Hydrogen selenide

Hydrogen selenide is an inorganic compound with the formula H₂Se. It is the simplest (and virtually the only) hydride of selenium. H₂Se is a colorless, flammable gas under standard conditions. It is the most toxic selenium compound^[3] with an exposure limit of 0.05 ppm over an 8-hour period.^[4]^[5] Even at extremely low concentrations, this compound has a very irritating smell resembling that of decayed horseradish or 'leaking gas', but smells of rotten eggs at higher concentrations.

Structure and properties

H₂Se adopts a "bent" structure with a H-Se-H bond angle of 91°. Consistent with this structure, three IR-active vibrational bands are observed: 2358, 2345, and 1034 cm⁻¹.

The properties of H₂S and H₂Se are similar, although the selenide is more acidic with pK_a = 3.89, and the second pK_a = 11.0 at 25 °C. Reflecting its acidity, H₂Se is soluble in water.

Preparation

Industrially, it is produced by treating elemental selenium at T > 300 °C with hydrogen gas.^[6] A number of routes to H₂Se have been reported, which are suitable for both large and small scale preparations. In the laboratory, H₂Se is usually prepared by the action of water on Al₂Se₃, concomitant with formation of hydrated alumina. A related reaction involves the acid hydrolysis of FeSe.^[7]

Al₂Se₃ + 6 H₂O ⇌ 2 Al(OH)₃ + 3 H₂Se

H₂Se can also be prepared by means of different methods based on the in situ generation in aqueous solution using boron hydride, Marsh test and Devarda's alloy. According to the Sonoda method, H₂Se is generated from the reaction of H₂O and CO on Se in the presence of Et₃N.^[8] H₂Se can be purchased in cylinders.

Reactions

Elemental selenium can be recovered from H₂Se through a reaction with aqueous sulfur dioxide (SO₂).

2 H₂Se + SO₂ ⇌ 2 H₂O + Se + S

Its decomposition is used to prepare highly pure Se metal.

Applications

H₂Se is commonly used in the synthesis of Se-containing compounds. It adds across alkenes. Illustrative is the synthesis of selenoureas from cyanamides.^[9]

H₂Se gas is used to dope semiconductors with selenium.

Safety

Hydrogen selenide is hazardous, being the most toxic selenium compound^[3] and far more toxic than its congener hydrogen sulfide. The threshold limit value is 0.05 ppm. The gas acts as an irritant at concentrations higher than 0.3 ppm, which is the main warning sign of exposure; below 1 ppm, this is "insufficient to prevent exposure", while at 1.5 ppm the irritation is "intolerable".^[5] Exposure at high concentrations, even for less than a minute, causes the gas to attack the eyes and mucous membranes; this causes cold-like symptoms for at least a few days afterwards. In Germany, the limit in drinking water is 0.008 mg/L, and the US EPA recommends a maximum contamination of 0.01 mg/L.^[6]^[10]

Despite being highly toxic, no human fatalities have yet been reported. It is suspected that this is due to the gas' tendency to oxidise to form red selenium in mucous membranes; elemental selenium is less toxic than selenides are.^[4]

References

1 2 3 4 5 "NIOSH Pocket Guide to Chemical Hazards #0336". National Institute for Occupational Safety and Health (NIOSH).
↑ "Hydrogen selenide". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH).
1 2 http://www.epa.gov/ttnatw01/hlthef/selenium.html, US Environmental Protection Agency, Air Toxins website
1 2 http://www.cdc.gov/niosh/idlh/7783075.html, Documentation of Immediately Dangerous to Life or Health Concentrations: Hydrogen Selenide, The National Institute for Occupational Safety and Health
1 2 http://www.cdc.gov/niosh/docs/81-123/pdfs/0336.pdf Occupational Health Guideline for Hydrogen Selenide, The National Institute for Occupational Safety and Health, 1978
1 2 Bernd E. Langner "Selenium and Selenium Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a23_525.
↑ Féher, F. In "Handbook of Preparative Inorganic Chemistry"; Brauer, E., Ed.; Academic: New York, 1963; 1, p 418.
↑ Sonoda, N.; Kondo K.; Nagano, K.; Kambe, N.; Morimoto, F. Angewandte Chemie International Edition English 1980, vol. 19, page 308
↑ Cohen, V.I. (1980). "A Convenient Synthesis of Mono-, N,N′-Di-, and Trisubstituted Selenoureas from Methyl Carbamimidothioates (S-Methylpseudothioureas)". Synthesis 1980: 60–63.
↑ https://www.osha.gov/dts/chemicalsampling/data/CH_246700.html, OSHA GENERAL INDUSTRY PEL: 0.05 ppm, 0.2 mg/m3 ,OSHA CONSTRUCTION INDUSTRY PEL: 0.05 ppm, 0.2 mg/m3 TWA

External links

Hydrogen compounds

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