Structure of Simpler Organic Compounds. 151 



" ring " formula, and it has been shown how, by means 

 of the formula, the number of possible isomerides of ben- 

 zene derivatives and their relationship to one another 

 can be explained. It has been further pointed out that 

 benzene differs in many respects from the unsaturated 

 aliphatic substance dipropargyl which is isomeric with 

 it. It is a very stable substance, and the ring nucleus of 

 six carbon atoms is only destroyed with difficulty. When 

 benzene or its derivatives are treated with nitric acid 

 they yield nitro-derivatives, and with sulphuric acid they 

 give rise to sulphonic acids. When a halogen is attached 

 to a carbon of the benzene nucleus it is not readily replaced 

 by hydroxyL When a hydroxyl group is directly attached 

 to a carbon atom, a " phenol " is obtained which differs 

 in many respects from an aliphatic hydroxyl derivative. 

 When an amino group is attached to a carbon atom of the 

 benzene nucleus, it can, on treatment with nitrous acid, 

 give rise to diazonium compounds, which undergo an 

 interesting series of condensations with other aromatic 

 derivatives, yielding characteristic products, some of 

 which are of considerable commercial value as dyestuffs 

 (aniline dyes). 



SOME TYPICAL AROMATIC SUBSTANCES. 



Hydrocarbons. 



Benzene. This hydrocarbon is obtained from the 

 lower boiling fractions produced by the distillation of coal 

 tar. It boils at 80-81. The ordinary commercial pro- 

 duct is contaminated with a substance, thiophene, C 4 H 4 S, 

 which can only be removed with some difficulty. The 

 removal is usually effected by shaking with concentrated 

 sulphuric acid, which sulphonates thiophene more readily 

 than it does benzene. The crude benzene obtained from 

 coal tar also contains certain quantities of the homologues 



