Optical Isomerism, and the Ring-Electron, 



433 



loft-handed rotation of a valency electron around the direc- 

 tion of a chemical bond. This suggestion does nor, receive 

 support from the arrangement shown in the figure which 

 seems to yield exactly the same number of isomerides as the 

 ordinary structural formulae. Et is true that, it is possible to 

 reverse in the diagram the magnetic polarity of one or more 

 pairs of electrons, but even if the arrangements so obtained 

 were stable, it is doubtful whether they would represent 

 different isomerides. If such a reversal of the magnetic 

 polarity were accompanied by a change in the nature of the 

 compound, it does not seem possible to explain the pheno- 

 menon of free mobility about a single bond which is assumed 

 in stereochemistry. It should, however, be mentioned that 

 although the prevailing view is that the single bond between 

 carbon atoms docs not fix the positions of ihe atoms connected 

 by it as regards rotation about the common axis, the contrary 

 opinion has been supported by Aberson (Cohen, 'Organic 

 Chemistry for Advanced Students,' pp. 116, 133). 



This raises a question in the theory of the cubical atom 

 which requires further elucidation. It is clear from fig. 3 

 that the electrons associated with group a are related to the 

 electrons of group b in a manner different from that in which 

 thev are related to the electrons of c or d. Lewis assumes 



that 



each } 



>air of electrons is drawn together so as to 



represent a single corner of " the model of the tetrahedral 

 carbon atom which has been of such signal utility throughout 

 the whole of organic chemistry/'' But even if the electrons 

 in a pair are drawn closer together than in the diagram, the 

 lines joining the centres of the pairs nre oriented differently 

 when a and h are compared with c and d. The difficulty 

 might be got over by supposing the electrons in a pair to 

 rotate about a point midway between them, or in the case of 

 the ring-electron one electron might be supposed to move 

 over the other, giving the arrangement 



instead of 



Dr. Langmuir has been kind enough to express his views 

 on this question in a letter to the author. "When a pair of 

 electrons acts as a bond between two adjacent atoms, the 

 relationship between the two electrons has certainly becom-- 



changed. The very fact that we never have one or 



three 



