ORGANIC SELENIUM COMPOUNDS Derivatives of Bivalent Selenium.—Ethylselenomercaptan,
a liquid of repulsive odour (b.p. 57° C), is obtained by distilling an alkali hydroselenide with an alkali ethylsulphate. Diethyl selenide (b.p. 108° C) and diethyl diselenide (b.p. 186° C) are prepared from potassium ethyl sulphate and potassium mono- and di-selenide respectively. These three liquids are also obtained by treating phosphorus pentaselenide with potassium ethyl sulphate and caustic potash.
The dialkyl sele nides are unsaturated compounds and combine further with alkyl iodides, dimethyl selenide and methyl iodide yielding trimethylsele nonium iodide,
a crystalline salt which is transformed by moist silver oxide into the basic trimethylselenonium hydrox ide,
From sodium selenophenoxide, NaSe•C6H5, and methyl iodide, phenyl methyl selenide,
is produced, and this substance combines with bromoacetic acid to form phenyl methylselenetine bromide (I.) a salt containing an asymmetric selenium atom; this substance has been resolved by W. J. Pope and Neville (1902) into two optically active isomerides. (See STEREOCHEMISTRY.) By the interaction of selenium tetrachloride and acetylacetone, G. T. Morgan and H. D. K. Drew (1920) obtained chloroacetylacetone and bimeric selenium acetylacetone (II.) in yellow plates melting at i5° C. The interaction of this organic derivative with sulphurous acid or alkali bisulphites affords an elegant method of preparing either selenodithionic acid,
or its alkali salts : Phenol condensed with selenium oxychloride gives rise to two isomeric trihydroxytriphenylselenonium chlorides,
SeCl; with selenious acid it yields dihydroxydiphenyl selenide (HO.C6H4)2Se (M. Takamatru, 1928). Ortho- and para-cresols also give rise to both types of selenium compound, whereas 9-naphthol furnishes only di-f3- hydroxydinaphthyl selenide (G. T. Morgan and F. Burstall, 1928).
The aromatic selen onic acids are obtained by (I) direct action of selenic acid on benzene and its homologues, or (2) permanganate oxidation of the aryl selenious acids or aryl diselenides. Benzeneselenonic acid,
is a hygroscopic syrup which in contact with moisture forms a crystalline hydrate. Diphenylselenone,
a colourless crystalline substance melting at 155° C, is produced by oxidation from diphenylselenoxide,
the latter being obtained from diphenylselenide dibromide.
Cyclic Selenium Derivatives.—Selenium takes part in the formation of various heterocyclic compounds. Selenophen (I.), the analogue of thiophen, has been prepared by H. V. A. Briscoe and J. B. Peel (1928) from the interaction of selenium and acetylene at 400° C; it is a colourless liquid boiling at
MM.
The following saturated selenohydrocarbons have been prepared by the interaction of sodium selenide and the appropriate alkylene dibromide : (i.) cycloselenopropane (III.), from trimethylene dibromide, is a colourless liquid boiling at 80-81° C/4o mm. and giving a crystalline mercurichloride; cycloselenobutane (tetra hydroselenophene) (II.), from tetramethylene dibromide, is a colourless liquid boiling at 135-6°/770 mm. ; (iii.) cycloselenopen tane (IV.), from pentamethylene dibromide, is a colourless liquid boiling at 158-161° C/76o mm. and having a disagreeable odour; it forms crystalline derivatives with the halogens and with mer curic chloride (Morgan and Burstall, 1929).
Selenium is pre cipitated from its inorganic compounds by hydrogen sulphide, and is redissolved by ammonium sulphide. The elimination of red selenium from its soluble compounds on reduction with sulphur dioxide, hydrazine, hydroxylamine, etc., is a characteristic test. The gravimetric estimation is made by weighing this pre cipitate on a tarred filter after boiling until the red selenium has turned black, the latter form being more easily filtered. Selenium may be estimated iodometrically, and selenious acid may be oxi dised to selenic acid by standard permanganate.