazonium salts (\(R-N_2^+Cl^-\)).
These salts are highly unstable because there is no resonance to stabilize the positive charge on the nitrogen. They decompose immediately, even at \(0^\circ \text{C}\), to evolve nitrogen gas and form carbocations, which then produce alcohols, alkenes, or alkyl halides.
\[ R-NH_2 + HNO_2 + HCl \xrightarrow{0-5^\circ \text{C}} [R-N_2^+Cl^-] \text{ (Unstable)} \xrightarrow{H_2O} ROH + N_2 \uparrow + HCl \]
Thus, Statement I is correct.

Analysis of Statement II:
Primary aromatic amines (like aniline) react with \(HNO_2\) at low temperatures (\(273\text{--}278 \text{ K}\)) to form aryl diazonium salts.
These are relatively stable at these low temperatures due to the dispersal of the positive charge through resonance with the benzene ring.
However, they are unstable above \(278 \text{ K}\) (\(5^\circ \text{C}\)). Upon warming (certainly above \(300 \text{ K}\)), they decompose to form phenols and nitrogen gas.
\[ Ar-N_2^+Cl^- + H_2O \xrightarrow{\text{Warm}} Ar-OH + N_2 \uparrow + HCl \]
Thus, Statement II is incorrect.
Step 3: Final Answer:
Statement I is correct but Statement II is incorrect.