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Explain the industrial and laboratory preparation of benzene and toluene. |
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Answer» Solution :Preparation of benzene : `(i)` Industrial preparation of benzene from coal tar : Coal tar is a viscous liquid obtained by the pyroluysis of coal . During fractional distillation , coal tar is heated and distills away its volatile compounds namely benzene, toluene, xylene in the TEMPERTURE range of `350` to `443K`. These vapours are collected at the upper PART of the fractionating column `(ii)` From acetylene : Acetylene on PASSING through a red- hot tube trimerises to give benzene. `underset("Acetylene")(3CH-=CH)overset("Red Hot Iron tube")underset(873K)(to)`  `(iii)` Laboratory methods of preparing benzene and toluene `(A)` Decarboxylaation of aromatic acid. When SODIUM benzoate in heated with sodalim, benzene vapours distil over. `underset("Sodium benzoate")(C_(6)H_(5)COONa)+NaOHoverset(CaO)(to)underset("Benzene")(C_(6)H_(6))+Na_(2)CO_(3)` `(b)` Preparation of benzene from phenol When phenol vapours are passed over zinc dust, then it is reduced to benzene. `underset("Phenol")(C_(6)H_(5)OH)+Zntounderset("Benzene")(C_(6)H_(6))+ZnO` `(c )` Wurtz -Fittig reaction : When a solution of bromo benzene and iodo methane in dry ether is treated with METALLIC sodium, toluene is formed. `underset("Bromo benzene")(C_(6)H_(5)Br)+2Na+underset("Iodo methane")(ICH_(3)overset("ether")(to))underset("Toluene")(C_(6)H_(6)CH_(3))+NaBR+NaI` `(d)` Friedel Craft's reaction : When benzene is treated with methyl chloride in the presence of anhydrous aluminium chloride,toluene is formed. `underset("Benzene")(C_(6)H_(6))+underset("Chloromethane")(CH_(3)Cl)overset("anhydrous"AlCl_(3))(to)underset("Toluene")(C_(6)H_(5)CH_(3))+HCl` |
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