Cyanides and Isocyanides — Revision Notes

⚡ 30-Second Revision

Cyanides (Nitriles): — $R-C equiv N$

- Prep: $R-X + KCN \rightarrow R-CN$; $R-CONH_2 xrightarrow{P_2O_5} R-CN$ - Hydrolysis: $R-CN xrightarrow{H_3O^+} R-COOH$ (or $R-CONH_2$) - Reduction: $R-CN xrightarrow{LiAlH_4} R-CH_2-NH_2$ - Grignard: $R-CN + R'MgX \rightarrow R-CO-R'$

Isocyanides (Isonitriles): — $R-N equiv C$

- Prep: $R-X + AgCN \rightarrow R-NC$; Carbylamine reaction ($R-NH_2 + CHCl_3 + KOH \rightarrow R-NC$) - Odor: Extremely foul - Toxicity: More toxic than nitriles - Hydrolysis: $R-NC xrightarrow{H_3O^+} R-NH_2 + HCOOH$ - Reduction: $R-NC xrightarrow{LiAlH_4} R-NH-CH_3$

Key Distinction: — KCN (ionic) gives nitriles; AgCN (covalent) gives isocyanides.

2-Minute Revision

Cyanides (nitriles, $R-C equiv N$) and isocyanides (isonitriles, $R-N equiv C$) are constitutional isomers differing in the attachment point of the $C equiv N$ unit to the R group. This structural difference leads to distinct properties and reactions.

Nitriles are typically prepared from alkyl halides using KCN (due to carbon's stronger nucleophilicity in the ionic $CN^-$) or by dehydrating primary amides. They hydrolyze to carboxylic acids and reduce to primary amines, extending the carbon chain by one.

Isocyanides are formed from alkyl halides using AgCN (due to nitrogen's lone pair availability in covalent AgCN) or via the Carbylamine reaction, a specific test for primary amines involving $CHCl_3$ and alcoholic KOH, yielding a foul-smelling product.

Isocyanides hydrolyze to primary amines and formic acid, and reduce to secondary amines. Remember their characteristic foul odor and higher toxicity compared to nitriles. The ambident nature of the cyanide ion is a key concept to grasp for NEET.

5-Minute Revision

Cyanides, or nitriles, are compounds with the general formula $R-C equiv N$, where the carbon of the cyano group is bonded to the alkyl/aryl group. Isocyanides, or isonitriles, are their isomers, $R-N equiv C$, with the nitrogen bonded to the R group. This seemingly small difference has profound chemical consequences.

Preparation:

Nitriles: — From alkyl halides ($R-X$) using alcoholic KCN (e.g., $CH_3Br + KCN \rightarrow CH_3CN$). KCN is ionic, and the carbon of $CN^-$ is the attacking nucleophile. Also, by dehydration of primary amides ($R-CONH_2$) using agents like $P_2O_5$ (e.g., $CH_3CONH_2 xrightarrow{P_2O_5} CH_3CN$). Aromatic nitriles can be made via Sandmeyer reaction ($Ar-N_2^+Cl^- + CuCN \rightarrow Ar-CN$).
Isocyanides: — From alkyl halides ($R-X$) using alcoholic AgCN (e.g., $CH_3Br + AgCN \rightarrow CH_3NC$). AgCN is covalent, and the nitrogen's lone pair is the attacking nucleophile. The most important method is the Carbylamine reaction (Hofmann isocyanide synthesis), a diagnostic test for primary amines. A primary amine ($R-NH_2$) reacts with chloroform ($CHCl_3$) and alcoholic KOH to produce a foul-smelling isocyanide ($R-NC$). Secondary and tertiary amines do not give this test.

Reactions:

Nitriles:

* Hydrolysis: To carboxylic acids ($R-COOH$) under acidic or basic conditions (e.g., $CH_3CN xrightarrow{H_3O^+} CH_3COOH$). Partial hydrolysis yields amides. * Reduction: To primary amines ($R-CH_2-NH_2$) using $LiAlH_4$ or catalytic hydrogenation (e.g., $CH_3CN xrightarrow{LiAlH_4} CH_3CH_2NH_2$). This extends the carbon chain. * Grignard Reaction: With $R'MgX$ to form ketones ($R-CO-R'$).

Isocyanides:

* Hydrolysis: To primary amines ($R-NH_2$) and formic acid ($HCOOH$) under acidic conditions (e.g., $CH_3NC xrightarrow{H_3O^+} CH_3NH_2 + HCOOH$). * Reduction: To secondary amines ($R-NH-CH_3$) using $LiAlH_4$ or catalytic hydrogenation (e.g., $CH_3NC xrightarrow{LiAlH_4} CH_3NHCH_3$).

Key Differences: Isocyanides have an extremely foul odor and are generally more toxic. The products of hydrolysis and reduction are distinct, which is a common point of confusion and testing in NEET. Always remember the ambident nature of the cyanide ion and how KCN and AgCN lead to different products.

Prelims Revision Notes

1
Nitriles (Cyanides): — $R-C equiv N$. Carbon of cyano group attached to R.

* Nomenclature: Alkane + nitrile (e.g., ethanenitrile for $CH_3CN$). Alkyl cyanide (e.g., methyl cyanide). * Preparation: * From Alkyl Halides: $R-X + KCN xrightarrow{\text{alcohol}} R-C equiv N$.

KCN is ionic, C-attack. * From Amides: $R-CONH_2 xrightarrow{P_2O_5, Delta} R-C equiv N + H_2O$. * From Diazonium Salts (Sandmeyer): $Ar-N_2^+Cl^- + CuCN/KCN \rightarrow Ar-C equiv N$. * Reactions: * Hydrolysis: $R-C equiv N xrightarrow{H_3O^+ \text{ or } OH^-, Delta} R-COOH$ (complete) or $R-CONH_2$ (partial).

* Reduction: $R-C equiv N xrightarrow{LiAlH_4 \text{ or } H_2/Ni} R-CH_2-NH_2$ (primary amine, chain extension). * Grignard: $R-C equiv N + R'MgX \rightarrow R-CO-R'$ (ketone). * Stephen Reduction: $R-C equiv N xrightarrow{SnCl_2/HCl} R-CHO$ (aldehyde).

1
Isocyanides (Isonitriles): — $R-N equiv C$. Nitrogen of isocyano group attached to R.

* Nomenclature: Alkyl isocyanide (e.g., methyl isocyanide). * Preparation: * From Alkyl Halides: $R-X + AgCN xrightarrow{\text{alcohol}} R-N equiv C$. AgCN is covalent, N-attack. * Carbylamine Reaction (Isocyanide Test): $R-NH_2 + CHCl_3 + 3KOH xrightarrow{Delta} R-N equiv C$.

Specific for primary amines. Product has extremely foul odor. * Reactions: * Hydrolysis: $R-N equiv C xrightarrow{H_3O^+, Delta} R-NH_2 + HCOOH$ (primary amine + formic acid). * Reduction: $R-N equiv C xrightarrow{LiAlH_4 \text{ or } H_2/Ni} R-NH-CH_3$ (secondary amine).

1
Key Concepts:

* Ambident Nucleophile: $CN^-$ can attack via C (with KCN) or N (with AgCN). * Toxicity: Both are highly toxic; isocyanides generally more so, with a characteristic foul smell. * Chain Extension: Nitrile synthesis from alkyl halides adds one carbon atom.

Vyyuha Quick Recall

Cyanides Come from Carbon attack (KCN), give Carboxylic acids & Chain-extended primary amines. Isocyanides Invite Incredible stench (foul odor), Involve Isomerism, and Invariably yield Interesting secondary amines (from reduction) or primary amines + formic acid (from hydrolysis) via AgCN or Carbylamine.