240 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



For simplification, the benzene nucleus is generally represented 

 as a regular hexagon, at each angle of which the existence 

 of a carbon atom and a hydrogen atom is recognised. 



The substitution products are then indicated by introducing only 

 the particular radicle or radicles. Further, the carbon Cl 

 atoms are generally numbered in order. The compound 

 is i-chloro-3-nitrobenzene. 



BENZENE AND ITS MONOSUBSTITUTION 



DERIVATIVES. 

 Benzene. 



Benzene was discovered by Faraday in 1825 in illuminating gas 

 prepared from oil, before the introduction of coal gas. The gases were 

 condensed to a liquid from which benzene was isolated by distillation. 

 Mitscherlich prepared benzene in 1834 by distilling benzoic acid with 

 lime and in 1845 it was found in coal tar by Hofmann. Berthelot 

 obtained benzene by passing acetylene through a red-hot tube. 



Preparation. 



Benzene is obtained from coal tar. Coal tar consists of a complex 

 mixture of aromatic compounds which are acid, basic and neutral in 

 character. It also contains unsaturated paraffins. The gases obtained 

 by the destructive distillation of coal are passed through vertical con- 

 densers connected to a trough. The condensed liquids collect in 

 the trough and are drawn off. A ton of coal yields from : 10-20 

 gallons of tar. 



Distillation of Coal Tar. 



Coal tar is distilled in wrought-iron stills and the distillate is 

 collected in the following fractions : 



(i) Light oil, or crude naphtha, which distils up to 150 and con- 

 tains benzene, toluene, xylene. It forms 3-5 per cent, 



