Cyanides

CYANIDES

Introduction

These are the derivatives of hydrocyanic acid (HCN), and is known to exist in two tautomeric forms

Two types of alkyl derivatives can be obtained. Those derived by replacement of H – atom of hydrogen cyanide by the alkyl groups are known as alkyl cyanides (R-C ≡ N). and those obtained by the replacement of H – atom of hydrogen isocyanide are known as alkyl isocyanides (R-N  C)

In IUPAC system, alkyl cyanides are named as “alkanenitriles” whereas aryl cyanides as “ arenecarbonitrile”.

Table : Nomenclature of cyanides

Methods of preparation of cyanides

1) From alkyl halides

When alkyl halides are treated in the solution NaCN (or) KCN , alkyl cyanides are obtained. In this reaction a new carbon – carbon bond is formed.

Example

Aryl cyanide cannot be prepared in this method because of their less reactivity towards nucleophilic substitution. Aryl cyanides are prepared using Sandmeyers reactions.

2) By dehydration of primary amides and aldoximes with P₂ O₅

3) By dehydration of ammonium carboxylates with P₂ O₅

This method suitable for large scale preparation of alkyl cyanides.

4) From Grignard reagent

Methyl magnesium bromide on treatment with cyanogen chloride (Cl - CN) forms ethanenitrile.

Properties Of Cyanides 

Physical Properties

The lower members (up to C₁₄ ) are colourless liquids with a strong characteristic sweet smell. The higher members are crystalline solids, They are moderately soluble in water but freely souble in organic solvents. They are poisonous.

They have higher boiling points than analogous acetylenes due to their high dipole moments.

Chemical properties

1. Hydrolysis

On boiling with alkali (or) a dilute mineral acid, the cyanides are hydrolysed to give carboxylic acids.

For example

2. Reduction

On reduction with LiAlH₄ (or) Ni / H₂ , alkyl cyanides yields primary amines.

3. Condensation reaction

a) Thorpe nitrile condensation

Self condensation of two molecules of alkyl nitrile (containing a–H atom) in the presence of sodium to form iminonitrile.

b) The nitriles containing α- hydrogen also undergo condensation with esters in the presence of sodamide in ether to form ketonitriles. This reaction is known as “ Levine and Hauser” acetylation

This reaction involves replacement of ethoxy (OC₂ H₅ )group by methylnitrile (- CH₂CN) group and is called as cyanomethylation reaction.