Cyanides and Isocyanides

CYANIDES AND ISOCYANIDES

Both alkyl cyanides (RCN) and alkyl isocyanides (RNC) are organicderivatives of hydrocyanic acid HCN. Alkali cyanides are ionic and cyanide ion is ambident in nature (can form covalent bond either from carbon or nitrogen).AgC = Nis covalent, hence lone pair on nitrogen is mainly available for covalent bond formation, resulting in predominant formation of isocyanides.

Illustration .How would you account for the fact that alkyl cyanides are soluble in water but alkyl isocyanides are insoluble in water?

Solution:Alkyl cyanides possess the tendency to form H – bonding with water which is absent with isocyanides

Methods of preparation of Cyanides

1.Dehydration of Amides:

High molecular weight acid amides are dehydrated to the corresponding cyanide by heat alone.

CH₃(CH₃)₆OCNH₂ CH₃(CH₂)₆CN

2. From RX:

RX + KCN ——> RCN + KX

This method is satisfactory only if R is1^oor2^ogroup. If it is3^ogroup, then it is converted into alkene.

CH₃CH₂Cl + KCN → CH₃CH₂CN + KCl

3. By Grignard’s reagent and Cyanogen chloride reaction:

RMgCl + CICIN → RCN + MgCl₂

This is best method for preparing3^oalkyl cyanides.

(CH₃)₃CMgCl + CICN → (CH₃)₃CCN + MgCl₂

4. From Diazonium salt

Methods of Preparation of Isocyanides

1.By heating an alkyl iodide with AgCN in aqueous ethanolic solution

Rl + AgCN → RNC + Agl

C₂H₅l + AgCN → C₂H₅NC + Agl

Ethylisocyanide

2.By carbylamine reaction

Heating a mixture of 1^oamine and chloroform with ethanolic potassium hydroxide

RNH₂+ CHCl₂+ 4KOH ——> RNC + 3KCl + 3H₂O

Mechanism proceeds via intermediate formation of dichloromethylene or, dichloro carbeneproduced from chloroform in alkaline solution. (Viaa-elimination)

CHCl₃+ KOH ———>KCl + H₂O + : CCl₂

Reduction of nitriles

The reduction of nitriles using hydrogen and a metal catalyst.The carbon-nitrogen triple bond in a nitrile can also be reduced by reaction with hydrogen gas in the presence of a variety of metal catalysts. Commonly used catalysts are palladium, platinum or nickel.

Physical properties of cyanide and isocyanide

The lower alkyl cyanides and isocyanides are colourless liquids. Cyanides are highly polar compounds.Cyanides and isocyanides posses high boiling points.

The lower alkyl cyanides and isocyanides are colourless liquids while the higher members are crystalline solids.

Alkyl cyanides usally posses fairly pleasent odour,resembling that of bitter almonds
Isocyanides posses very unpleasent odour

• Cyanides are highly polar compounds and possess dipolemoments of the order of 4D.
• Isocyanides are also polar but they possess comparatively lesser dipolemoments.
• The dipole moments of isocyanides are of the order of 3D

Both cyanides and isocyanides possess high boiling points

· Their boiling points are higher than those of alkyl halides of comparable molecular masses

· Ex:_355KCH₃​CN​,_332KCH₃​NC​,_249.5KCH₃​Cl​

The lower alkyl cyanides are fairly soluble in water due to formation of hydrogen bonds

The solubility of alkyl cyanides in water decreases with increase in size of the alkyl group All cyanides are highly soluble in organic solvents

Chemical properties of cyanide and isocyanide

1. Hydrolysis:

RNC + 2H₂O RNH₂+ HCO₂H

CH₃NC + 2H₂O CH₃NH₂+ HCO₂H

RNC are not hydrolysed by alkalis.

2. Reduction:

RNCR NHCH₃

2^oamine

CH₃NC CH₃NHCH₃

Methyl isocyanide Dimethyl amine

3. When alkyl isocyanides are heated for a long time, they arrange to form cyanide

RNC → R CN

CH₃CH₂NC → CH₃CH₂CN

4. With non metals:

(i) RNC + X₂———> RNCX₂

CH₃NC + Cl₂——> CH₃NCCl₂

(ii) RNC + S ———> RNCS

Alkyl isothiocyanates

CH₃NC + S ———> CH₃NCS

5. Oxidation with HgO:

RNC + HgO → RNCO + Hg

Akyl isocyanates

CH₃NC + HgO → CH₃NCO + Hg