136 Biological Chemistry. 



of the type C 6 H 5 R, C 6 H 4 RR', etc. On oxidation these sub- 

 stances are converted into carboxylic acids C 6 H 5 (CO OH), 

 C 6 H 4 (CO OH) 2 . From the former of these, C 6 H 4 CO OH 

 benzene can be obtained when the calcium salt is distilled 

 with lime 



(C 6 H 5 CO.O) 2 Ca + Ca(OH) 2 = 2CaCO s + H 2 + C 6 H 6 



Furthermore, when a substitution product such as bromo- 

 or nitro-benzene is treated in various ways, the substitut- 

 ing group can be replaced and altered, but throughout all 

 the various series of reactions the six-carbon-atom nucleus 

 of the benzene remains intact. From these facts, there- 

 fore, it is obvious that benzene must have a constitution 

 different from that of an ordinary hydrocarbon. It is 

 now generally represented by what is known as a " ring " 

 formula in the following way : 



A ring formula (but somewhat different to the above) 

 was first given to benzene by Kekule", to whom the funda- 

 mental conceptions of the constitution of the aromatic 

 substances are due. 



It will be observed in this formula that the carbon 

 atoms are all represented as being tervalent and not 

 quadrivalent, and as the adoption of the quadrivalency of 

 the carbon atom is one of the basal conceptions upon 

 which the theory of structural formulae of the organic 

 compounds has been built up, some further explanation 

 of this formulae is necessary. It has been assumed by 

 Armstrong and Baeyer that the fourth valency of the 

 carbon atom is directed towards the centre of the ring, 



