STEREOCHEMISTRY OF ELEMENTS OTHER THAN CARBON 4-Covalent Elements.—In all optically active compounds known up to 1899 the molecular dissymmetry was dependent upon the spatial arrangement of the valencies of carbon. In that year a great advance was made through the discovery by Pope and Peachey that quaternary ammonium salts of the type N(R1R2R3 could be resolved into optical antipodes, for this indicated that the valency configurations of other elements besides carbon could have sufficient permanence to make their stereochemical in vestigation possible, and thus a wide field of research was opened. Further work on quaternary am monium salts showed, (I) that molecular dissymmetry could no longer be demonstrated when two of the hydrocarbon radicals were identical, proving that the ion has a plane of symmetry; (2) that different methods of formation of a given salt, such as never yielded isomeric modifications, indicating that the valencies linking the four hydrocarbon radicals to the nitrogen atom are inter-equivalent. Only two configurations of the ammonium ion satisfy these conditions, namely, the pyramidal configuration XXXI. (fig. 19) and the tetrahedral configuration XXXII., and of these the first has been definitely disproved by showing that a spirocyclic ammonium salt of the type XXXIII. (fig. 2o) can be resolved into optically active components; for it can easily be seen that, if the two rings in this compound were attached to the base of a square pyramid, the molecule would have a plane of symmetry and the compound would be non-resolvable. In the ammonium ion the nitrogen atom must accordingly have the tetrahedral configuration XXXII. corresponding with the asym metric configuration XXXIV. of its spirocyclic derivative XXXIII.
The amine-oxides are another class of compounds of 4-covalent nitrogen (see VALENCY), and in these also the radicals are tet rahedrally disposed about the nitrogen atom, for amine-oxides of the type XXXV. have been obtained in optically active forms by Meisenheimer (1908).
Phosphorus and arsenic have also been shown to have the tet rahedral configuration when in the 4-covalent state—phosphorus one of the isomers (that of which XXIX. represents one of the antimeric components) is molecularly dissymmetric.
The ketodilactone of benzophenonetetracarboxylic acid, XXX., resolved by Mills and Nodder (1921) is of the type through the optical activity of a phosphine-oxide, XXXVI. (Meisenheimer and Lichtenstadt, 1911), and arsenic through that of an arsine-sulphide, XXXVII. (Mills and Raper, 1925). In the first short period (see PERIODIC LAW), besides carbon and nitro gen, the elements beryllium, boron and oxygen can assume the 4-covalent state, and there is evidence that the atoms of each of these elements then have the four radicals tetrahedrally dis posed about them. This was proved for beryllium by the pro (Mills and Gotts, 1926), and for boron in a similar way by Boeseken and Meulenhoff (1924), and the tetrahedral symmetry of 4-covalent oxygen is indicated by the work of Morgan and Bragg on basic beryllium acetate (1923). The molecular dis
symmetry of silicon compounds of the type SiR1R2R3R4 has been established by Kipping, and the investigation of the con figuration of 4-covalent copper and zinc by the method used for 4-covalent beryllium has shown that their valencies are also tetrahedrally distributed.
It thus appears to be the rule that when an atom is linked di rectly to four other atoms these are arranged around it tetrahe drally. This rule is however not universal : thus, many ,com pounds of 4-covalent platinum containing a complex of the type occur in two, evidently stereoisomeric, modifications. A and B may be chemically similar, as ammonia and ethylamine, the complex then being a bivalent ion, or chemically diverse, as pyridine and chlorine, when the complex is electrically neutral. This isomerism is clearly incompatible with a tetrahedral arrange ment of the radicals about the platinum atom and is to be ex plained most simply by supposing that 4-covalent platinum has a plane configuration and that the isomers are cis- and trans modifications of the type 3-Covalent Elements.—The stereochemical relationships of 3-co-ordinated elements are clearest in the case of sulphur. Sul phonium salts [SR1R2R3]X, in which three groups are covalently linked to the sulphur atom, were shown to be molecularly dis symmetric by Pope and by Smiles (1900) ; the sulphonium ion therefore has a non-planar configuration (fig. 21).
Other compounds of 3-covalent sulphur are the sulphinic 0 esters, , the sulphoxides, >S--K), and the OEt and these have each been shown to have a similar non-planar configuration, since optically active repre sentatives of the three classes have been obtained, principally by Phillips and Kenyon (1925-27). The sulphoxides therefore differ from the ketones in configuration, probably as shown in diagrams which are given in fig. 21; the difference assumed in the oxygen linking is indicated by the electronic theory of valency and is confirmed by the parachor (q.v.). Optically active selenonium salts were obtained by Pope and Neville (1902) ; hence 3-covalent selenium also has a non-planar configuration.
The configuration of the tri valent nitrogen atom is of special interest on account of the number and importance of the com pounds in which the element is present in this state. The numer ous attempts made to obtain op tically active compounds of the _ _ _ type have all given negative results, but it is scarcely safe to conclude from this that these compounds have a planar configuration, for it is possible that they are non-planar but racemize rapidly, and the configuration of this important type is still an open question. However, in compounds of trivalent nitrogen of the type —X :N.Y—, where the nitrogen atom is directly linked to two other atoms only, and therefore to one of them by a double bond, the evidence for the non-planar dis position of its valencies is quite definite. Thus, oximes of the