Benzene Reactions

Summary of benzene reactions

Benzene 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 C_nH_2n-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 60^oC .
       
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 .