Cyanides and Isocyanides — Core Principles

Core Principles

Cyanides, also known as nitriles, are organic compounds featuring a cyano group ($-C equiv N$) where the carbon atom is directly bonded to an alkyl or aryl group (R-CN). They are important synthetic intermediates, often used to extend carbon chains.

Isocyanides, or isonitriles, are constitutional isomers of cyanides, with the nitrogen atom of the $-N equiv C$ group bonded to the R group (R-NC). This structural difference leads to distinct properties and reactivity.

Nitriles are typically prepared from alkyl halides using KCN or by dehydration of amides. Isocyanides are formed from alkyl halides using AgCN, or famously via the Carbylamine reaction from primary amines, chloroform, and KOH.

Both classes of compounds are polar and have higher boiling points than corresponding alkanes. Chemically, nitriles hydrolyze to carboxylic acids (or amides) and reduce to primary amines. They also react with Grignard reagents to form ketones.

Isocyanides hydrolyze to primary amines and formic acid, and reduce to secondary amines. Isocyanides are characterized by their extremely foul odor and high toxicity, generally surpassing that of nitriles.

The ambident nature of the cyanide ion ($CN^-$) explains the formation of nitriles with KCN and isocyanides with AgCN, a key concept for NEET.

Important Differences

vs Isocyanides

Aspect	This Topic	Isocyanides
Functional Group	$-C equiv N$ (cyano group)	$-N equiv C$ (isocyano group)
Bonding to R group	Carbon of cyano group attached to R ($R-C equiv N$)	Nitrogen of isocyano group attached to R ($R-N equiv C$)
Nomenclature (IUPAC)	Alkanenitrile or alkyl cyanide	Alkanisonitrile or alkyl isocyanide
Preparation from R-X	With KCN (ionic, C-attack)	With AgCN (covalent, N-attack)
Odor	Some have almond-like smell (e.g., benzonitrile), HCN has bitter almond smell	Extremely foul and offensive odor
Toxicity	Highly toxic	Generally more toxic than nitriles
Hydrolysis Product	Carboxylic acid ($R-COOH$) or amide ($R-CONH_2$)	Primary amine ($R-NH_2$) and formic acid ($HCOOH$)
Reduction Product	Primary amine ($R-CH_2-NH_2$)	Secondary amine ($R-NH-CH_3$)
Carbon of functional group	Electrophilic (susceptible to nucleophilic attack)	Nucleophilic (has a lone pair and formal negative charge)

Cyanides (nitriles) and isocyanides (isonitriles) are constitutional isomers with the same molecular formula but different atomic connectivity. Nitriles feature a carbon-carbon bond ($R-C equiv N$), while isocyanides have a carbon-nitrogen bond ($R-N equiv C$). This fundamental difference dictates their synthesis (KCN vs. AgCN with alkyl halides), physical properties (odor, toxicity), and chemical reactivity. Nitriles hydrolyze to carboxylic acids and reduce to primary amines, extending the carbon chain. Isocyanides hydrolyze to primary amines and formic acid, and reduce to secondary amines. The Carbylamine reaction is a specific test for primary amines, yielding foul-smelling isocyanides.