94 MOLECULAR STRUCTURE 



in benzole acid is not allowable, the remaining possibilities 



are: H, = H e and H e = H, (4) 



H 6 =H e H^H, . . . . . (5) 



H t = H e H e = Hy (6) 



H t = H r H c = H e (7) 



H 4 = H, Hu = H. (8) 



H & = H, H. = Hr (9) 



These hydrogen atoms must, then, have identical positions 

 in benzene, and we have already 



H ft = H 6 = H c = H, (i) 



so that combination of (i) with the possibilities given by 

 (4) to (9) leads to the conclusion 



H a = H 6 = H c = H d = H e = Hy. 



(6) The second fact, the existence of three disubstituted 

 products, also assumed by Kekule' on the ground of the 

 data then existing, is included in the foregoing as a necessary 

 consequence. Besides the three oxybenzoic acids, 



C 6 (COOH) a (OH) t H c _ /; C 6 (COOH) a H 6 (OH) e H,,., 

 and C 6 (COOH) H,, c (OH) d H e>/ , 



two other isomers are conceivable, with the hydroxyl group 

 in the place of the hydrogen atoms H g , H/ respectively. 

 But we have seen that one of the six combinations (4) to 

 (9) necessarily holds, and this makes the other two isomers 

 impossible, since one of these equivalences must hold for 

 benzoic acid, C 6 (COOH) a H 6 _/, viz. : 



H 6 = R e and H c = H, (4) 



HI, = H e H^ = H/ (5) 



H t = H. = Hr (6,9) 



H,, = H/ and H. = H e (7) 



H 6 = H/ H d = H e (8) 



(c) The third fact, that the benzene substitution products, 

 not containing asymmetrical substituting groups, cannot be 

 separated into optically active isomers (proved at least for 

 the trisubstituted compounds), shows that compounds of the 



