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Alcoholic Potassium Cyanide (KCN)

The primary reagent for synthesizing Alkyl Cyanides (Nitriles).

By chemca Team • Updated Jan 2026

Alcoholic Potassium Cyanide (KCN) is a key reagent in organic synthesis used to convert Alkyl Halides into Alkyl Cyanides (Nitriles). This reaction is crucial for increasing the carbon chain length (Ascent of Series).

1. Reaction with Alkyl Halides

Formation of Cyanides

When an alkyl halide is heated with an alcoholic solution of Potassium Cyanide, the major product formed is Alkyl Cyanide (Nitrile).

$$ R-X + KCN \xrightarrow{C_2H_5OH, \Delta} \underset{\text{Alkyl Cyanide}}{R-C \equiv N} + KX $$

Example: Ethyl Bromide to Propane Nitrile.

$$ CH_3CH_2Br + KCN \rightarrow CH_3CH_2CN + KBr $$

2. Mechanism: Why Cyanide?

Ambident Nucleophile

The cyanide ion ($CN^-$) is an Ambident Nucleophile, meaning it has two nucleophilic centers: Carbon and Nitrogen.

$$ [:C \equiv N:]^- $$

Reason for KCN forming Cyanide:

• Ionic Nature: KCN is an ionic compound ($K^+CN^-$). In solution, it dissociates to give free cyanide ions.
• Attack Site: Both C and N can donate electrons. However, the $C-C$ bond formed (if attacked via Carbon) is stronger and more stable than the $C-N$ bond (if attacked via Nitrogen).
• Therefore, the attack occurs preferentially through the Carbon atom, resulting in Alkyl Cyanides.

3. Comparison with Silver Cyanide (AgCN)

Formation of Isocyanides

If Silver Cyanide ($AgCN$) is used instead of KCN, the major product is Alkyl Isocyanide (Isonitrile).

$$ R-X + AgCN \xrightarrow{\Delta} \underset{\text{Alkyl Isocyanide}}{R-N \equiv C} + AgX $$

Reason: $AgCN$ is predominantly Covalent. The carbon atom is bonded to Silver and is not free to attack. Nucleophilic attack can only take place through the lone pair on Nitrogen, forming the isocyanide product.

Reagent	Bond Nature	Nucleophilic Atom	Product
KCN	Ionic	Carbon (Mainly)	Cyanide ($R-CN$)
AgCN	Covalent	Nitrogen	Isocyanide ($R-NC$)

4. Synthetic Utility (Ascent of Series)

Reactions of Alkyl Cyanides

The cyanide group ($ -CN $) introduces an extra carbon atom and can be converted into various functional groups.

1. Complete Hydrolysis: Yields Carboxylic Acid.

$$ R-CN \xrightarrow{H_3O^+, \Delta} R-COOH + NH_3 $$

2. Partial Hydrolysis: Yields Amide.

$$ R-CN \xrightarrow{Conc. HCl / H_2O} R-CONH_2 $$

3. Complete Reduction (Mendius Reaction): Yields Primary Amine.

$$ R-CN \xrightarrow{LiAlH_4 \text{ or } Na/C_2H_5OH} R-CH_2NH_2 $$

4. Stephen Reduction: Yields Aldehyde.

$$ R-CN \xrightarrow{1. SnCl_2/HCl, 2. H_3O^+} R-CHO $$

Knowledge Check

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