STEREO-CHEMISTRY AND PHYSIOLOGY 467 



consider those cases in which isomerism is due to the presence 

 in the molecule of a double bond between two atoms. The 

 Le Bel-van't Hoff hypothesis assumed that, in the case of a single 

 bond between two carbon atoms, free rotation of the atoms about 

 their common axis was possible ; but when we come to a com- 

 pound containing a double bond, this postulate is abandoned, 

 and it is assumed instead that the double linking of the atoms 

 introduces an element of rigidity into the molecule. We may 

 retain the tetrahedral symbols in this case also, and represent 

 the compound ethylene, CHaiCHz, by two tetrahedra laid edge 

 to edge instead of apex to apex, as in the previous cases. 



When we substitute other atoms or radicles for the four 

 hydrogen atoms, we shall find that under certain conditions 

 we have a system which is capable of two space arrangements. 

 The necessary conditions are simple : we need only ensure that 

 the two groups attached to each carbon atom differ from one 

 another. In other words, isomerism is to be expected in its 

 simplest form in the case of an ethylene derivative. 



R' R' 



\ / 



C:C 



/ \ 

 R R 



The two possible special arrangements are those represented by : 



