38 SECTIONAL ADDRESSES 



fundamental principles on which the conception was based. A stage of 

 some importance in the development of stereochemistry was therefore 

 marked by the synthesis, by Perkin and Pope, and the subsequent resolu- 

 tion of methylryc/ohexylideneacetic acid, for this was the first representa- 

 tive to be synthesised of a type of compounds, bearing a certain con- 

 figurational relationship to allene, in which it is clearly more natural to 

 consider the dissymmetry of the molecule as a whole than to refer it to 

 the presence of an asymmetric atom. 



The present period is one that has been marked by particularly notable 

 advances. On the one hand , progress in molecular physics and in crystallo- 

 graphy has given us a knowledge of atomic dimensions and of the configura- 

 tion of simple molecules and ions which, as far as we can see, could never 

 have been obtained by the methods of classical stereochemistry. It is 

 the progress thus attained which chiefly distinguishes the present period 

 from those that have gone before. 



During this same period the more purely chemical methods of investi- 

 gation of molecular configuration have also been yielding results of great 

 interest, some of them in directions in which advances had been quite 

 unanticipated. 



We have only to recall the discovery of molecular dissymmetry in the 

 diphenyl series dependent on the restriction of rotation about a single 

 bond, the discovery of optically active sulphinic esters and sulphoxides, 

 and of optically active salts of nitroparaffins, the determination of the true 

 configuration of aldoximes and ketoximes and the demonstration of the 

 non-planar strainless character of six- and higher-membered alicyclic rings 

 to see how far from being exhausted is the usefulness of the methods of 

 classical stereochemistry. 



The electronic theory of valency, which was one of the first-fruits of 

 the application of the new knowledge of atomic structure to chemical 

 problems, has greatly increased the clearness of our ideas of the various 

 types of chemical combination, and the octet rule in particular has enabled 

 us to gain a fuller understanding of the constitution of many atomic 

 groupings. 



The octet theory has not only led to a greatly increased clearness in 

 our views of the nature of valency. We have only to supplement it by 

 a simple three-dimensional interpretation and it serves also to indicate 

 the relative directions of the valencies in space. 



If we consider the general results of the stereochemical investigation of 

 compounds in which we have to infer that a central atom is associated 

 with an octet of electrons corresponding with the electron group of highest 

 principal quantum number in the succeeding inert gas, we find it clearly 

 indicated that there is something in the arrangement of this octet which 

 is related to a tetrahedral configuration. 



If this octet really does correspond with that of the inert gas, it is what 

 we might expect if the orbits of the electrons constituting the inert gas 

 octet might in certain circumstances be related to a tetrahedral system 

 of axes. 



The tridimensional extension of the octet theory can be simply 

 represented, of course in a purely diagrammatic manner, by placing the 



