Q.Write the reactions of

Concept: Nucleophilic Substitution Reactions (Diazotisation and its consequences)

The key difference lies in the stability of the diazonium salt formed as an intermediate.

Reasoning:

1. Aliphatic primary amines react with nitrous acid ($\ce{HNO2}$, generated in situ from $\ce{NaNO2 + HCl}$) to form a highly unstable aliphatic diazonium salt. This salt immediately decomposes, releasing nitrogen gas ($\ce{N2}$) and forming a carbocation, which then gives a mixture of alcohols, alkenes, and alkyl halides.
2. Aromatic primary amines (e.g., aniline) react with nitrous acid at low temperatures (0–5°C) to form a stable arenediazonium salt. This salt is a key synthetic intermediate and does not decompose spontaneously at that temperature.

Reactions: …

The Concept: Why Nitrous Acid Attacks Amines

Nitrous acid ($\ce{HNO2}$) is generated in situ by reacting sodium nitrite ($\ce{NaNO2}$) with a mineral acid like $\ce{HCl}$ or $\ce{H2SO4}$ at low temperatures ($0-5^\circ\text{C}$). The key reactive species is the nitrosonium ion ($\ce{NO+}$), which is a strong electrophile.

Primary amines ($\ce{R-NH2}$) have a lone pair on nitrogen that can attack the electrophilic $\ce{NO+}$. This initial attack leads to an N-nitroso intermediate, which then undergoes tautomerization and elimination to form a diazonium salt ($\ce{R-N2+}$). The fate of this diazonium salt depends entirely on whether the R group is aliphatic or aromatic.

1. Reaction with Aliphatic Primary Amines

When an aliphatic primary amine (e.g., $\ce{CH3CH2NH2}$) reacts with nitrous acid, the diazonium salt forms but is extremely unstable. It spontaneously decomposes to give a carbocation and nitrogen gas ($\ce{N2}$).

The carbocation then reacts with water (the solvent) to form an alcohol, or can undergo elimination to give an alkene, or rearrangement to give a more stable carbocation. The major product is usually a mixture of alcohols and alkenes.

General reaction:

Example with ethylamine:

The nitrogen gas evolution is visible as bubbles — this is a classic test for a primary aliphatic amine.

2. Reaction with Aromatic Primary Amines

Aniline ($\ce{C6H5NH2}$) and other aromatic primary amines react similarly at first, forming the diazonium salt. However, the aromatic diazonium ion is stabilized by resonance with the benzene ring. The positive charge is delocalized into the $\pi$-system, making it much more stable than its aliphatic counterpart.

Reaction at $0-5^\circ\text{C}$:

This is benzenediazonium chloride — a pale yellow solution that is stable only at low temperatures. If warmed above $5^\circ\text{C}$, it decomposes to give phenol and nitrogen gas:

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Method: Reaction of Amines with Nitrous Acid (Diazotization & Deamination)

This is a concept-first method — understand the why before the what.

Why nitrous acid ($\ce{HNO2}$) is special

• Nitrous acid is unstable and is prepared in situ by reacting $\ce{NaNO2}$ with a mineral acid (like $\ce{HCl}$ or $\ce{H2SO4}$).
• It generates the nitrosonium ion ($\ce{NO+}$), which is the actual electrophile.
• The reaction outcome depends entirely on whether the amine is aliphatic or aromatic, and whether it is primary, secondary, or tertiary.

(i) Aromatic Primary Amine → Diazotization

Reaction:

Aniline ($\ce{C6H5NH2}$) reacts with $\ce{HNO2}$ at 0–5°C to form a diazonium salt.

Key points:

• Temperature is critical — above 5°C, the diazonium salt decomposes to phenol.
• The product is a stable diazonium salt (at low temperature), which is a key intermediate for coupling reactions (azo dyes).
• Mechanism: $\ce{NO+}$ attacks the lone pair on the amino nitrogen → loss of water → formation of diazonium group ($\ce{-N2+}$).

(ii) Aliphatic Primary Amine → Deamination (No Diazonium Salt)

Reaction:

An aliphatic primary amine (e.g., ethylamine, $\ce{C2H5NH2}$) reacts with $\ce{HNO2}$ to give a mixture of products — mainly alcohols, alkenes, and alkyl halides.

(plus some $\ce{C2H5Cl}$ if $\ce{HCl}$ is used, and some $\ce{C2H4}$)

Key points:

• The diazonium salt formed is unstable even at low temperature — it spontaneously decomposes to give a carbocation.
• The carbocation then undergoes:
  • Nucleophilic attack by water → alcohol
  • Elimination → alkene …

Here is a breakdown of the common mistakes students make when writing the reactions of amines with nitrous acid ($HNO_2$), and how to avoid them.

The Core Concept (The "Why")

Nitrous acid ($HNO_2$) is unstable and is always prepared in situ (in the reaction mixture) by reacting sodium nitrite ($NaNO_2$) with a mineral acid (like $HCl$ or $H_2SO_4$). The key difference in reactivity comes from the stability of the intermediate diazonium ion formed.

• Aliphatic primary amines form highly unstable aliphatic diazonium salts that immediately decompose to give a mixture of products (mostly alkenes and alcohols via carbocation rearrangement).
• Aromatic primary amines form stable arenediazonium salts at low temperatures (0-5°C), which are valuable intermediates in organic synthesis.

Common Mistake #1: Forgetting the In Situ Preparation of $HNO_2$

The Mistake: Students write the reaction as if $HNO_2$ is a stable reagent added directly from a bottle. They write:

$RNH_2 + HNO_2 \rightarrow ...$

Why it's wrong: $HNO_2$ is highly unstable and decomposes rapidly. In every exam reaction, you must show it being generated.

How to Avoid: Always write the reaction showing the source of $HNO_2$:

• Reagent: $NaNO_2 + HCl$ (or $H_2SO_4$)
• Condition: Cold (0-5°C) for aromatic amines.

Correct representation:

Common Mistake #2: Writing the Same Product for Both Aromatic and Aliphatic Amines

The Mistake: Students write that both give a diazonium salt that can be used for substitution.

Why it's wrong: Aliphatic diazonium salts are so unstable they decompose instantly at any temperature. They cannot be isolated or used for substitution reactions.

How to Avoid: Memorize the fate of each:

Correct representation for aliphatic:

Common Mistake #3: Forgetting the Temperature Condition for Aromatic Amines

The Mistake: Writing the aromatic reaction without specifying temperature, or writing it at room temperature.

Why it's wrong: At room temperature, the arenediazonium salt decomposes to give phenol ($C_6H_5OH$) and nitrogen gas. You lose the valuable intermediate.

How to Avoid: Always write 0-5°C (ice-cold condition) for the formation of the diazonium salt. If the question asks for the reaction with nitrous acid, the stable diazonium salt is the expected product.

Correct:

Common Mistake #4: Writing the Diazonium Salt Structure Incorrectly

The Mistake: Writing the diazonium group as $-N=N-$ (azo group) or as $-NH_2$ (amine).

Why it's wrong: The diazonium group is $-N_2^+$ (a nitrogen-nitrogen triple bond with a positive charge on the terminal nitrogen). The azo group ($-N=N-$) is a different functional group found in azo dyes.

How to Avoid: Draw the structure carefully: …