96 MOLECULAR STRUCTURE 



larger (2.3, 4.5, 6.1) and smaller (1.2, 3.4, 5.6) edges, 

 very suggestive of Kekule's formula with alternate single 

 and double linkages. 



The second principle, that three diclerivatives, C G H 4 X 2 , 

 exist, settles the choice, since only the second possibility, 

 the regular hexagon, provides the three isomers : 



XX X H X H 



H H H X and H H 



H H H H H X 



distinguished as i . 2 (ortho), 1 . 3 (meta), and r . 4 (para), 

 whilst 1.5= 1.3 and i . 6 = 1.2. With the irregular 

 hexagon we should have four isomers, since 1.5 = 1.3, but 

 i . 6 is not = 1.2. 



The position of the carbon atoms is indeterminate, except 



that it must leave untouched the symmetry of the whole 



with respect to the plane in which the hydrogen atoms lie, 



and the identical position of the hydrogen atoms, whilst 



the number of diderivates remains three. An arrangement 



jj jj indicated by the formula for benzene 



C C satisfies these conditions completely, as 



H C C H does any in which the carbon atoms are 



C C in a regular hexagon in the same plane 



as the hydrogen atoms, and with the 



same centre ; either, therefore, the above, or the same figure 



rotated 30. The symbol may, without losing its validity, 



be brought into accordance with the theory of valency in 



either of the following ways : 



H H H H 



\ / \ / 



C C C C 



H C^X^C-II and H ( ^C-H 



\7\> V V 



C C C C 



/ \ / \ 



H H H H 



due respectively to Glaus and to Armstrong and Baeyer 

 (the so-called centric formula) ; in the latter it is assumed 



