H CO 



V 



II 

 c 



/\ 



COOH H 



428 Mr. J. E. Marsh on Van't Hoff's Hypothesis 



Now von Baeyer has pointed out that the carbon atoms in 

 question are asymmetric ; and it will be clear that if one of 

 them, in fig. 2 for example, is replaced by its image the for- 

 mula so obtained is not identical with the original formula. 

 It is not, however, related to the original compound also as 

 image to object, but is in fact that of the other geometrical 

 isomer (fig. 3). 



Again, if we consider the case of two carbon atoms doubly 

 linked and united each to two different groups besides, the 

 case for example of maieic and fumaric acids (figs. 4 and 5), 



H COOH 

 \ / 



C 



II 



c 



R ^COOH 



Fig. 4. Fig. 5. 



we find that if one of the doubly linked carbon atoms is 

 replaced by its image, it gives rise to the other geometrical 

 isomer. The carbon atoms in question are in fact asymmetric, 

 and there is here no question of optical activity. Maieic and 

 fumaric acids are in fact devoid of rotatory power. Hence 

 we require an extension of the original conception to include 

 cases where the asymmetric carbon atom, in virtue of its 

 asymmetry, gives rise not to optically different or opposite 

 isomers, but to geometrically different or opposite isomers ; 

 and I think the modification and extension of the original 

 conception, as elucidated in the preceding pages, may fairly 

 be expressed in the conclusions following. 



1. If in the formula of a compound a carbon atom, being 

 replaced by its image, gives rise to the formula of a com- 

 pound different from the original one, such carbon atom will 

 be asymmetric. 



2. If the asymmetric carbon atom is replaced by its image, 

 and if the formula so obtained is related to the original for- 

 mula also as image to object, the two isomers will be optically 

 opposite, i. e. will have equal and opposite rotatory power. 



'6. If the asymmetric carbon atom is replaced by its image, 

 and the formula so obtained is not related to the original 

 formula also as image to object, the isomers will be geome- 

 trically opposite and not necessarily possessed of rotatory 

 power, the rotatory power being absent from all isomers in 

 the case where the image of each of the geometrical isomers 

 is identical with the object. 



