Chemistry:Selenium dioxide

Selenium dioxide

Names
Other names Selenium(IV) oxide Selenous anhydride
Identifiers
CAS Number	7446-08-4 Y
3D model (JSmol)	monomer: Interactive image polymer: Interactive image
ChEMBL	ChEMBL3183452
ChemSpider	22440 Y
EC Number	231-194-7
PubChem CID	24007
RTECS number	VS8575000
UNII	9N3UK29E57 Y
UN number	3283
InChI InChI=1S/O2Se/c1-3-2 Y Key: JPJALAQPGMAKDF-UHFFFAOYSA-N Y InChI=1/O2Se/c1-3-2 Key: JPJALAQPGMAKDF-UHFFFAOYAQ
SMILES monomer: O=[Se]=O polymer: O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O[Se](=O)O
Properties
Chemical formula	SeO 2
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)
Solubility in water	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	1 Pa (124.5 °C (256.1 °F; 397.6 K)) 1 kPa (228 °C (442 °F; 501 K)) 100 kPa (315 °C (599 °F; 588 K))
Band gap	3.29 eV
Magnetic susceptibility (χ)	−27.2×10−6 cm3/mol
Dipole moment	2.62±0.05 D
Structure
Crystal structure	Tetragonal
Space group	P42/mbc
Point group	4/mmm
Lattice constant	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
Std enthalpy of formation (ΔfH⦵298)	−225.4 kJ/mol
Enthalpy of fusion (ΔfH⦵fus)	17.6 kJ/mol
Hazards
Main hazards	Toxic by ingestion and inhalation
GHS pictograms
GHS Signal word	Danger
GHS hazard statements	H301+331Category:GHS errors, H373, H410
GHS precautionary statements	P260, P264, P270, P271, P273, P301+310+330Category:GHS errors, P304+340+311Category:GHS errors, P314, P391, P403+233, P405, P501
NFPA 704 (fire diamond)	0 4 1
Flash point	Non-flammable
Threshold limit value (TLV)	0.2 mg/m3 (TWA)
Lethal dose or concentration (LD, LC):
LD50 (median dose)	48 mg/kg (oral, rat) 4 mg/kg (dermal, rabbit)
LCLo (lowest published)	5890 mg/m3 (rabbit, 20 min) 6590 mg/m3 (goat, 10 min) 6590 mg/m3 (sheep, 10 min)
NIOSH (US health exposure limits):
PEL (Permissible)	0.2 mg/m3
REL (Recommended)	0.2 mg/m3
IDLH (Immediate danger)	1 mg/m3 (as Se)
Related compounds
Other cations	Ozone Sulfur dioxide Tellurium dioxide
Related selenium oxides	Selenium trioxide
Related compounds	Selenous acid Selenium disulfide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Selenium dioxide is the chemical compound with the formula SeO
2. 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.

Properties

Solid SeO
2 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. 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. The monomeric form adopts a bent structure very similar to that of sulfur dioxide with a bond length of 1.61 Å. in selenium oxydichloride give the trimer [Se(O)O]
3. 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 SO
2 tends to be molecular and SeO
2 is a one-dimensional chain, TeO
2 is a cross-linked polymer.

SeO
2 is considered an acidic oxide: it dissolves in water to form selenous acid. 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:

SeO
2 + 2 NaOH → Na
2SeO
3 + H
2O

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:

2 H
2O
2 + Se → SeO
2 + 2 H
2O

3 Se + 4 HNO
3 + H
2O → 3 H
2SeO
3 + 4 NO

H
2SeO
3 ⇌ SeO
2 + H
2O

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.

Uses

Organic synthesis

SeO₂ is an important reagent in organic synthesis. Oxidation of paraldehyde (acetaldehyde trimer) with SeO₂ gives glyoxal and the oxidation of cyclohexanone gives 1,2-cyclohexanedione. The selenium starting material is reduced to selenium, and precipitates as a red amorphous solid which can easily be filtered off. 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:

Allylic oxidation

This can be described more generally as:

R
2C=CR′–CHR″
2 + [O] → R
2C=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.

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.