Nitro Benzene

Nitro Benzene  

 

 

 


                It was first isolated by Mitcherlich in 1834. It has the odour of bitter almonds and is sold as an artificial oil of bitter almonds under the name oil of Mirabane.

       

 

Methods of Preparation

               

1. Labaratory method

                In the laboratory nitrobenzene is prepared by the action of a mixture of conc. HNO3 and conc. H2SO4 on benzene at the ordinary temperature.

NO2

                              

       

                          +  HNO3                                   +   H2O

         

             Benzene                               Nitrobenzene

       

50 ml of concentrated nitric acid and 50 ml of concentrated sulphuric acid  acid are taken in a round bottomed  flask and cooled. 50 ml of benzene is added to this mixture slowly by instalments adding 2-3 ml at a time. The temperature , during the addition of benzene is not allowed  to increase above 55Oc. when the whole the benzene is added,the flask is heted on a water bath to a temperature of 60oC about an hour. The flask is then cooled and transferred to a separating funnel. Nitrobenzene forms the upper layer. Lower layer of acids is drawn off, nitrobenzene washed with water several times to remove acids. It is then allowed to stand over the fused calcium chloride and distilled using an air condenser.

                        Pure nitrobenzene distils at a temperatures 208-2110C

 

2.  Commericial Method

                On a large scale also, nitrobenzene is prepared by the action of a mixture of nitric acid and sulphuric acid on benzene. At the end , the spent acids are withdrawn and the nitrobenzene layer given the necessary washings.

 

Physical Properties

 

1.          Nitrobenzene is a pale yellow liquid when freshly prepared but it darkens somewhat with age.

2.          It boils at 210 Oc and has a specific gravity 1.087.

3.          It is insoluble in water but is soluble in alcohol and ether.

4.          It has an odour like that of bitter almonds.

5.          It is volatile with steam.

 

Chemical properties

 

Nitrobenzene is a typical compound and gives all the general reactions.

 

A.            Reactions due to NO2

 

1. Replacement reactions

The nitro groups are firmly attached to the  nucleus . Thus it is very difficult to replace it.

 

2. Reduction reactions        

 

a. In acidic medium        

                 

When reduced with a tin and hydrochloric acid,it is converted to aniline.

 

  C6H5NO2      +    6H                      C6H5NH2  +2H20

 Nitrobenzene                                    Aniline

 

 The conversion is believed to proceed through the following stages

 

                O

C6H5  - N         +    2H                 C6H5       N=O

                  O                                          + 2H

                                                       

 

                        C6H5NH2                     C6H5NHOH

                    Aniline                      Phenyl hydroxyl        

                                                             amine

                                                      

b.  In alkaline medium

Alkaline reducing agents like zinc dust and sodium hydroxide or stannous chloride and sodium hydroxide , reduce nitrobenzene successively to azoxybenzene,azobenzene and hydrazobenzene.       

                               O

               

C6H5  - N         +    2H                 C6H5       N=O

                  O                                          + 2H

                                                       

 

                        C6H5NH2                     C6H5NH-OH

                    Aniline                      Phenyl hydroxyl        

                                                             amine

                                                      

c.  In neutral medium

Neutral reducing agents like zinc dust and water or aluminium-mercury couple reduce it to phenyl hydroxylamine.

 

                C6H5NO2                   C6H5N=O              C6H5NHOH

 

B. Reactions of benzene nuclues:

Nitrobenzene undergoes the usual reactions of the benzene nucleus . The incoming group occupies the meta position with respect to the nitro group.

 

          NO2                                         NO2  

FeCl3

               +      Cl2                            

                                                        Cl

                                        m-Chloronitrobenzene 

 

   N02                                                                         NO2  

Conc. H2SO4

                                                                        +  HNO3                                    

                                                                       

                                                                 NO2

m-Dinitrobenzene       

 

 

   N02                                                                         NO2  

                                                                        +  H2SO4                                   

                                                               

                                                                 SO3H       

 m-Nitrobenzene

sulphonic acid    

 

 It is further noted that the reactions of nitrobenzene towrds the above reagents is much less as compared to that of benzene.

  

Uses

A.    It has been extensively used for making the shoe polishes as it dissolves dyes and penetrate the leather well.

B.    It is used as a scent for cheap soaps.

C.    It has also been used for making the floor polishes.

D.    It has also been used as an oxidising agents in organicx synthesis.

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