Abstract
The historical survey traces the development of the electronic theory of valency; defines the polar, nonpolar, and semi-polar double bond with special reference to the stereochemistry and valency of sulphur in the sulphinic esters and sulphoxides; in which compounds the sulphur-oxygen linkage is shown to be a semi-polar double bond. A brief account of the valency of the organic compounds of selenium and tellurium describes quite different types of stereochemical and valency relations from those of sulphur: namely the cis-trans isomerism of the dimethyl telluronium dihydroxide and the failure up to the present time to separate enantiomorphously related forms of selenoxides. Three mixed selenoxides were examined with a view to provide stereochemical evidence of the existence of a semi-polar double bond between the selenium and oxygen atoms in selenoxides. These attempts were not successful, although Dr Sugden has shown that the parachor measurements of selenoxides are consistent with the possession of a semi-polar double bond between the oxygen and selenium atoms. Many organic compounds of selenium are unstable in solution, and under the action of heat, to a much greater extent than the analogous sulphur compounds; thus evidence is put forward to show that selenoxides and selenide dihalic have a labile structure which in the case of solenoxides leads to rapid racemisation. Experiments were performed with the semi-aromatic selenides and their additive compounds to investigate the nature of these structures. The results appear to afford an explanation of the polymerisation of the a-dimethyl telluronium dihydroxide recently described by Drew. The thermal decomposition of semi-aromatic selenides can be developed as a method for the production of new organic compounds of selenium for the continuation of this research.