SECTION B.— CHEMISTRY. 



SOME ASPECTS OF STEREOCHEMISTRY 



ADDRESS BY 



DR. W. H. MILLS, F.R.S., 



PRESIDENT OF THE SECTION. 



It is many years since the opening address to this Section was devoted to 

 the subject of Stereochemistry. I think, therefore, that it might be useful 

 at this present time to consider some of the problems connected with 

 molecular configuration and review them in the light of present knowledge. 



Looking back on the history of stereochemistry we can distinguish a 

 succession of well-marked phases in its development. 



There was the initial phase, Pasteur's discovery of Icevo tartaric acid, 

 his consequent recognition that every optically active substance must 

 have its antipode, and his establishment of the doctrine of molecular 

 dissymmetry. 



The theory of van 't Hojff and Le Bel of the relation between molecular 

 dissymmetry and structure in carbon compounds marked the beginning 

 of a second phase. It originated soon after the structure theory had 

 developed sufficiently to provide an adequate basis for it. In the form 

 in which it was presented by van 't Hoff — the theory of the tetrahedral 

 distribution of the valencies of the carbon atom — it provided a frame- 

 work into which we have been able to fit practically all that we know of 

 the stereochemistry of carbon. For the ensuing quarter of a century 

 stereochemical progress was largely made up of applications of the theory 

 of the asymmetric carbon atom, and the conception still retains its 

 usefulness. 



A new stage was marked by Pope's discovery that the valencies of 

 other elements besides carbon had sufficient configurational stability to 

 give rise to mirror-image isomerism, and a further advance was attained 

 when Werner brought within the scope of stereochemical investigation 

 those complex compounds of the transitional elements which at that 

 time seemed to lie outside the domain of the ordinary laws of valency. 



By the optical resolution of compounds of this class he established the 

 theory of co-ordination and at the same time demonstrated the association 

 of the co-ordination number 6 with octahedral configuration. 



The rule of the asymmetric carbon atom had provided so simple and 

 reliable a guide to indicate when molecular dissymmetry was to be 

 expected in carbon compounds, and the number of stereomers corre- 

 sponding with a given molecular structure could be so simply determined 

 with its aid that a certain tendency had arisen among organic chemists to 

 think too much in terms of asymmetric atoms and lose sight of the more 



