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| Names | |
|---|---|
| Other names Selenium(IV) oxide Selenous anhydride | |
| Identifiers | |
3D model (JSmol) |
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| ChEMBL | |
| ChemSpider |
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| ECHA InfoCard | 100.028.358 |
| EC Number |
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| RTECS number |
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| UNII | |
| UN number | 3283 |
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| Properties | |
| SeO2 | |
| Molar mass | 110.96 g/mol |
| Appearance | White crystals, turn slightly pink with trace decomposition[1] |
| Odor | rotten radishes |
| Density | 3.954 g/cm3, solid |
| Melting point | 340 °C (644 °F; 613 K) (sealed tube) |
| Boiling point | 350 °C (662 °F; 623 K) subl. |
| 38.4 g/100 mL (20 °C) 39.5 g/100 ml (25 °C) 82.5 g/100 mL (65 °C) | |
| Solubility | soluble inbenzene |
| Solubility inethanol | 6.7 g/100 mL (15 °C) |
| Solubility inacetone | 4.4 g/100 mL (15 °C) |
| Solubility inacetic acid | 1.11 g/100 mL (14 °C) |
| Solubility inmethanol | 10.16 g/100 mL (12 °C) |
| Vapor pressure | 1.65 kPa (70 °C) |
| Acidity (pKa) | 2.62; 8.32 |
| −27.2·10−6 cm3/mol | |
Refractive index (nD) | > 1.76 |
| Structure | |
| see text | |
| trigonal (Se) | |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards | Toxic by ingestion and inhalation[2] |
| GHS labelling: | |
   | |
| Danger | |
| H301,H331,H373,H410 | |
| P260,P264,P270,P271,P273,P301+P310,P304+P340,P311,P314,P321,P330,P391,P403+P233,P405,P501 | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
LCLo (lowest published) | 5890 mg/m3 (rabbit, 20 min) 6590 mg/m3 (goat, 10 min) 6590 mg/m3 (sheep, 10 min)[3] |
| Safety data sheet (SDS) | ICSC 0946 |
| Related compounds | |
Otheranions | Selenium disulfide |
Othercations | Ozone Sulfur dioxide Tellurium dioxide |
| Selenium trioxide | |
Related compounds | Selenous acid |
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |
Selenium dioxide is thechemical compound with theformula SeO2. This colorless solid is one of the most frequently encountered compounds ofselenium. It is used in making specialized glasses as well as a reagent in organic chemistry.[4]
Solid SeO2 is a one-dimensionalpolymer, the chain consisting of alternating selenium andoxygen atoms. Each Se atom is pyramidal and bears a terminal oxide group. The bridging Se-O bond lengths are 179 pm and the terminal Se-O distance is 162 pm.[5] The relativestereochemistry at Se alternates along the polymer chain (syndiotactic). In the gas phase selenium dioxide is present as dimers and other oligomeric species, at higher temperatures it is monomeric.[6] The monomeric form adopts a bent structure very similar to that ofsulfur dioxide with a bond length of 161 pm.[6] The dimeric form has been isolated in a low temperature argon matrix and vibrational spectra indicate that it has a centrosymmetric chair form.[5] Dissolution of SeO2 inselenium oxydichloride give the trimer [Se(O)O]3.[6] Monomeric SeO2 is a polar molecule, with the dipole moment of 2.62 D[7] pointed from the midpoint of the two oxygen atoms to the selenium atom.
The solid sublimes readily. At very low concentrations the vapour has a revolting odour, resembling decayed horseradishes. At higher concentrations the vapour has an odour resembling horseradish sauce and can burn the nose and throat on inhalation. Whereas SO2 tends to be molecular and SeO2 is a one-dimensional chain,TeO2 is a cross-linked polymer.[5]
SeO2 is considered anacidic oxide: it dissolves in water to formselenous acid.[6] Often the termsselenous acid andselenium dioxide are used interchangeably. It reacts with base to formselenitesalts containing theSeO2−
3 anion. For example, reaction withsodium hydroxide producessodium selenite:
Selenium dioxide is prepared by oxidation ofselenium by burning in air or by reaction with nitric acid orhydrogen peroxide, but perhaps the most convenient preparation is by the dehydration ofselenous acid.
The natural form of selenium dioxide, downeyite, is a very rare mineral. It is only found at a small number ofburning coal banks, where it forms around vents created from escaping gasses.[8]
SeO2 is an important reagent inorganic synthesis. Oxidation ofparaldehyde (acetaldehyde trimer) with SeO2 givesglyoxal[9] and the oxidation ofcyclohexanone gives1,2-cyclohexanedione.[10] The selenium starting material isreduced to selenium, and precipitates as a redamorphous solid which can easily be filtered off.[10] This type of reaction is called aRiley oxidation. It is also renowned as a reagent forallylic oxidation,[11] a reaction that entails the following conversion
This can be described more generally as;
where R, R', R" may bealkyl oraryl substituents.
Selenium dioxide can also be used to synthesize 1,2,3-selenadiazoles from acylated hydrazone derivatives.[12]
Selenium dioxide imparts a red colour toglass. It is used in small quantities to counteract the colour due toiron impurities and so to create (apparently) colourless glass. In larger quantities, it gives a deep ruby red colour.
Selenium dioxide is the active ingredient in somecold-bluing solutions.
It was also used as a toner inphotographic developing.
Selenium is anessential element, but ingestion of more than 5 mg/day leads tononspecific symptoms.[13]
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