Selenium dioxide
Chemical compound
From Wikipedia, the free encyclopedia
Selenium dioxide is the chemical compound with the formula SeO2. This colorless solid is one of the most frequently encountered compounds of selenium. It is used in making specialized glasses as well as a reagent in organic chemistry.[7]
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| Names | |
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| Other names
Selenium(IV) oxide Selenous anhydride | |
| Identifiers | |
3D model (JSmol) |
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| ChEMBL | |
| ChemSpider | |
| ECHA InfoCard | 100.028.358 |
| EC Number |
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PubChem CID |
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| RTECS number |
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| UNII | |
| UN number | 3283 |
CompTox Dashboard (EPA) |
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| Properties[1] | |
| SeO2 | |
| Molar mass | 110.969 g·mol−1 |
| Appearance | White tetragonal needles or powder |
| Odor | rotten horseradish |
| Density | 3.95 g/cm3, solid |
| Melting point | 360 °C (680 °F; 633 K) (sealed tube) |
| Boiling point | 315 °C (599 °F; 588 K) (sublimation point) |
| 263.4 g/100mL (22 °C (72 °F; 295 K), forms selenous acid) | |
| Solubility in ethanol | soluble |
| Solubility in acetone | slightly soluble |
| Solubility in methanol | soluble |
| Vapor pressure |
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| Band gap | 3.29 eV[2] |
| −27.2×10−6 cm3/mol | |
| 2.62±0.05 D | |
| Structure[2] | |
| Tetragonal | |
| P42/mbc | |
| 4/mmm | |
a = 8.37 Å, b = 8.37 Å, c = 5.08 Å α = 90°, β = 90°, γ = 90° | |
Lattice volume (V) |
356.23 Å3 |
Formula units (Z) |
8 |
| Thermochemistry[1] | |
Std enthalpy of formation (ΔfH⦵298) |
−225.4 kJ/mol |
Enthalpy of fusion (ΔfH⦵fus) |
17.6 kJ/mol |
| Hazards | |
| Occupational safety and health (OHS/OSH): | |
Main hazards |
Toxic by ingestion and inhalation[3] |
| GHS labelling:[4] | |
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| Danger | |
| H301+H331, H373, H410 | |
| P260, P264, P270, P271, P273, P301+P310+P330, P304+P340+P311, P314, P391, P403+P233, P405, P501 | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
Threshold limit value (TLV) |
0.2 mg/m3 (TWA) |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose) |
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LCLo (lowest published) |
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| NIOSH (US health exposure limits):[a] | |
PEL (Permissible) |
0.2 mg/m3 |
REL (Recommended) |
0.2 mg/m3 |
IDLH (Immediate danger) |
1 mg/m3 (as Se) |
| Related compounds | |
Other cations |
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Related selenium oxides |
Selenium trioxide |
Related compounds |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Properties
Solid SeO2 is a one-dimensional polymer, the chain consisting of alternating selenium and oxygen atoms. Each Se atom is pyramidal and bears a terminal oxide group. The bridging Se−O bond lengths are 1.79 Å and the terminal Se−O distance is 1.62 Å.[8] The relative stereochemistry 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.[9][page needed] The monomeric form adopts a bent structure very similar to that of sulfur dioxide with a bond length of 1.61 Å.[9][page needed] The dimeric form has been isolated in a low temperature argon matrix and vibrational spectra indicate that it has a centrosymmetric chair form.[8] Dissolution of SeO2 in selenium oxydichloride give the trimer [Se(O)O]3.[9][page needed]
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.[8]
SeO2 is considered an acidic oxide: it dissolves in water to form selenous acid.[9] Often the terms selenous acid and selenium dioxide are used interchangeably. It reacts with base to form selenite salts containing the SeO2−3 anion. For example, reaction with sodium hydroxide produces sodium selenite:
- SeO2 + 2 NaOH → Na2SeO3 + H2O
Preparation
Selenium dioxide is prepared by oxidation of selenium by burning in air or by reaction with nitric acid or hydrogen peroxide, but perhaps the most convenient preparation is by the dehydration of selenous acid:[citation needed]
- 2 H2O2 + Se → SeO2 + 2 H2O
- 3 Se + 4 HNO3 + H2O → 3 H2SeO3 + 4 NO
- H2SeO3 ⇌ SeO2 + H2O
Occurrence
The natural form of selenium dioxide, downeyite, is a very rare mineral. It is only found at a small number of coal-seam fires, where it forms around vents created from escaping gasses.[10]
Uses
Organic synthesis
SeO2 is an important reagent in organic synthesis. Oxidation of paraldehyde (acetaldehyde trimer) with SeO2 gives glyoxal[11] and the oxidation of cyclohexanone gives 1,2-cyclohexanedione.[12] The selenium starting material is reduced to selenium, and precipitates as a red amorphous solid which can easily be filtered off.[12] This type of reaction is called a Riley oxidation. It is also renowned as a reagent for allylic oxidation, a reaction that entails the following conversion:[13]
This can be described more generally as:
- R2C=CR′−CHR″2 + [O] → R2C=CR′−C(OH)R″2
where R, R', R" may be alkyl or aryl substituents.
Selenium dioxide can also be used to synthesize 1,2,3-selenadiazoles from acylated hydrazone derivatives.[14]
As a colorant
Selenium dioxide imparts a red colour to glass. It is used in small quantities to counteract the colour due to iron 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 some cold-bluing solutions.
It was also used as a toner in photographic developing.
Safety
Selenium is an essential element, but ingestion of more than 5 mg/d leads to nonspecific symptoms.[15]
Notes
- PEL, REL, and IDLH values are based on sodium selenide, which is an order of magnitude more toxic than other selenium compounds.[6]
