154 Biological Chemistry. 



If, however, no carrier is present, and toluene is treated 

 in direct sunlight or at its boiling point with chlorine or 

 bromine, substitution takes place in the " side-chain " and 

 derivatives of the type C 6 H 5 CH 2 C1, benzyl chloride, 

 C 6 H 5 -CHC1 2 , benzal chloride, and C Q H 6 CC1 3 , benzo- 

 trichloride, are obtained. 



The various halogen derivatives are mostly liquids 

 which can be purified by distillation. It may be recalled 

 here (see p. 146) that the halogen when in the " side-chain " 

 is readily replaced by a hydroxyl group, on treatment 

 with caustic alkalis, whereas this is not the case when it 

 is attached to the benzene nucleus. 



Nitro Compounds. 



Nitrobenzene. This compound is made by treating 

 benzene with a mixture of nitric and sulphuric acid, under 

 conditions which do not allow too great a rise in tem- 

 perature. It is a liquid which boils at 205. By treating 

 it with a mixture of nitric and sulphuric acid at higher 

 temperatures, the chief product produced is the m-dinitro 

 benzene, C 6 H 4 (NO 2 ) 2 , melting point 90 ; small quantities 

 of the o- and ^-compounds are formed at the same time. 



Amino Compounds. 



Aniline, C 6 H 5 NH 2 . This is produced from nitro- 

 benzene by reduction, tin and hydrochloric acid being 

 generally employed for the purpose. With these reagents, 

 the aniline separates in the form of a tin double salt, 

 which is decomposed by excess of caustic alkalis, and the 

 free aniline is then distilled off in a current of steam. It 

 boils at 184, and forms salts in the same way as other 

 amines. The hydrogens of the amino group are replace- 

 able by alkyl radicles. These substitution products can be 



