Introduction to benzene reactionsBenzene is considered as the parent compound of aromatic hydrocarbons . Faraday in the year 1825 isolated from illuminating gas cylinders . In 1834 Mitscherlich  proposed the name benzene and the structure was proposed by Kekule . Aromatic hydrocarbons contain less number of hydrogens compared with aliphatic hydrocarbons . Benzene fits in the general formula CnH2n-6 .

Methods of preparation of benzene :

1. Benzene is prepared in the laboratory by distillation of sodium benzonate with soda lime .

                        C6H5COONa     +  NaOH        C6H6    +    Na2CO3 

2. Reduction of phenol : Benzene can be obtained by the distillation of phenol with zinc dust .

                       C6H5OH   +  Zn      C6H6     +   ZnO 

3. Polymerisation of actylene : When acetylene gas is passed through red hot iron or copper tubes it polymerises and gives benzene .

                      3 C2H2     C6H6

Reactions of Benzene
Benzene undergoes substitution reactions readily .

1. Halogenation : Benzene reacts with bromine or chlorine in the presence of Lewis acids like FeCl3 , AlCl3 , etc . to give corresponding halo-benzene .


Similarly with bromine , bromobenzene is formed .

2. Nitration : Benzene undergoes nitration when heated with a mixture of 1:1 ( by volume ) concentrated nitric acid and concentrated sulphuric acid ( nitration mixture ) at a temperature below 60oC .


3. Sulphonation : Benzene reacts with fuming sulphuric acid and gives benzene sulphonic acid . 


4. Friedel - Crafts alkylation and acylation : Benzene reacts with alkyl halides and acyl halides in the presence of Lewis acids ( AlCl3 , FeCl3 ) and gives alkyl benzenes and acyl benzenes .


Benzene reacts with acetyl chloride in the presence of anhydrous aluminium chloric and gives acetophenone  .

Addition reactions
1. Halogenation : In the presence of finely divided nickel , under pressure , benzene undergoes hydrogenation to give cyclohexane .

2. Addition of halogens : Benzene undergoes addition reactions with chlorine or bromine in the presence of sunlight and gives hexachloro hexane or hexabromo cyclohexane .


3. Ozonolysis : One mole of benzene reacts with three moles of ozone and gives an ozonide which hydrolysis in presence of zinc and gives 3 moles of glyoxal .

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    May 2013