AROMATIC COMPOUNDS 243 



with water. It is dried by being shaken with calcium chloride, filtered through 

 glass wool and distilled, using an air condenser, the fraction boiling from 204- 

 208 being collected. A small residue of dinitrobenzene may remain in the 

 flask. 



Properties. 



Nitrobenzene is a pale yellow liquid which possesses an odour of 

 bitter almonds. It boils at 205 and can be frozen to a solid which 

 melts at 3. It is frequently used for scenting soap but chiefly in the 

 preparation of aniline and benzidine : it is sometimes used as a 

 solvent. 



Benzene Sulphonic Acid. C 6 H 5 . SO 3 H. 



Benzene slowly dissolves in warm concentrated sulphuric acid and 

 is converted into benzene sulphonic acid (sulphoriation) : 



HO ,o /\s=o. 



\ f = H 2 + 

 HO/ \ V 



Preparation. 



2 c.c. of benzene are mixed with 7 c.c. of concentrated sulphuric 

 acid and carefully heated with constant shaking. The benzene which 

 at first floats on the surface gradually dissolves. A clear solution 

 is obtained on pouring a portion of the cooled mixture into water. 

 Sodium benzene sulphonate separates if some be poured into a satu- 

 rated solution of sodium chloride. 



On a larger scale the benzene and sulphuric acid are carefully heated 

 under a reflux condenser with constant stirring. The reaction product is 

 poured into water (or salt solution if the sodium salt be required) and the 

 solution neutralised with calcium carbonate. The calcium sulphate is filtered 

 off and the filtrate is evaporated until the calcium salt crystallises. The other 

 salts or the free acid are prepared from the calcium salt by double decomposi- 

 tion with potassium carbonate, etc., or sulphuric acid. 



Properties. 



Benzene sulphonic acid is a hygroscopic solid which is readily 

 soluble in water and melts at 50. The solution is strongly acid. 



It forms salts with the metallic carbonates, or oxides. These salts 

 generally crystallise well. 



On heating, the sulphonic acid is decomposed. The sulphonic acid 

 group may be removed by heating it in a sealed tube with concentrated 

 hydrochloric acid or with strong sulphuric acid in a current of steam ; 

 the hydrocarbon is regenerated : 



C q H 5 S0 3 H + H 3 = CpHg 



