AROMATIC COMPOUNDS. 



Aromatic compounds are mostly contained in the fragrant and 

 peculiar smelling oils, resins, etc., which are present in the flowers, leaves 

 and other parts of plants and which often ooze out when the bark is 

 broken. Though only a few aromatic compounds are actually found in 

 animals, yet they are essential to their life. Turpentine, india-rubber, 

 tannin, oil of lemon, oil of cloves, cinnamon and numerous plant pig- 

 ments are aromatic compounds. They derive their name from their 

 origin in these sweet-smelling natural substances. 



Another great source of aromatic compounds is coal tar, which is 

 a complex mixture, but contains benzene, the parent substance from 

 which all the other compounds can be derived. 



In some respects the aromatic resemble the fatty or aliphatic 

 compounds, but in other respects they are totally different. They 

 differ by having a greater carbon content and in being more resistant 

 to oxidation, reduction, etc. The more complex substances can 

 be oxidised and converted into simpler substances, but these simpler 

 substances are resistant and are found to possess a stable nucleus com- 

 posed of six atoms of carbon. This nucleus is not easily oxidised to 

 compounds containing fewer carbon atoms, but on oxidation it is con- 

 verted into carbon dioxide and water. The presence of a nucleus of 

 six carbon atoms is thus a characteristic of aromatic compounds. 



Another characteristic is the formation of nitro compounds and 

 sulphonic acid derivatives by the direct action of nitric and sulphuric 

 acids. 



The Structure of Aromatic Compounds. 



Our representation of the structure of the aromatic compounds is 

 based upon the theory of Kekule, which was published in 1865. 



Benzene, C fi H 6 , is the simplest aromatic compound containing six 

 carbon atoms. Its properties do not correspond to the properties of 

 an unsaturated hydrocarbon, such as : 



CH=C CH 2 CH a C=CH 



or 

 CH 3 C=C CH 2 C=CH, 



