Introduction to Cyanides and Isocyanides: Definition, Nomenclature, Preparation, Examples

Introduction to Cyanides and Isocyanides: Hydrocyanic acid \(\left( {{\rm{HCN}}} \right)\) has two isomeric forms and cyanides, and isocyanides are the derivatives of this hydrocyanic acid. Cyanides are produced by the replacement of the hydrogen atom of \({{\rm{HCN}}}\) by an alkyl or aryl group, which is generally represented as \({\rm{RCN,}}\) where \( {\rm{R}}\) represents the aryl or alkyl group. This alkyl or aryl group can also be attached to the nitrogen atom of \({\rm{ – CN}}\) group, which is known as isocyanide. Both cyanides and isocyanides are the functional isomers of each other. The general nomenclature of these cyanides is ‘alkyl cyanides’, and isocyanides are named ‘alkyl isocyanides’. Isocyanides are also known as carbylamines. In this article, we will learn about Cyanides and Isocyanides in detail.

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Cyanides Overview

Cyanide \({\rm{RCN}}\) is a potentially deadly toxic chemical that is highly reactive and occurs in various forms. Cyanide is very harmful to the brain and heart, and it hinders the cell bodies to use oxygen. Isocyanides are a popular component of organic reactions; due to their functional group tolerance, the isocyanide multicomponent reactions can be used to synthesise various organic compounds. They have high levels of regioselective, chemoselective and stereoselective activities.

Cyanides are a good synthetic route to prepare amides, acids, and amines; hence cyanides are used to synthesise these compounds. In organic reactions, cyanides can be used as intermediates to increase the number of carbon atoms in the chain as one more carbon atom is added due to the conversion of alkyl halides to the alkyl cyanide. Acetonitrile \({\rm{(C}}{{\rm{H}}_{\rm{3}}}{\rm{C}} \equiv {\rm{N)}}\) is popularly used as an industrial solvent, and it is also used in the manufacturing of textiles, nitriles, rubber, etc. Cyanides have a pleasant smell, whereas; isocyanides are extremely unpleasant. The dipole moment of cyanide is more than isocyanides.

Cyanides Definition

A chemical compound that contains the \({\rm{ – C}} \equiv {\rm{N}}\) group is called cyanide. This group consists of a carbon atom triple-bonded to a nitrogen atom and is commonly known as the cyano group. The cyanide group is present as \({\rm{C}}{{\rm{N}}^{\rm{ – }}}\) anion in inorganic compounds. Hydrocyanic acid or hydrogen cyanide, or \({\rm{HCN,}}\) is obtained by acidification of cyanide salts and is a highly volatile liquid that is produced on a large scale industrially. Organic cyanides are usually called nitriles, where the \({\rm{ – CN}}\) group is linked by a covalent bond to the carbon atom of the alkyl or aryl group. For example, in acetonitrile \(\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{CN}}} \right)\) the cyanide group is bonded to the methyl group. Although nitriles generally do not release cyanide ions, the cyanohydrins do and are thus rather toxic.

What are Isocyanides?

An isocyanide is an organic compound containing the functional group \({\rm{ – N}} \equiv {\rm{C}}\) and is also called isonitrile or carbylamine. The name isocyanide refers that it is the isomer of the related nitrile \(\left( { – {\bf{C}} \equiv {\bf{N}}} \right).\) The alkyl or aryl is connected to the isocyanide group through the nitrogen atom and not via the carbon atom.

Nomenclature of Cyanides and Isocyanides

Ambident Nature of the Cyanide Group

The cyanide \(\left( { – {\bf{C}} \equiv {\bf{N}}:} \right)\) group can be attached to the carbon atom of the alkyl or aryl group either through the carbon or through the nitrogen atom. Such a type of group which can be linked through two different sites is known as an ambident group.

IUPAC Nomenclature of Cyanides

1. The longest chain of carbon atoms that contains the \({\rm{ – CN}}\) group is selected as the parent chain.
2. The carbon of the \({\rm{ – CN}}\) group is counted along with the parent chain.
3. Thus, the parent alkane is identified, and the suffix ‘nitrile’ is added to the name of the parent alkane. For example, the IUPAC nomenclature of \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}} \equiv {\rm{N}}\) is named ethanenitrile.

IUPAC Nomenclature of Isocyanides

1. In the IUPAC system, the isocyanides are generally named alkyl carbylamine. For example, IUPAC name of \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{NC}}\) is Methylcarbylamine.
2. In the common nomenclature system, the isocyanides are named alkyl isocyanides or aryl isocyanides. For example, \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{NC}}\) is named Methyl isocyanide.

Preparation of Cyanides and Isocyanides

Methods of Preparation of Cyanides

1. Dehydration of Amides: Cyanides are prepared by dehydration of high molecular weight acid amides to form the corresponding cyanide by heating.

2. From Alkyl halide: Cyanides are prepared from alkyl halides (RX) but only when R is \({{\rm{1}}^ \circ }\) or \({{\rm{2}}^ \circ }\) group. In the case of the \({3^ \circ }{\rm{R}}\) group, it gets converted to an alkene.

3. For preparing \({3^ \circ }\) alkyl cyanides, Grignard’s reagent and Cyanogen chloride reaction is one of the best methods.

4. From Diazonium salt: Aryl nitrile is prepared by treating diazonium salts with cuprous cyanide or KCN and copper powder. The chemical reaction involved in this process is given below:

Methods of Preparation of Isocyanides

1. Alkyl isocyanide is prepared by heating an alkyl iodide with AgCN in an aqueous ethanol solution.

2. By carbylamine Reaction

The alkyl or aryl isocyanide is prepared by a carbylamine reaction by heating a mixture of primary amine and chloroform with ethanolic potassium hydroxide. The reaction mechanism proceeds via intermediate formation of dichloromethylene or dichloro carbene which is produced from chloroform in alkaline solution via \(\alpha – \)elimination.

Physical Properties of Cyanides and Isocyanides

1. Alkyl cyanides are soluble in water, while alkyl isocyanides are insoluble because cyanides possess the tendency to form \({\rm{H–}}\)bonding with water which is absent with isocyanides. Lower members of alkyl cyanides are liquids that are soluble in water, but solubility decreases as molecular weight increases.
2. Alkyl isocyanides are poisonous, have an unpleasant smell, with lower boiling points than respective isomeric cyanides.
3. Alkyl isocyanides \(\left( {{\rm{RNC}}} \right)\) are not very soluble in water because nitrogen atoms are not having a lone pair of electrons that could be available for hydrogen bonding.

Uses of Cyanides and Isocyanides

1. Cyanide was used as a chemical warfare agent during the Franco-Prussian War \(\left( {1870 – 1871} \right).\) Cyanides are very toxic.
2. Isocyanates are broadly used in painting, shipbuilding, firefighting, electrical wire protection, cement, rubbers, and filaments.
3. Isocyanides and cyanides are widely used in chemical reactions because of their chemoselectivity, regioselectivity, and stereoselectivity. 

Summary

Both cyanides and isocyanides are the isomeric forms derived from Hydrocyanic acid \(\left( {{\rm{HCN}}} \right)\) and are produced by the replacement of a hydrogen atom of \({{\rm{HCN}}}\) by an alkyl or aryl group that is generally represented as \({{\rm{RCN}}}\) where \({\rm{R}}\) represents the aryl or alkyl group. This alkyl or aryl group can also be attached to the nitrogen atom of \({\rm{ – CN}}\) group, which is known as isocyanide. In the common nomenclature system, the names of cyanides are the alkyl cyanides. Isocyanides are named alkyl isocyanides and are also known as isonitriles. The alkyl or aryl cyanide \(\left( {{\rm{RCN}}} \right)\) is a very toxic chemical that is highly reactive and occurs in different forms. 

Isocyanides are a popular component of organic reactions; due to their functional group tolerance, high levels of regioselective, chemoselective and stereoselective activities, they are widely used in organic synthesis. Cyanides are used to prepare amides, acids, and amines. In organic reactions, cyanides can be used as intermediates to increase the number of carbon atoms in the chain as one more carbon atom is added due to the conversion of alkyl halides to the alkyl cyanide. For example, acetonitrile is represented by the chemical formula \(\left( {{\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{C}} \equiv {\rm{N}}} \right)\) and \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{NC}}\) is named methylcarbylamine according to IUPAC nomenclature. Hence, both cyanides and isocyanides are isomeric functional groups.

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