Cyanides and isocyanides are the derivatives of hydrogen cyanide, in which hydrogen is replaced by alkyl group. Hydrogen cyanide is known to exist in the following two tautomeric forms:
H—C≡N 
H—N≡C
Hydrogen cyanide Hydrogen isocyanide
Therefore, the C≡N group may attach to alkyl or aryl group through either the carbon or the nitrogen and, hence, two types of compounds may be formed.
The compounds obtained by the replacement of the H-atom of hydrogen cyanide by an alkyl or aryl group are called cyanides or nitriles. The functional group in these compounds is ─CN group. On the other hand, the compounds obtained by the replacement of the H-atom of hydrogen isocyanides by an alkyl or aryl group are called isocyanides or isonitriles. These are also called carbylamines.
In cyanides, both the C and the N atoms of the cyano group are sp hybridized and the R─C≡N bond angle is 180º (linear). Thus, C≡N group can attach to alkyl or aryl group either through carbon or nitrogen. Such a group which can be linked through two different atoms is called the ambident group.
Preparation of Cyanides and Isocyanides: Following methods are used to prepare cyanides and isocyanides, these are-
(1) From alkyl halides: Alkyl cyanides can be prepared by reaction between alkyl halides and alcoholic solution of NaCN or KCN.
R—X + KCN (alco) ———> R—CN + KX
e.g. C2H5—Br + KCN (alco) ——–> C2H5—CN + KBr
Ethyl bromide propanenitrile
N.B: Ary cyanides cannot be prepared by this method because aryl halides do not undergo nucleophilic substitution reaction.
However, when alkyl halides are heated with alcoholic solution of AgCN, alkyl isocyanides are formed as the major product.
CH3—CH2—Br + AgCN (alcoholic) ——> CH3—CH2—NC + AgBr.
Ethyl bromide Ethyl isocyanide
(2) From acid amides: Alkyl and aryl cyanides are prepared by dehydration of acid amides with phosphorus pentaoxide or thionyl chloride (SOCl2).
CH3—CO—NH2 ——–> CH3—CN + H2O
Acetamide methyl cyanide
C6H5—CO—NH2 —P2O5/ SOCl2—> C6H5—CN
Benzamide Phenyl cyanide
(3) From diazonium salts: Aryl cyanides can be prepared by treating benzene diazonium chloride with NaCN or KCN in the presence of cuprous cyanide or copper powder as catalyst.
N.B.: This method is a special case of Sandmeyer’s reaction.
(4) From amines: Isocyanides can be prepared by treating primary amines with CHCl3 in presence of KOH. This reaction is called Carbylamine reaction.
Physical properties of cyanides and isocyanides: The lower members of the family are colourless liquids while the higher members are crystalline solids. Alkyl cyanides have fairly pleasant odour while the corresponding isocyanides have intolerable smell. Both are polar in nature. Hence, they are soluble in water. There boiling and melting points are high due the formation of hydrogen bonding with water.
Chemical Properties of cyanides and isocyanides: The important chemical reactions of cyanides and isocyanides are as follows:
1) Hydrolysis: Alkyl cyanides can be hydrolyzed under acidic and basic conditions to give amides. The further hydrolysis of amides gives carboxylic acids and ammonia.
Isocyanides can be hydrolysis by dilute acids to form primary amines and formic acid.
N.B. Alkyl isocyanides are not hydrolyzed by alkalies.
2) Reduction: Alkyl cyanides can be reduced to primary amines with hydrogen in presence of Pt, Ni, lithium aluminium hydride or Na/ C2H5OH.
R—C≡N + H2 ——(Pt or Ni)——> R—CH2NH2
Alkyl cyanide Primary amine
R—C≡N + H2 —–(LiAlH4or Na/C2H5OH)——-> R—CH2NH2
Alkyl cyanide Primary amine.
On the other hand, isocyanides are reduced to secondary amine.
R—NC + H2 —-Pt or Ni—–> R—NH—CH3
C6H5—NC + 2H2 —-Pt or Ni——> C6H5—NH—CH3
On partial reduction alkyl cyanide and phenyl cyanide give aldehydes in presence of acidified stannous chloride at room temperature.
3) Reaction with Grignard reagent: Grignard reagents add to the alkyl nitriles forming intermediate imino salts which on subsequent hydrolysis give ketones.
4) Addition reactions: Isocyanides (but not cyanides) react with halogens sulhpur, mercuric oxide, ozone, etc. to give addition compounds. e.g.
