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A dioxygen molecule can add or lose electrons during chemical transformations, forming molecular ions O₂^-, O₂^2-, O₂⁺. Their stability is predicted by MO method. They are called peroxide type compounds since these species have two-bonded atoms of oxygen (O—O).

Table 1. Molecular orbitals of dioxygen, O₂^-, O₂^2-, O₂⁺ (MO method)

 

Molecular orbitals	O2+	O2	O2-	O22-
sр*	—	—	—	—
pр* pр*	­	­ ­	­¯ ­	­¯ ­¯
pр pр	­¯ ­¯	­¯ ­¯	­¯ ­¯	­¯ ­¯
ss	­¯	­¯	­¯	­¯
ss*	­¯	­¯	­¯	­¯
ss	­¯	­¯	­¯	­¯
Bond order	2.5		1.5
O oxidation state	+ ½		- ½	-1
Distance O—O, nm	0.112	0.1207	0.132	0.149
Dissociation energy, kJ/mol

 

Adding one electron onto p*_2p molecular orbital, the molecule of oxygen forms a molecular ion O₂^-, which is named superoxide-ion, and its compounds are superoxides. The process O₂ + e^- = O₂^- is accompanied by the liberation of heat (DH°₂₈₉ = -48.1 kJ/mol) and decreases the bond order to 1.5 (the third electron is added to unpaired electrons of antibonding p*_2p orbitals of the О₂ molecule).

Superoxides can be obtained directly only for the most active reductants (alkali metals below potassium in IA group):

K + O₂ = KO₂

Dioxygen forms an ion О₂⁺ (bond order 2.5) at elimination of one electron from the p*_2p molecular orbital. This ion is named a dioxygenyl-ion. Dioxygen is only oxidized by very strong oxidants, for example by Pt(VI) fluorid e:

О₂ + PtF₆ = О₂[Pt⁵⁺F₆]

Dioxygenil hexafluoroplatinate (V)

These salts are also synthesized by heating a mixture of О₂, F₂ and a powdered element to 150-500°C:

О₂ + 3F₂ + M = О₂⁺[MF₆],

where M = As, Sb, Bi, Nb, Pt, Au, Ru, Rh.

A molecule О₂ can accept two electrons forming the peroxide-ion О₂^2-, derivatives of which are named peroxides. Additional electrons are added onto p*_2p MO. Thus the bond order lowers down to 1. Absence of the unpaired electrons determines diamagnetism of peroxides. Peroxides are formed during combustive oxidation of some active metals:

2Na + O₂ = Na₂О₂

Ва + О₂ = ВаО₂,

Hydrogen Peroxide H₂O₂

 

It is one of the most important compounds of oxygen.

Production of peroxides in industry:

1. Electrolysis of 50%-Н₂SO₄ solution or some sulfates (in particular, (NН₄)₂SO₄) at high current density and cooling. Anode oxidation of hydrogen sulfate-ions to peroxodisulfuric acid, Н₂S₂O₈, occurs on a platinum anode:

cathode: 2H⁺ + 2е = Н₂­

anode: 2НSO₄^- = Н₂S₂O₈ + 2е

 

The slight heating of H₂S₂O₈ solution leads to its hydrolysis:

Н₂S₂O₈ + Н₂O = Н₂O₂ + Н₂SO₄

In industry Н₂O₂ is produced and sold with concentration up to 60%, as more concentrated solutions are impossible to transport due to their high explosiveness. Concentrated to 30% and stabilized Н₂О₂ solution is named perhydrol. 3% solution of Н₂О₂ is used for medical purposes. World production of Н₂О₂ is a 0.5 million of tons per year.

2. Chemical methods are based on organic compounds oxidation with oxygen. Н₂О₂ together with other useful product - acetone are obtained at catalytic oxidation of isopropanol:

(СН₃)₂СНОН + О₂ ® СН₃СОСН₃ + Н₂О₂

Preparation in a laboratory:

It is easily obtained from BaO₂ at interaction with diluted Н₂SO₄:

ВаО₂ + Н₂SO₄ = Н₂О₂ + ВаSO₄¯

Structure. A molecule of Н₂О₂ is nonlinear, two bonds O—H are located in two planes:

Owing to the asymmetrical location of bonds the O—H molecule Н₂О₂ is strongly polar (electric dipole moment m = 0.7•10^-29 C•m), it exceeds water polarity m = 0.61•10^-29 C•m. The presence of unshared electron pairs of oxygen atoms in hydrogen peroxide creates a possibility for donor-acceptor interaction.

Properties. Instability of solutions. The Н₂О₂ of high purity and its solutions at STP are quite stable and can be stored for a long time, but at elevated temperatures, UV-irradiation, and also in the presence of ions of transition metals Н₂О₂ intensively decomposes:

2Н₂О₂ = 2Н₂О + О₂

Decomposition of pure Н₂О₂ and its concentrated solutions proceeds violently, explosion-like.

Additives (stabilizers, usually Na₃PO₄) are used to prevent Н₂О₂ decomposition in aqueous solutions. Stabilizers bind the ions of metals and as a result break undesirable catalytic process of decomposition. Solutions Н₂О₂ are stored in a cool place in dark vessels. It is worth knowing that even the minor content of alkalis leached from ordinary glass bottles substantially speed up the decomposition of Н₂О₂.

In the blood of a man and animals and juices of plants there is a specific enzyme of catalase,which decomposes Н₂О₂. The active component of catalase is ion of iron bound to complex organic molecules. One molecule of catalase decomposes at STP approximately 100 thousand molecules of Н₂О₂ per 1 sec.

Physical properties. Properties of liquid and solid Н₂О₂, as well as its solutions are determined by strong hydrogen bonds, which cause association of molecules. Therefore pure Н₂О₂ at STP is a syrup-like viscous, pale-blue, odourless liquid, which is almost 1.5 times heavier than water (r = 1.44 g/cm³ at 25°С), t_boil = 150.2°C (to determine t_boil Н₂О₂ at atmospheric pressure is impossible, as already at 90°C it decomposes).

Н₂О₂ is miscible in water at any proportion due to H-bonds formation.

Hydrogen peroxide is a very weak acid in aqueous solutions:

Н₂О₂ Н⁺ + НО₂^- К = 2.24•10^-12

but it is stronger than water. Dissociation at the second stage,

НО₂^- Н⁺ + О₂^2-

virtually does not occur. It is suppressed by the presence of water — a substance that dissociates with the formation of hydrogen ions to a greater extent than hydrogen peroxide.

Peroxides of metals which belong to the class of salts (not oxides) can be obtained by the interaction of a hydrogen peroxide solution with alkalis:

Н₂О₂ + Ва(ОН)₂ = ВаО₂ + 2Н₂О

Unlike normal oxides which form salt and water with acids, peroxides form salt and hydrogen peroxide in similar reactions:

ВаО₂ + Н₂SO₄ = BaSО₄ + Н₂О₂

ВаО + Н₂SO₄ = BaSО₄ + Н₂О

SnО₂ + 2Н₂SO₄ = Sn(SО₄)₂ + 2Н₂О

In aqueous solution the peroxides of metals, being the salts of a weak acid, are unstable. It is obvious at its strong hydrolysis and decomposition of Н₂О₂in alkaline medium:

Na₂О₂ + 2Н₂О Û 2NaОН + Н₂О₂

2Н₂О₂ ® 2Н₂О + О₂

The hydrogen atoms in hydrogen peroxide can be substituted not only by a metal but, for example, by acid residues with the formation of peroxide compounds of various types:

 

 

Peroxodisulfuric peroxosulfuric acids

 

peroxonitric acid (aquafortis)

НО—ОН

hydrogen peroxide

 

Bond O—O is weak due to the mutual electrostatic repulsion of two unshared pairs of electrons of each bound atom of oxygen. It is almost three times weaker than the bond O—H. Reactions of O—O bond destruction with the formation of compounds of oxygen (-2) or 0 are typical. Therefore the most characteristic Н₂О₂ transformations are namely redox reactions, where Н₂О₂ can be an oxidant or reductant with the following E°’s:

Н₂О₂ + 2Н⁺ + 2е^- = 2Н₂О E° = 1.78 V

О₂ + 2Н⁺ + 2е^- = Н₂О E° = 0.68 V

Oxidizing activity (E° = 1.78 V) of H₂O₂ is considerably stronger, than its reducing agent strength (E°= 0.68 V).

Indeed, peroxides are strong oxidants, water is the product of reduction:

2KI + Н₂О₂ + Н₂SO₄ = I₂ + 2Н₂О + K₂SO₄

KNO₂ + H₂O₂ = KNO₃ + H₂O

2K₃[Cr(OH)₆] + 3Н₂О₂ = 2K₂CrO₄ + 8Н₂О + 2KOH

Hydrogen peroxide can demonstrate also reducing properties (only with strong oxidants), oxygen is the product of oxidation of Н₂О₂, for example:

Н₂О₂ + Cl₂ = О₂ + 2НCl

Н₂О₂ + О₃ = 2О₂ + Н₂О

3Н₂О₂ + 2KMnО₄ = 3О₂ + 2MnО₂ + 2KOH + 2Н₂О

5Н₂О₂ + 2KMnО₄ + 3H₂SO₄ = 5О₂ + 2MnSO₄ + K₂SO₄ + 3Н₂О

The last reaction is used in chemical analysis for quantitative determination of Н₂О₂.

Н₂О₂ decomposition belongs to the disproportionation reactionstype (autooxidation-autoreduction):

2 Н₂О₂ = 2 Н₂О + О₂.

 

in the atmosphere into ozone and thereby provides a protective shield against short-wavelength UV light:

Practical activity of a man often predetermines destruction of ozone layer. Chlorine presence in atmosphere stands here in the first place. One molecule of chlorine is able to annihilate up to 100 thousands molecules of О₃ (comparison: 1 molecule NO destroys 10 molecules О₃ only). One launch of spaceship «Shuttle» destroys 0.3% О₃ from its common quantity in an earthly atmosphere.

 

Properties. О₃ is a toxic gas of a dark blue color with a strong characteristic smell, b.p. = -110°C, m.p. = -192.7°C. Liquid ozone has a navy blue color, solid ozone is black.

The molecule of О₃ diamagnetic, it has bent structure, it is polar (m = 0.17•10^-29 Cm). Length of bond between the atoms of oxygen (0.128 nm) has intermediate value compared to single (0.149 nm) and double (0.121 nm) bonds. Therefore the bond order in a molecule О₃ equals 1.5. The valence angle value (116.5°) is the evidence of sp²-hybridization of valence orbitals of the central oxygen atom. It is also the substantiation of delocalized three-center p-bond formation in О₃ molecule. Thus the central sp²-hybridized atom of oxygen forms 2 s-bonds with neighboring atoms. Its nonhybridized 2р_z-orbital is located perpendicularly to the plane of three oxygen atoms like 2р_z-orbitals of neighboring oxygen atoms forming together delocalized three-centered p- bond.

The asymmetrical structure of molecule О₃ predetermines its polarity. Therefore the energy of О₃ intermolecular interaction in the condensed state exceeds the latter of О₂ molecules. Consequently, t_boil and t_melt of ozone is higher than in О₂. These factors explain also ozone’s better solubility in polar solvents like water (45 volumes of О₃ is soluble in 100 volumes of water at STP).

О₃ easily decomposes and forms dioxygen and atomic oxygen, therefore it has a high oxidizing activity:

О₃ = О₂ + О

That is why О₃ easily oxidizes various substances, transforming itself into О². О₂ molecule remains as a stable fragment of unstable ozone molecule:

2Ag + О₃(OО₂) = Ag₂O + О₂

¯1e ­2e

PbS + 4О₃ = PbSO₄ + 4О₂

2KI + О₃ + H₂O = I₂ + О₃ + 2KOH

The latter reaction is used as the method of determination of ozone in quantitative chemical analysis.

In some case ozonides are the products of reactions with participation of ozone:

К + О₃ = КО₃

4КОН + 4О₃ = 4КО₃ + О₂ + 2Н₂О

Such compounds contain a molecular ion О₃^-. Unlike О₃ an О₃^-ion has an unpaired electron that predetermines its paramagnetizm. The ozonides of alkali metals have a red color and are very strong oxidants.

 

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