AROMATIC COMPOUNDS 239 



but point to the presence of six CH groups and a symmetrical arrange- 

 ment of the six carbon and six hydrogen atoms in the molecule. A 

 structure in which the six carbon atoms are united in a closed chain or 

 ring and joined by alternate single and double bonds, a hydrogen atom 

 being united to each carbon atom, satisfies the tetra valency of the 

 carbon atom and it gives a symmetrical structure, thus : 



H 

 c 



HC CH 



I I! 



HC CH 



Y 



H 



The symmetry of the molecule can be represented by a regular 

 hexagon at each angle of which there is a CH group ; thus : 



CH 



CH 



CH 



This structure represents benzene as an unsaturated compound. In 

 some respects benzene behaves like an olefine, but not in all respects, 

 e.g. it forms addition compounds with the halogens and with hydrogen, 

 but not with the halogen acids and sulphuric acid, nor does it decolorise 

 permanganate in the cold. 



It explains the formation of derivatives by the substitution of 

 hydrogen atoms by other elements or groups, and on account of its 

 symmetry explains the formation of one monosubstitution derivative, 

 three disubstitution derivatives, and so on. 



The alternate linking of the carbon atoms by double and single 

 bonds would point to the existence of two isomeric monosubstitution 

 derivatives. To overcome this difficulty Kekule assumed that there 

 was a continual alternation between the double and single bonds, and 

 that the double bonds are not like the double bonds in unsaturated 

 aliphatic compounds. 



The closed chain or ring structure has been accepted by all 

 chemists, but some chemists have suggested a different figure such as 

 a prism. This does not explain the substitution deriva- 

 tives as well as the centric formula proposed by Arm- 

 strong, in which the extra bonds to make the carbon 

 atoms tetra valent simply point to the centre of the 

 figure, and come into play only under certain conditions, 

 e.g. amongst the hydroaromatic compounds. 



/T\ 



