LECTURE XIV. 



THEORY OF THE AROMATIC COMPOUNDS DETERMINATION OF POSITION 

 OF SUBSTITUTED ATOMS OR GROUPS QUINONES ARTIFICIAL 

 DYES RING COMPOUNDS CONSTITUTION OF THE ALKALOIDS- 

 SYNTHESES CONDENSATION PROCESSES. 



TAKING the quadrivalence of carbon as his starting-point, 

 Kekule points out that in the fatty compounds the carbon 

 atoms are linked together by one valency of each. 1 In the 

 case of benzene, the next simplest assumption is made, in 

 accordance with which the carbon atoms are linked together 

 by one and by two valencies alternately, so as to form a closed 

 chain or ring. Of the twenty-four affinities of the six carbon 

 atoms, eighteen are employed in linking carbon to carbon, 

 thus : 



6 6 



.4+ .2 - 18 



2* 2 



Six valencies then remain which are satisfied by the six 

 hydrogen atoms of the benzene. Hence, according to Kekule, 

 benzene may be represented by means of a regular hexagon 

 whose sides are composed of single and of double lines alter- 

 nately, the CH groups occupying the corners. 



This conception is designed to illustrate, in the first place, 

 the relatively great stability of benzene as compared with the 

 hydrocarbons of the fatty series, which consist of open carbon 

 chains with, for the most part, singly linked carbon atoms. It 

 further illustrates the fact, which is of such great importance 

 with respect to the aromatic compounds, that the six hydrogen 

 atoms of benzene are symmetrically disposed in the molecule 

 that is, that they are identical in function. 



1 Bull. Soc. Chim. [2] 3, 104 ; Annalen. 137, 129 ; Lehrbuch der 

 organischen Chemie, 2, 493. 



