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 = N is 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)6 OCNH2  CH3(CH2)6 CN  

2. From RX:  

RX + KCN ——> RCN + KX  

This method is satisfactory only if R is 1o or 2group. If it is 3o group, 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 preparing 3o alkyl cyanides.

(CH3)3CMgCl + CICN →  (CH3)3 CCN + 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 1o amine and chloroform with ethanolic potassium hydroxide  

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

 

Mechanism proceeds via intermediate formation of dichloromethylene or, dichloro carbene produced from chloroform in alkaline solution. (Via a-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:  

 RNC R NHCH3  

                  2o amine

CH3NC CH3NHCH

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