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Preparation of Cyanides and Isocyanides

Synthesis of Nitriles ($R-CN$) and Isonitriles ($R-NC$).

By chemca Team • Updated Jan 2026

Cyanides (Nitriles) and Isocyanides (Isonitriles) are functional isomers. Their method of preparation often depends on the nature of the nucleophile used in substitution reactions.

1. Preparation of Cyanides (Nitriles)

A. From Alkyl Halides (with KCN)

Alkyl halides react with alcoholic Potassium Cyanide ($KCN$) to form Alkyl Cyanides as the major product.

$$ R-X + KCN \text{ (alc.)} \xrightarrow{\Delta} R-CN + KX $$

Reason: $KCN$ is predominantly ionic ($K^+ + CN^-$). The cyanide ion is an ambident nucleophile, but the $C-C$ bond is stronger than the $C-N$ bond. Therefore, attack occurs through Carbon.

B. From Amides (Dehydration)

Amides are dehydrated to nitriles by heating with strong dehydrating agents like Phosphorus Pentoxide ($P_2O_5$) or Thionyl Chloride ($SOCl_2$).

$$ R-CONH_2 \xrightarrow{P_2O_5, \Delta} R-CN + H_2O $$

C. From Diazonium Salts (Sandmeyer Reaction)

Aryl cyanides are prepared by treating benzene diazonium chloride with Cuprous Cyanide ($CuCN$) and $KCN$.

$$ C_6H_5N_2^+Cl^- \xrightarrow{CuCN/KCN} C_6H_5CN + N_2 $$

2. Preparation of Isocyanides (Isonitriles)

A. From Alkyl Halides (with AgCN)

Alkyl halides react with Silver Cyanide ($AgCN$) to form Alkyl Isocyanides as the major product.

$$ R-X + AgCN \text{ (alc.)} \xrightarrow{\Delta} R-NC + AgX $$

Reason: $AgCN$ is predominantly covalent. The carbon atom is not free to attack. Nucleophilic attack occurs through the lone pair on Nitrogen, forming the isocyanide ($R-N \equiv C$).

B. From Primary Amines (Carbylamine Reaction)

Primary amines (aliphatic or aromatic) react with Chloroform and alcoholic KOH to form foul-smelling Isocyanides.

$$ R-NH_2 + CHCl_3 + 3KOH \xrightarrow{\Delta} R-NC + 3KCl + 3H_2O $$

This reaction is specific for $1^\circ$ amines only.

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