Mr. J. E. Marsh on Van't Hoff's Hypothesis. 427 



I refer to the case where the asymmetric carbon atom forms 

 one of a closed chain or ring of atoms. As examples may be 

 mentioned, conine, a pipecoline, a ethyl piperidine, camphor, 

 and the terpenes. 



Dealing with the whole class preferably by an example, let 

 us consider the case of conine (fig. 1). 



Now, if we replace the carbon atom to which the propyl 

 group is attached by its image, effected by exchange of place of 

 two of the groups attached to it, we obtain a formula related 

 to the original formula also as the image to its object, and 

 not superposable on it. Conine is of course active, and yet 

 this carbon atom is not attached to four different groups, for 

 it is attached by two bonds to the same group, namely to 

 the whole group of atoms in the molecule. Hence we are led 

 to this extension of the original conception, namely, that the 

 asymmetric carbon atom is not only one to which four 

 different groups are attached, but more generally one which 

 when replaced by its image in the formula of a compound 

 gives rise to the formula of a different compound. The latter 

 in fact includes the former as a particular case. 



There is, I think, a further extension of the conception of 

 the asymmetric carbon atom necessary, an extension by which 

 is excluded in certain cases the possibility of the existence of 

 active bodies in spite of the presence of asymmetric carbon. 

 Dealing with this case also preferably by an example, let us 

 consider the case of the compound hexahydroterephthalic 

 acid (v. Baeyer, Ann. ccxlv. p. 128). This compound presents 

 a peculiar case of geometrical isomerism (figs. 2 and 3) 

 analogous to fumaric and maleic acids. 



H COOH 



C 

 CH 2 CH 2 



I i 



CH 2 CH 2 

 \ / 

 C 

 / \ 



COOH X H 

 Fig. 2. Fig. 3. 



The two carbon atoms to which the carboxyl groups are 

 attached are in fact doubly linked, not directly as in the 

 case of maleic and fumaric acids, but indirectly through the 

 intervention of other groups. 



H 



COOH 



C 



/ 



CH 2 

 i 



X CH 2 



i 



1 

 CH 2 



CH 2 



x c 



/ 



/ 

 II 



"COOH 



