Oxygen compounds

See also: Category:Oxygen compounds

In almost all known compounds of oxygen, the oxidation state of oxygen is -2. The oxidation state -1 is found in a few compounds such as peroxides. Compounds containing oxygen in other oxidation states are very uncommon: -1/2 (superoxides),-1/3 (ozonides), 0 (elemental, hypofluorous acid), +1/2 (dioxygenyl), +1 (dioxygen difluoride) and +2 (oxygen difluoride). The most familiar oxygen-containing compound is H₂O. Other well-known examples include silica (found in sand, glass, rock, etc.), and the compounds of carbon and oxygen, such as carbon dioxide (CO₂), alcohols (R-OH), carbonyls, (R-CO-H or R-CO-R), and carboxylic acids (R-COOH). Oxygenated radicals such as chlorates (ClO₃⁻), perchlorates (ClO₄⁻), chromates (CrO₄²⁻), dichromates (Cr₂O₇²⁻), permanganates (MnO₄⁻), and nitrates (NO₃⁻) are strong oxidizing agents in and of themselves. Phosphorus is biologically important in its oxygenated form as the phosphate (PO₄³⁻) ion. Many metals bond with oxygen atoms, such as iron in iron(III) oxide (Fe₂O₃), commonly called rust.

There are known compounds of oxygen with almost all the other elements occurring in nature. The list of known compounds of oxygen includes some of the rarest elements: technetium (TcO₄⁻), promethium (Pm₂O₃), neptunium (NpO₂), plutonium (PuO₂); but also some of the least reactive such as xenon (XeO₃), gold (Au₂O₃) and platinum (PtO₂). Of the synthetic elements that have known oxides are: americium (AmO₂), curium (CuO₂), berkelium (BkO₃), californium (Cf₂O₃), einsteinium (Es₂O₃).

One unexpected oxygen compound is dioxygen hexafluoroplatinate O₂⁺PtF₆⁻. It was discovered when Neil Bartlett was studying the properties of platinum hexafluoride (PtF₆).^[1] He noticed a change in color when this compound was exposed to atmospheric air. Bartlett reasoned that xenon should be oxidized by PtF₆. This led him to the discovery of xenon hexafluoroplatinate Xe⁺PtF₆⁻.Epoxides are ethers in which the oxygen atom is part of a ring of three atoms. O₂²⁺ is another cation as in O₂F₂, it is only formed in the presence of stronger oxidants than oxygen, which limits this cation to oxygen fluorides, e.g. oxygen fluoride.^[2]

When dissolved in water, many metallic oxides form alkaline solutions while many oxides of nonmetals form acidic solutions. For example, sodium oxide in solution forms the strong base sodium hydroxide while phosphorus pentoxide in solution forms phosphoric acid.^[3]

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Oxides and peroxides

  Although oxygen molecules are not generally reactive at room temperature they do react with certain strong inorganic reducing substances, such as ferrous sulfate in aqueous solution.^[3] Oxygen also reacts spontaneously with many organic compounds at or below room temperature in a process called autoxidation.^[3] Other substances need to be heated before they will react with oxygen in bulk but some, such as iron, readily forms iron oxide, or rust, Fe₂O₃. The production of free oxygen by photosynthetic bacteria some 3.5 billion years ago precipitated iron out of solution in the oceans to be deposited as Fe₂O₃ in the economically-important iron ore hematite.

Due to its electronegativity, oxygen forms chemical bonds with almost all other free elements at elevated temperatures to give corresponding oxides. The only elements known to escape the possibility of combination with oxygen are a few of the noble gases and fluorine. So-called noble metals (common examples: gold, platinum) resist direct chemical combination with oxygen, and substances like gold(III) oxide must be formed by an indirect route.

Peroxides retain some of oxygen's original molecular structure. White or light yellow sodium peroxide (Na₂O₂) is formed when metallic sodium (Na) is burned in oxygen. Each oxygen atom in its peroxide ion may have a full octet of 4 pairs of electrons.^[4] Superoxides are a class of compounds that are very similar to peroxides, but with just one unpaired electron for each pair of oxygen atoms.^[4]. These compounds form from oxidation of alkali metals with larger ionic radii (K, Rb, Cs). For example, Potassium superoxide ( KO₂) is an orange-yellow solid formed when potassium (K) reacts with oxygen.

Hydrogen peroxide (H₂O₂) can be produced by passing a volume of 96 to 98% hydrogen and 2 to 4% oxygen through an electric discharge.^[3] A more commercially viable method is allow autoxidation of an organic intermediate; 2-ethylanthrahydroquinone dissolved in an organic solvent is oxidized to H₂O₂ and 2-ethylanthraquinone.^[3] The 2-ethylanthraquinone is then reduced and recycled back into the process.

Silicates and silica

Most chemically combined oxygen is locked in a class of minerals called silicates (which in turn are the major component of rocks and clays). The basic structure of silicates consists of two parts; units of silicon surrounded by four oxygen anions in a tetrahedral arrangement and units of metal-oxygen polyhedra that contain metal cations (examples: aluminium, calcium, iron and sodium).^[4] Both units are linked together by sharing oxygen anions, forming complex polymers in the process.

Water- soluble silicates in the form of Na₄SiO₄, Na₂SiO₃, and Na₂Si₂O₅ are used as detergents and adhesives.^[4] Na_xSi_xO_x with a higher ratio of SiO₂ to Na₂O has a greater molecular weight and a lower solubility. Silica is a crystalline polymer with the chemical formula (SiO₂)_n. Quartz is the mineral form of silica in nature and the most common deposits of quartz are in sand.

In organic compounds

Most of the thousands of organic compounds that contain oxygen are not made by direct action of oxygen. Many of the compounds that are directly created by a reaction with oxygen are commercially important. Examples and the reactions that form them include:^[4]

  Acetone ((CH₃)₂CO) and phenol (C₆H₅OH) are used as feeder materials in the synthesis of many different substances. Ethylene oxide (C₂H₄O) is used to make the antifreeze ethylene glycol.

C₂H₄ + 1/2 O₂ -catalyst-> C₂H₄O

Peracetic acid (CH₃(COOH)O) is the feeder material used to make various epoxy compounds.

CH₃CHO + O₂ -catalyst-> CH₃(COOH)O

Among the most important classes of organic compounds that contain oxygen are (where "R" is an organic group): alcohols (R-OH) such as ethanol, glycerol and glucose; ethers (R-O-R') such as diethyl ether; ketones (R-CO-R) such as acetone; aldehydes (R-CO-H) such as formaldehyde and glutaraldehyde; carboxylic acids (R-COOH) such as acetic acid in vinegar and citric acid; esters (R-COO-R') such as ethyl acetate and methyl salicylate]], amides (R-C(O)-NR₂) such as acetamide. There are several important organic solvents that contain oxygen, among which: acetone, methanol, furan, tetrahydrofuran, dioxane, ethylacetate.

Of the organic compounds with biological relevance, carbohydrates (such as glucose) contain a large amount of oxygen. All fatty acids (such as oleic acid) and aminoacids contain oxygen (due to the presence of carboxyl group). Furthermore, seven of the amino acids incorporate oxygen in the side-chain too: serine, tyrosine, threonine, glutamic acid, glutamine, aspartic acid and asparagine.

Oxygen also occurs in phosphate groups in the biologically important energy-carrying molecules ATP and ADP and in the backbone of RNA and DNA.

References

• Cook, Gerhard A.; Lauer, Carol M. (1968). "Oxygen", in Clifford A. Hampel: The Encyclopedia of the Chemical Elements. New York: Reinhold Book Corporation, 499-512. LCCN 68-29938. 

See also

• Oxygen