Chemical Properties of Cyanides and Isocyanides: Cyanides and Isocyanides are functional isomers of each other derived from Hydrocyanic acid \((\text {HCN})\). When the hydrogen atom of \(\text {HCN}\) is replaced by an alkyl or aryl group, cyanide is formed, which is generally represented as \(\text {RCN}\), where \({\rm{R}}\) represents the aryl or alkyl group. When this alkyl or aryl group is attached to the nitrogen atom of \(-\text {CN}\) group, it is known as isocyanide.

Generally, these cyanides are named as ‘alkyl cyanides’, and isocyanides are named as ‘alkyl isocyanides.’ Isocyanides are commonly known as carbylamines. Cyanides are highly reactive and very toxic chemicals and are very harmful to living cells and tissues. Isocyanides are very common in organic reactions; due to their functional group tolerance, they can be used to synthesise various organic compounds.

Define Cyanides

A chemical compound containing the \(-\text {C} \equiv \text {N}\) group is called cyanide. This group consists of a carbon atom and a nitrogen atom that are bonded through a triple bond and is commonly known as the cyano group. Cyanides are used to synthesise amides, acids, and amines. In inorganic compounds, this group is present as \(\mathrm{CN}^{-}\) anion.

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Cyanide salts are highly volatile liquid, and hydrocyanic acid or hydrogen cyanide, or \(\text {HCN}\), is obtained by acidification of cyanide salts. Organic cyanides are also called nitriles which are represented as \(\text {RCN}\), where \(\text {R}\) represents the aryl or alkyl group, and \(\text {CN}\) group is linked by a covalent bond to the carbon atom of this alkyl or aryl group.

Like in Acetonitrile \(\left(\mathrm{CH}_{3} \mathrm{CN}\right)\) the cyanide group is bonded to the carbon atom of the methyl group. In organic reactions, cyanides can also 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.

What are Isocyanides?

An organic compound containing the functional group \(-\text {N} \equiv \text {C}\) is known as isocyanide and is also called isonitrile or carbylamine. It is the isomer of the related nitrile \((-\text {C} \equiv \text {N})\). The alkyl or aryl group in isocyanide group is connected through the nitrogen atom and not via the carbon atom.

Different Chemical Properties of Cyanides and Isocyanides

Both cyanides and isocyanides undergo many chemical reactions. Some of them are discussed below in detail:

1. Hydrolysis of Cyanides and Isocyanides

Cyanides and isocyanides undergo hydrolysis reactions. Cyanides are usually hydrolysed by both acids and alkalis. Firstly, amides are formed that leads to the formation of corresponding carboxylic acid and ammonia. Both the hydrolysis reactions (under acidic and basic conditions) are shown below:

Basic Hydrolysis:

Isocyanides do not show the same hydrolysis as cyanides. Rather, isocyanides are hydrolysed by dilute acids and not by alkalis. Isocyanides get hydrolysed to give primary amine and formic acid, as shown in the chemical reaction below:

2. Reduction of Cyanides and Isocyanides

Reduction in the presence of Nickle \((\text {Ni})\) or Platinum \((\text {Pt})\):

Cyanides undergo reduction in the presence of Nickle \((\text {Ni})\) or Platinum \((\text {Pt})\) to form primary amines.

\({\text{R}} – {\text{C}} \equiv {\text{N}} + {{\text{H}}_{\text{2}}}\xrightarrow{{{\text{Ni}}\,{\text{or}}\,{\text{Pt}}}}{\text{RC}}{{\text{H}}_{\text{2}}}{\text{N}}{{\text{H}}_{\text{2}}}\)

On the other hand, Isocyanides undergo reduction in the presence of Nickle \((\text {Ni})\) or Platinum \((\text {Pt})\) to form secondary amines.

\({\text{R}} – {\text{N}} \equiv {\text{C}} + 2{{\text{H}}_{\text{2}}}\xrightarrow{{{\text{Ni}}\,{\text{or}}\,{\text{Pt}}}}{\text{RNHR}}\)

Stephen’s Reduction

Through Stephen’s Reduction reaction, cyanides undergo a reduction in the presence of Stannous chloride \(\left(\mathrm{SnCl}_{2}\right)\) and Hydrochloric acid \((\mathrm{HCl})\). In this reaction both these compounds react in-situ to produce two nascent hydrogen atoms.

\(\mathrm{SnCl}_{2}+2 \mathrm{HCl} \rightarrow \mathrm{SnCl}_{4}+2 \mathrm{H}\)

Further, these nascent hydrogens and Hydrochloric acid react with cyanides to give imine hydrochloride. This imine hydrochloride is further hydrolysed and boiled to give an aldehyde. The chemical equation that follows this reaction is given below:

\(R – {\text{C}} \equiv {\text{N}} + 2{\text{H}}\xrightarrow{{{\text{Ether}},\,290 – 295\,{\text{K}}}}{\text{RCH}} = {\text{NH}}.{\text{HCl}}\)

\({\text{RCH}} = {\text{NH}}.{\text{HCl}}\xrightarrow{{{{\text{H}}_{\text{2}}}{\text{O,}}\,{\text{Boil}}}}{\text{RCHO}} + {\text{N}}{{\text{H}}_{\text{4}}}{\text{Cl}}\)

3. Addition Reaction with Grignard’s Reagent

Cyanides undergo an addition reaction with Grignard reagent’s \((\text {RMgX})\) that forms an addition product (Imine salt) and this intermediate then get hydrolysed to form a ketone.

This ketone can further react again with Grignard’s reagent to form tertiary alcohols, as shown below:

Like Cyanides, Isocyanides also undergo an addition reaction with ozone, halogen, and mercuric oxide. 

• Isocyanides react with Sulphur to form Isothiocyanate.
• Isocyanides react with mercuric oxide \((\text {HgO})\) to form Isocyanate.

• Isocyanides react with Chlorine to form Imino carbonyl chloride.
• Isocyanides react with Ozone to form Isocyanate.

Conversion of Isocyanides to Cyanides

On heating Isocyanides at \(250^{\circ} \mathrm{C}\), it gets converted into the corresponding Cyanides. As shown in the equation given below:

\({\text{R}} – {\text{N}} \equiv {\text{C}}\xrightarrow{{{\text{Heat}}}}{\text{R}} – {\text{C}} \equiv {\text{N}}\)

Summary

In short, cyanides and isocyanides are the isomeric forms derived from Hydrocyanic acid \((\text {HCN})\) and are formed when a hydrogen atom of \(\text {HCN}\) gets replaced by an alkyl or aryl group that is generally represented as \(\text {RCN}\) where \(\text {R}\) represents the aryl or alkyl group. This alkyl or aryl group can also be attached to the nitrogen atom of the cyanide group, which is known as isocyanide.

Cyanide is a very toxic chemical that is very reactive and occurs in different forms. There are various chemical reactions that cyanides and isocyanides go through. Cyanides get hydrolysed in both acidic as well as basic conditions to form carboxylic acid, and ammonia and Isocyanides gets hydrolysed in the presence of dilute acids to give primary amine and formic acid.

Reduction of cyanides and isocyanides occur in the presence of \(\mathrm{Ni}\) and \(\mathrm{Pt}\); another famous reduction reaction known as Stephen’s reduction converts cyanides to aldehyde. Grignard’s reagent is used for addition reactions of cyanides and isocyanides. Thus, there are many chemical reactions occurring in the case of cyanides and isocyanides, and they are very important for the interconversion of functional groups in organic reactions.

FAQs

Q.1. What are cyanides and isocyanides?
Ans: Cyanides are the chemical compounds containing the \(-\text {C} \equiv \text {N}\) group which consists of a carbon atom and a nitrogen atom that is bonded through a triple bond and is commonly known as the cyano group. In inorganic compounds, this group is present as \(\text {CN}^{-}\) anion.

The functional isomer of cyanide is known as \(-\text {N} \equiv \text {C}\) isocyanide and is also called isonitrile or carbylamine. It is the isomer of the related nitrile \((-\text {C}\equiv \text {N})\). The alkyl or aryl group in isocyanide group is connected through the nitrogen atom and not via the carbon atom.

Q.2. How are isocyanides prepared from alkyl halides?
Ans: Alkyl isocyanide can be prepared from alkyl halides by carbylamine reaction. Carbylamine reaction is also known as the Hofmann isocyanide synthesis. In this reaction, potassium hydroxide reacts with chloroform to produce dichlorocarbene which then converts primary amines to isocyanides. As this reaction is only effective for primary amines, it is used as a chemical test for the presence of primary amines.

Q.3. Are cyanides and isocyanides functional isomers?
Ans: Yes, cyanides and Isocyanides are functional isomers of each other. Isocyanide is the isomer of the cyanide or nitrile \((-\text {C} \equiv \text {N})\), as denoted by the prefix ‘isocyano’. The organic part is connected through the nitrogen atom in the isocyanide group, and not via the carbon as in cyanide. Isocyanides are used as building blocks for the synthesis of other compounds like amino acids.

Q.4. Are cyanides and isocyanides Metamers?
Ans: Metamers are compounds having the same molecular formula and differ in the alkyl groups attached to the heteroatom. Cyanide and isocyanide have the same molecular formula but differ in function group as they differ in the attachment of different groups with the main chain.

The cyanide functional group is \(-\text {C} \equiv \text {N}\). It means the linking site for cyanide is a carbon atom. The isocyanide functional group is \(-\text {NC}\), where the attachment occurs through nitrogen, and this gives rise to functional isomerism. Thus, cyanides and isocyanides are not metamers, and they are functional isomers.

Q.5. What is the functional group of isocyanide?
Ans: An organic compound containing the functional group \(-\text {N} \equiv \text {C}\) is known as isocyanide and is also called isonitrile or carbylamine. It is the isomer of the related nitrile \((-\text {C} \equiv \text {N})\). The alkyl or aryl group in the isocyanide group is connected through the nitrogen atom and not via the carbon atom.

Q.6. What is the difference between isocyanide and isocyanide?
Ans: The key difference between cyanide and isocyanide is the atom linked to the alkyl or aryl group. The alkyl or aryl group in the isocyanide group is connected through the nitrogen atom and in cyanide it is linked through the carbon atom.

Cyanides are the chemical compounds containing the \(-\text {C} \equiv \text {N}\) group which consists of a carbon atom and a nitrogen atom that is bonded through a triple bond and is commonly known as the cyano group. On the other hand, isocyanide is the functional isomer of cyanide and is represented as \(-\text {N} \equiv \text {C}\). Isocyanide is also called isonitrile or carbylamine. It is the isomer of the related nitrile \((-\text {C} \equiv \text {N})\).

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