Which class of amines reacts with nitrous acid (HNO2) to form diazonium salts in aromatic systems and alcohols in aliphatic systems, a characteristic test in organic chemistry?

Introduction / Context:
Reactions of amines with nitrous acid are very important in organic chemistry, especially for distinguishing primary, secondary and tertiary amines. Aromatic primary amines can form stable diazonium salts, while aliphatic primary amines give alcohols and nitrogen gas. This question asks which class of amines shows the characteristic reaction with nitrous acid that leads to diazonium salt formation in aromatic systems and is widely used as a diagnostic test.

Given Data / Assumptions:

• We are considering the reaction of amines with nitrous acid (HNO2).
• Aromatic primary amines can form diazonium salts in cold conditions.
• Aliphatic primary amines give alcohols with evolution of nitrogen gas.
• Secondary and tertiary amines behave differently with nitrous acid.

Concept / Approach:
The classic distinction is that primary amines (especially primary aromatic amines) react with nitrous acid at low temperature to form diazonium salts, such as benzene diazonium chloride. Aliphatic primary amines form unstable diazonium salts that decompose to alcohols and nitrogen gas. Secondary amines generally form N nitrosoamines with nitrous acid. Tertiary amines can form salts but do not give diazonium salts in the same characteristic way. Quaternary ammonium salts do not show the diazotisation reaction. Therefore, primary amines are the correct class that shows this well known reaction.

Step-by-Step Solution:

Verification / Alternative check:
Standard organic chemistry texts emphasise that the diazotisation reaction is a characteristic test for primary aromatic amines and is carried out with sodium nitrite and hydrochloric acid at 0 to 5 degrees Celsius to generate nitrous acid in situ. They also state that secondary amines form N nitrosoamines and tertiary amines do not give diazonium salts. Laboratory tests for primary amines often involve the evolution of nitrogen gas and formation of stable diazonium salts in aromatic cases. These points confirm that primary amines are the required answer.

Why Other Options Are Wrong:

• Secondary amines: React with nitrous acid to form N nitrosoamines, not diazonium salts.
• Tertiary amines: May form addition products or salts, but they do not undergo classical diazotisation to give diazonium salts.
• All of these: Incorrect because only primary amines show the characteristic diazonium salt formation reaction.
• Quaternary ammonium salts: Already fully substituted and do not show typical amine reactions with nitrous acid.

Common Pitfalls:
Students often memorise that amines react with nitrous acid without distinguishing between the classes. This can lead to choosing options such as all of these. It is important to remember the specific products: primary amines give diazonium salts or alcohols plus nitrogen, secondary amines give N nitrosoamines, and tertiary amines essentially do not give diazotisation products. Keeping a small comparison table for these reactions helps avoid confusion.

Final Answer:
The class of amines that reacts with nitrous acid to form diazonium salts in aromatic systems is Primary amines.