242 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



This reaction can be observed by adding a few drops of bromine to 

 a few c.c. of benzene in a test tube. The evolution of hydrobromic 

 acid is slow. If a piece of aluminium mercury couple or some iron 

 filings be added to a few c.c. of benzene in another test tube and then 

 a few drops of bromine, the reaction is more rapid. 



Benzene is not acted upon by iodine, but iodobenzene has been 

 prepared by heating benzene with iodine and iodic acid : 

 5 C 6 H 6 + 4 i + HI0 3 = 5 c tt H 5 i + 3 H 2 0. 



Iodobenzene is obtained through the diazo reaction (p. 249). 



Properties. 



These halogen derivatives are colourless liquids with a faint but not 

 disagreeable smell. They boil without decomposition, are insoluble in 

 water but soluble in organic solvents. 



Reactions. 



These compounds are more stable than the corresponding aliphatic 

 halogen compounds. The halogen atom is not replaceable by OH 

 groups or other groups. 



Nitrobenzene. C 6 H 6 NO 2 . 



Benzene is not acted upon by dilute nitric acid, but it is converted 

 into nitrobenzene by the action of concentrated nitric acid (nitration) : 



ON0 2 . 

 



This reaction is one of the principal reactions which benzene and 

 its derivatives undergo and in which aromatic compounds differ from 

 aliphatic compounds. 



Preparation. 



3 or 4 drops of benzene are added to a mixture of 2 c.c. of con- 

 centrated sulphuric acid and I c.c. of concentrated nitric acid and 

 warmed. Nitrobenzene is formed and is recognised by its smell of 

 bitter almonds, which remains after removing the excess of acid by 

 alkali. 



On a larger scale nitrobenzene may be prepared as follows : 

 100 gm. of concentrated nitric acid are added with shaking to 150 gm. of 

 concentrated sulphuric acid in a 500 c.c. flask and cooled by placing under 

 running water. 50 gm. of benzene are added in portions of 2 c.c. to the 

 cold mixture of acids ; after each addition the mixture is thoroughly shaken. 

 There is an energetic reaction and the contents of the flask must be kept below 

 50 by immersing it in cold water. The addition of benzene should take at 

 least half an hour. The reaction is completed by heating the contents of the flask 

 under an air condenser in a water-bath at 60. Tne nitrobenzene floats as an 

 oil on the surface. It is separated from the acid by means of a tap funnel 

 and shaken with water. The heavier layer of nitrobenzene is separated, 

 washed with excess of sodium carbonate solution to remove acid and again 



