152 Biological Chemistry. 



toluene and xylene (see below), the last traces of which 

 cannot be completely separated by fractional distillation. 

 On freezing the liquid, benzene can be caused to separate 

 as a crystalline mass, melting point 5 '4, which is filtered 

 off. By repeated separation in crystalline form the ben- 

 zene can be obtained pure with boiling point 80*5. 

 Pure benzene can also be obtained by the distillation of 

 calcium benzoate with lime. 



Toluene, C 6 H 5 (CH 3 ), is the next higher homologue of 

 benzene, and is also contained in the lower fractions of 

 coal tar, from which it can be separated in a crude form 

 by fractional distillation. It can also be obtained from 

 balsam of Tolu and other resins by distillation. The pure 

 product boils at 110. It can also be obtained syntheti- 

 cally from benzene by two methods which are of general 

 application for the preparation of alkyl substitution pro- 

 ducts of benzene. 



(a) By the so-called Friedel and Crafts' reaction, which 

 consists in the treatment of benzene with an alkyl halo- 

 gen compound in the presence of anhydrous aluminium 

 chloride 



C,H 6 + CH 3 C1 = HC1 + C 6 H 5 -CH 3 



More than one hydrogen of the benzene nucleus can 

 be replaced in this way 



C 6 He + 2CH 3 C1 = 2HC1 + C 6 H 4 - (CH 3 ) a 



The mechanism of this reaction is somewhat complex, and 

 need not be further discussed here. 



(b) By the method already referred to (p. 142), by 

 the treatment of a mixture of a benzene halogen derivative 

 and an alkyl halogen derivative with metallic sodium 

 (see p. 55). 



Toluene can also be obtained by the distillation of the 

 calcium salt of toluic acid, C 6 H 4 CH 2 COOH, with lime, 



