Nitro & Cyno Compounds

Nitro alkanes are derivatives of alkanes. They are isomeric to nitrites (esters) classified as primary, secondary and tertiary depending on the nature of carbon atom to which nitro group is linked.

 2058_equation.JPG

Primary nitro alkane        Secondary Nitro alkane            Tertiary nitro alkane

—NO2 group is an ambident group. If it attacks through nitrogen. It is called nitro and if it attacks through oxygen atom, it is called nitrite. Hence nitrites and nitro compounds are isomers.

What are ambident nucleophiles?

Nucleophiles which can attack from two sites such as CN-, NO2- are called ambident nucleophiles

Evidences show that nitrogen is attached to one of the oxygen atoms by a double bond and to the other by a dative bond. The resonance hybrid is shown as under which confirms the spectroscopic evidence that both nitrogen – oxygen bonds have same bond length.

 

  2075_equation.JPG

          Resonating forms                             Hyrbid structure    

Out of three hybrid orbitals of nitrogen one overlaps with alkyl group and two with oxygens while the unhybridized p orbital of N – atom containing a pair of electrons and lying perpendicular to the plane of hybrid orbitals overlaps sideway with half filled 2 p – orbitals of two oxygen atoms. This forms π-bond above and below the plane of molecule.

  • Preparation of Nitro Compounds

(i) From alkyl halides:  

Alkyl halides react with silver nitrite in ethanolic solution to give nitro compounds. Alkyl nitrite is formed in minor quantity. This reaction is used to prepare  1nitro compounds primarily while 2oand 3o halides give major proportion of alkenes due to β – elimination. Contrary to this alkali nitrites give alkyl nitrites as major product. This is due to ionic nature of alkali nitrite.  

But if the reaction is carried out in solvents like DMF or DMSO, then even NaNO2 or KNO2 give good yield (about 60%) of nitro compound.  

Reactions:  

R—I + AgNO2 ——> RNO2 + Agl

C2H5l + AgNO2  ——> C2H5NO2 + Agl  

                                  Nitroethane

         1058_equation.JPG  

 

 

(ii) Nitration:  

Nitro derivatives of aromatic compounds like nitrobenzene are produced when benzene is allowed to react with nitrating mixture.(conc. HNO3/conc.H2SO4).            

 1740_benzene.JPG

999_benzene.JPG  

Mechanism:  

Generation of nitronium ion                            

Attack of NO2 on benzene molecule              

 1841_benzene.JPG            

Loss of proton:  

            43_Nitrobenzene.JPG

                                                 Nitrobenzene  

Direct nitration of alkane involves vapour phase nitration at high temperature.  

R — H + HONO2 ———> R — NO2 + H2O

                          675 K          low yield  

Problem faced in the method is that at such high temperature, a mixture of nitro alkanes is formed due to C – C cleavage.  

e.g. CH3CH2CH3 + HNO3 ——> CH3CH2CH2NO2 + CH3CH2NO2 + CH3NH2 + other products   

(iii) From amines:                 

3o nitroalkanes can be produced as follows:  

                    CH3                                                       CH3

                      |                                                            |

CH3 ———— C ———— NH2 ————> CH3 ———— C ———— NO2 + 2H2

                      |                                                            |

               3o butylamine                                        (83% yield)

Distinguish test between nitroalkanes & alkyl nitrites

1. Nitroalkane on reduction with  H2/Ni produce 1o amines while alkyl nitrites produce alcohols and NH3                                                         

 CH3CH2NO2 \small \overset{[6H]}{\rightarrow} CH3CH2NH2 + H2O

                                   1o amine                                                                                          

CH3CH2 — O — N = O \small \overset{[6H]}{\rightarrow} CH3CH2OH2 + NH3 + H2O

Ethyl nitrite

2.Nitroalkanes do not get hydrolysed in basic conditions while nitrites produce alcohols  

763_sodium salt.JPG  

CH3 O – N = O + NaOH ——> CH3OH + NaNO2 

  • Cyanides & Isocyanides

Both alkyl cyanides (RCN) and alkyl isocyanides (RNC) are organic derivatives of hydrocyanic acid HCN. Alkali cyanides are ionic 1167_Alkyl cyanides.JPG 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  

2296_Alkyl cyanides possess.JPG

  • Methods of preparation of Cynaides  

1. Dehydration of Amides:  

 1098_Dehydration of Amides.JPG  

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

CH3(CH3)6 OCNH2 \small \overset{Heat}{\rightarrow} 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

 1471_DIAZONIUM SALT.JPG

  • 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  

1727_ethanolic potassium hydroxide.JPG  

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

CHCl3 + KOH ———>KCl + H2O + : CCl2  

1681_mechanism.JPG

  • Properties of Isocyanides 

1. Alkyl isocyanides are poisonous, unpleasant smelling, with lower boiling points than isomeric cyanides.

2. RNC are not very soluble in water, nitrogen atom not having a lone pair of electrons available for hydrogen bonding.  

Reactions:  

1. Hydrolysis:                         

RNC + 2H2O \small \overset{Acid}{\rightarrow} RNH2 + HCO2H

CH3NC + 2H2O \small \overset{Acid}{\rightarrow} CH3NH2 + HCO2H         

RNC are not hydrolysed by alkalis.  

2. Reduction:  

 RNC\small \overset{H_2/Pt}{\rightarrow} R NHCH3  

                  2o amine

CH3NC \small \overset{H_2/Pt}{\rightarrow}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  

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