Q.Assertion: KCN reacts with methyl chloride to give methyl isocyanide.
Reason: $\mathrm{CN^-}$ is an ambident nucleophile.

Concept: Nucleophilic Substitution (Ambident Nucleophile)

KCN provides the cyanide ion ($\mathrm{CN^-}$), which is ambident — it can attack through either the carbon or the nitrogen atom. With methyl chloride ($\mathrm{CH_3Cl}$), the attack occurs via the carbon end (the more nucleophilic site), giving methyl cyanide ($\mathrm{CH_3CN}$), not methyl isocyanide. Methyl isocyanide ($\mathrm{CH_3NC}$) would require attack through nitrogen, which is less favourable with a primary alkyl ha …

The key to this question lies in understanding two things: what an ambident nucleophile is, and how the reaction of KCN with an alkyl halide actually proceeds. Let’s break it down.

1. What does “ambident nucleophile” mean?

   An ambident nucleophile is a species that has two different atoms capable of donating a lone pair and acting as a nucleophile. The cyanide ion, $\mathrm{CN^-}$, is the classic example. It can attack through the carbon atom (forming a C–C bond) or through the nitrogen atom (forming a C–N bond). The structure is $\mathrm{^{-}C \equiv N:}$ — the negative charge is mainly on carbon, but the nitrogen also has a lone pair.

2. Which atom actually attacks in the reaction with methyl chloride?

   Methyl chloride ($\mathrm{CH_3Cl}$) undergoes an $\mathrm{S_N2}$ reaction with $\mathrm{CN^-}$. In $\mathrm{S_N2}$ reactions, the nucleophile attacks the electrophilic carbon from the back side. The carbon atom of $\mathrm{CN^-}$ is more nucleophilic (more polarizable, less electronegative) than the nitrogen atom. So the attack occurs through carbon, giving methyl cyanide (also called acetonitrile), $\mathrm{CH_3CN}$.

   The reaction is:

3. When does methyl isocyanide form?

   Methyl isocyanide ($\mathrm{CH_3NC}$) is formed when the attack happens through the nitrogen atom. This requires a different reagent — typically silver cyanide ($\mathrm{AgCN}$) or a similar metal cyanide where the metal coordinates to carbon, forcing attack through nitrogen. With KCN, the carbon attack dominates overwhelmingly.

4. Evaluating the assertion and reason: …

Method: Ambident Nucleophile Analysis

Concept: An ambident nucleophile is a nucleophile that can attack from two different atoms, giving two different products.

Step 1: Identify the nucleophile

• $\mathrm{CN^-}$ (cyanide ion) is an ambident nucleophile.
• It can attack through:
  • Carbon atom → gives cyanide ($\mathrm{R-CN}$)
  • Nitrogen atom → gives isocyanide ($\mathrm{R-NC}$)

Step 2: Check the reaction given

• Methyl chloride ($\mathrm{CH_3Cl}$) reacts with KCN.
• Major product depends on the reaction conditions:
  • With KCN (ionic, in aqueous/alcoholic medium) → attack through carbon → methyl cyanide ($\mathrm{CH_3CN}$)
  • With AgCN (covalent) → attack through nitrogen → methyl isocyanide ($\mathrm{CH_3NC}$)

Step 3: Evaluate Assertion …

Common Mistakes in This Assertion-Reason Question

Mistake 1: Assuming KCN always gives isocyanide

The error: Many students think KCN always produces isocyanide (R–NC) with any alkyl halide.

Why it's wrong: The product depends on the alkyl halide structure and reaction conditions:

• With methyl chloride ($\mathrm{CH_3Cl}$), KCN gives methyl cyanide ($\mathrm{CH_3CN}$), not methyl isocyanide.
• Isocyanide formation is favoured only with primary alkyl halides under specific conditions (e.g., using AgCN, not KCN).

How to avoid: Remember that KCN is ionic ($\mathrm{K^+CN^-}$) and the $\mathrm{CN^-}$ ion attacks through the carbon end (the more nucleophilic site), giving cyanide (R–CN). For isocyanide, use AgCN (covalent, attacks through nitrogen).

Mistake 2: Confusing KCN with AgCN

The error: Treating KCN and AgCN as interchangeable reagents.

Why it's wrong:

How to avoid: Memorise this distinction — it's a classic exam trap. KCN → cyanide, AgCN → isocyanide.

Mistake 3: Misinterpreting "ambident nucleophile"

The error: Thinking that because $\mathrm{CN^-}$ is ambident, it must give both products.

Why it's wrong: Ambident means the ion has two nucleophilic sites (C and N), but which site attacks depends on the reagent's nature (ionic vs covalent), not just the fact that it's ambident.

How to avoid: Understand that "ambident" describes potential, not outcome. The actual product depends on reaction conditions and the counterion.

Mistake 4: Overlooking the Assertion-Reason logic …