234 SCIENCE PROGRESS 



The Ethylenic Linkage 

 Let us consider a substance of the following configuration : 



a \ ' ,/V N / C 



> c ; \ c < 



In such a configuration, according to the Van't Hoff hypothesis, 

 a, b, c, and d all lie in one plane, which is at right angles to the 

 plane containing the linkages joining the two carbons. So, the 

 structure does have a plane of symmetry, and it is identical with 

 its mirror-image ; and so no optical activity is to be expected. 



On the other hand, according to Le Bel's ideas (and also 

 according to the ideas set forth above), the four groups a, b, c, 

 and d may not, and very probably will not, lie in the same plane. 

 The structure thus may become asymmetric and enantiomor- 

 phous ; and the possibility of optical activity arises. 



In fact it was at one time expected to get optically active 

 substances of such a configuration ; and Le Bel x himself carried 

 out a number of experiments with the hope of isolating them. 

 Similar researches were made by Anschiitz and Walden ; but all 

 of them were unsuccessful ; and now it is generally agreed that 

 there is no possibility of optical activity in such compounds. 



It appears that this was considered as a great difficulty in the 

 way of accepting Le Bel's views. Now please notice the tacit 

 assumption made here, that asymmetry and the consequent 

 enantiomorphism necessarily imply optical activity, which 

 assumption appears to the writer to be unjustifiable. According 

 to the ideas set forth above, there must be something else 

 present besides enantiomorphism, viz. the unsymmetrical spatial 

 distribution of the linkages, and the screw-spiral arrangement of 

 the radicals round the carbon. This is obviously not the case 

 here ; for the two linkages of each carbon, by which it is joined 

 to the other carbon, may be regarded as acting along practically 

 the same line. And so there is no real difficulty in reconciling 

 the absence of optical activity, which is an experimental fact, 

 and the presence of enantiomorphism, demanded by Le Bel's 

 hypothesis. 



The case of the acetylenic linkage is simpler still, and need 

 not be further considered. 



1 For references to the original papers, see Stewart's Stereochemistry, p. 158. 



