Selenium chain compounds

The largest number of selenium atoms in covalent selenium chain compounds isolated so far appears to be three, and even so, the representative compounds known are relatively few in number. In the case of tellurium, the maximum number is two. Rbeinboldt (191) has recently reviewed preparative methods for di- and triselenides, ditellurides, and compounds containing mixed sulfur-selenium and sulfur-tellurium chains. 

The ability of thienothiophenes and selenoloselenophenes to transmit substituent effects has been determined by H and l3C NMR, and also calculated by the CNDO/2 method [82CS(20)208 83CS(22)22]. As expected, resonance effects are better transmitted between the 2- and 5-positions in 8.185 than in 8.186, since only in the former is direct conjugation between these positions possible. Both resonance and inductive/field effects were greater in the sulfur- relative to the selenium-containing compounds, but in general, replacement of sulfur by selenium caused only minor perturbations of data, which follows from the fact that the main mode of transmission of substituent effects in five-membered heterocycles must be via the carbon chain (cf. Chapter 6, Section 9.C). 

The three lowest members of the corresponding selenium series are known. Cyanogen triselenide, or selenium diselenocyanate Se(SeCN)2, was prepared by Verneuil 219) in 1886, by treatment of potassium selenocy-anate with oxidizing agents the yellow crystals melt at 132° and are quite stable in the dark. Three analogs containing mixed sulfur-selenium chains have been described, namely diselenium dithiocyanate Se2(SCN)2 and the compound SeS(SCN)2 by Baroni 16) in 1936, and selenium dithiocyanate Se(SCN)2 by Ohlberg and vander Meulen 180) in 1953. 

Conversion of epoxides into olefins. Epoxides are converted into olefins with retention of configuration by treatment with KSeCN in H2O-CH3OH at 25-65°. Selenium is deposited. The reaction fails in aprotic solvents (DMSO, DMF). Yields are high in the case of straight-chain compounds, but epoxides... 

Diselenium dichloride acts as a solvent for selenium. Similarly disulphur dichloride is a solvent for sulphur and also many other covalent compounds, such as iodine. S Clj attacks rubber in such a way that sulphur atoms are introduced into the polymer chains of the rubber, so hardening it. This product is known as vulcanised rubber. The structure of these dichlorides is given below ... 

When a compound that can form several modifications crystallizes, first a modification may form that is thermodynamically unstable under the given conditions afterwards it converts to the more stable form (Ostwald step rule). Selenium is an example when elemental selenium forms by a chemical reaction in solution, it precipitates in a red modification that consists of Se8 molecules this then converts slowly into the stable, gray form that consists of polymeric chain molecules. Potassium nitrate is another example at room temperature J3-KN03 is stable, but above 128 °C a-KNOs is stable. From an aqueous solution at room temperature a-KN03 crystallizes first, then, after a short while or when triggered by the slightest mechanical stress, it transforms to )3-KN03. 

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