Q.What is the product when $C_6H_5CH_2NH_2$ reacts with $HNO_2$?

The key idea is that primary aliphatic amines react with nitrous acid ($HNO_2$) to form unstable diazonium salts, which spontaneously decompose to give a carbocation that can react further.

Reasoning steps:

1. $C_6H_5CH_2NH_2$ is benzylamine — a primary amine. $HNO_2$ is generated in situ from NaNO$_2$ + HCl.
2. The amine undergoes diazotization to form an aliphatic diazonium salt: $C_6H_5CH_2N_2^+Cl^-$.
3. This aliphatic diazonium salt is highly unstable and immediately loses $N_2$, producing a benzyl carbocation ($C_6H_5CH_2^+$). …

Concept and Intuition

This question tests your understanding of nucleophilic substitution reactions — specifically, the reaction of primary amines with nitrous acid. The key here is that $HNO_2$ is unstable and generated in situ (usually from NaNO₂ + dilute HCl). It reacts with a primary amine to form a diazonium salt.

But here’s the critical distinction:

• Aromatic primary amines (like aniline, $C_6H_5NH_2$) form stable diazonium salts at low temperatures (0–5°C), which can be used in coupling reactions.
• Aliphatic primary amines (like benzylamine, $C_6H_5CH_2NH_2$) form highly unstable diazonium salts that immediately decompose, even at low temperatures, to give a carbocation — and then a mixture of products.

Benzylamine is special because it’s a benzylic primary amine: the $–CH_2NH_2$ group is attached directly to a benzene ring. The resulting carbocation ($C_6H_5CH_2^+$) is resonance-stabilized, making it relatively stable and leading to a cleaner product than with simple alkyl amines.

Step-by-Step Solution

1. Formation of nitrous acid

$HNO_2$ is prepared in the reaction mixture by mixing sodium nitrite with a dilute acid (typically HCl or $H_2SO_4$):

2. Diazotization of benzylamine

The primary amine group reacts with $HNO_2$ to form an unstable diazonium salt:

This is the same type of reaction as with any primary amine — the $–NH_2$ group is converted to $–N_2^+$.

3. Spontaneous decomposition

Unlike aromatic diazonium salts, this aliphatic diazonium salt is extremely unstable and decomposes instantly, even at 0°C, releasing nitrogen gas:

The loss of $N_2$ is driven by the formation of an extremely stable $N\equiv N$ triple bond.

4. Carbocation reacts with water …

Concept: Diazotisation & Sandmeyer Reaction / Decomposition of Primary Amines

Method: Diazotisation followed by Decomposition (for aliphatic vs. aromatic amines)

Why this method?

$HNO_2$ (nitrous acid) reacts differently with primary aliphatic amines vs. primary aromatic amines.

• Aliphatic primary amines → give nitrogen gas ($N_2$) and a carbocation, which forms an alcohol/alkene mixture.
• Aromatic primary amines → form stable diazonium salts at low temperature (0–5°C).

Here, $C_6H_5CH_2NH_2$ is benzylamine — a primary aliphatic amine (the $NH_2$ is on a $CH_2$ group, not directly on the ring).

Steps of the method:

1. Identify the amine type

   • $C_6H_5CH_2NH_2$ = benzylamine (primary aliphatic amine).

2. Reaction with $HNO_2$

   • $HNO_2$ is generated in situ from $NaNO_2 + HCl$ (cold).
   • The amine reacts to form an unstable diazonium salt (aliphatic diazonium salts are unstable even at low temperature).

3. Immediate decomposition

   • The diazonium salt spontaneously decomposes to give:
     • Nitrogen gas ($N_2$) — seen as effervescence.
     • A carbocation ($C_6H_5CH_2^+$). …

The Reaction

The compound is benzylamine ($C_6H_5CH_2NH_2$).

When treated with nitrous acid ($HNO_2$), the product is benzyl alcohol ($C_6H_5CH_2OH$), along with nitrogen gas ($N_2$).

Why?

Benzylamine is a primary aliphatic amine (the $NH_2$ is attached to a $CH_2$ group, not directly to the benzene ring).

$HNO_2$ converts primary aliphatic amines into diazonium salts, which are unstable and spontaneously decompose to give a carbocation. This carbocation reacts with water to form an alcohol.

The balanced equation:

Common Mistakes & How to Avoid Them

1. ✗ Mistaking benzylamine for an aromatic amine

• Error: Students think the $NH_2$ is attached directly to the benzene ring (like aniline), so they predict a diazonium salt as the product.
• Why it’s wrong: In benzylamine, the $NH_2$ is on a side chain — it’s aliphatic, not aromatic. Aromatic diazonium salts are stable at low temperatures; aliphatic ones decompose instantly.
• How to avoid: Always check where the $NH_2$ is attached. If it’s on a carbon that is part of an alkyl chain (even if that chain is attached to a ring), treat it as aliphatic.

2. ✗ Forgetting that $N_2$ gas is released

• Error: Writing only the alcohol as the product, omitting nitrogen gas.
• Why it’s important: $N_2$ evolution is a key test for primary aliphatic amines — it’s often asked in exams.
• How to avoid: Memorize: primary aliphatic amine + $HNO_2$ → alcohol + $N_2$ + $H_2O$. Always include $N_2$ in the product list.

3. ✗ Writing the wrong alcohol

• Error: Writing phenol ($C_6H_5OH$) instead of benzyl alcohol.
• Why it’s wrong: The carbocation forms on the $CH_2$ group, not on the ring. Water attacks that carbon, giving $C_6H_5CH_2OH$.
• How to avoid: Track the carbon that originally held the $NH_2$ — that’s where the $OH$ ends up.

4. ✗ Confusing with the reaction of aniline

• Error: Applying the same logic as for aniline ($C_6H_5NH_2$), which gives a stable diazonium salt at 0–5°C.
• Why it’s wrong: Aniline is an aromatic primary amine — its diazonium salt is stable in cold conditions. Benzylamine is aliphatic, so no stable diazonium salt forms.
• How to avoid: Distinguish clearly:
  • Aromatic primary amine → stable diazonium salt (cold) …