Nitro and 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.

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.

          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  1^o nitro compounds primarily while 2^oand 3^o 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

(ii) Nitration:  

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

Mechanism:  

Generation of nitronium ion                            

Attack of NO₂ on benzene molecule              

Loss of proton:  

                                                 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:                 

3^o nitroalkanes can be produced as follows:  

                    CH₃                                                       CH₃

                      |                                                            |

CH₃ ———— C ———— NH₂ ————> CH₃ ———— C ———— NO₂ + 2H₂O 

                      |                                                            |

               3^o butylamine                                        (83% yield)

Distinguish test between nitroalkanes and alkyl nitrites

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

 CH₃CH₂NO₂  CH₃CH₂NH₂ + H₂O

                                   1^o amine                                                                                          

CH₃CH₂ — O — N = O  CH₃CH₂OH₂ + NH₃ + H₂O

Ethyl nitrite

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

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

Cyanides and Isocyanides

Both alkyl cyanides (RCN) and alkyl isocyanides (RNC) are organic derivatives 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 1^o or 2^o group. If it is 3^o 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 3^o 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 1^o 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  

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  RNH₂ + HCO₂H

CH₃NC + 2H₂O CH₃NH₂ + HCO₂H         

RNC are not hydrolysed by alkalis.  

2. Reduction:  

 RNC R NHCH₃  

                  2^o amine

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  

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