Hydrogen polyoxide

Chemical structure of water, the simplest hydrogen polyoxide

Hydrogen polyoxides (also known as oxidanes, oxohydrogens, or oxyhydrogens) are chemical compounds that consist only of hydrogen and oxygen atoms, are bonded exclusively by single bonds (i.e., they are saturated), and are acyclic (have molecular structures containing no cycles or loops). They can, therefore, be classed as hydrogen chalcogenides.

The simplest possible hydrogen polyoxide (the parent molecule) is water, H₂O. The general structure of the class of molecules is some number of oxygen atoms single-bonded to each other in a chain. The oxygen atom at each end of this oxygen skeleton is attached to a hydrogen atom. Thus, these compounds form a homologous series with chemical formula H
2O
n in which the members differ by a constant relative molecular mass of 16 (the mass of each additional oxygen atom). The number of oxygen atoms is used to define the size of the hydrogen polyoxide (e.g., hydrogen pentoxide contains a five-oxygen backbone).

An oxidanyl group is a functional group or side-chain, that like a hydrogen polyoxide, consists solely of single-bonded oxygen and hydrogen atoms: for example, a hydroxy (oxidyl) or hydroperoxy (dioxidanyl) group.

Specific examples

Hydrogen polyoxides containing up to five oxygen atoms have been produced experimentally.

• Water (H₂O) is the most common hydrogen polyoxide, occurring widely on Earth's surface.
• Hydrogen peroxide (H₂O₂) is a common disinfectant and readily decomposes to form water and oxygen.
• Trioxidane (H₂O₃) is rare and readily decomposes into water and singlet oxygen.
• Hydrogen tetroxide (H₂O₄) has been synthesized by reaction among peroxy radicals at low temperature.^[1]
• Hydrogen pentoxide (H₂O₅) is a byproduct of trioxidane production^[2] and has also been synthesized by reaction among peroxy radicals at low temperature.^[1]

Hydrogen polyoxides containing up to 10 oxygen atoms have been studied theoretically, but those containing more than five oxygens are expected to be extremely unstable.^[3]

Ionization

All the hydrogen polyoxides are known or expected to autoionise when in liquid form, with the acidic hydrogen being solvated by other of the neutral polyoxide molecule(s).

H₂O_n ⇌ H⁺ + HO_n^–

2H₂O_n ⇌ H₃O_n + HO_n^–

See also

• Polysulfide
• Peroxide

References

1. Levanov, Alexander V.; Sakharov, Dmitri V.; Dashkova, Anna V.; Antipenko, Ewald E.; Lunin, Valeri V. (2011). "Synthesis of Hydrogen Polyoxides H2O4 and H2O3 and Their Characterization by Raman Spectroscopy". European Journal of Inorganic Chemistry: 5144–5150. doi:10.1002/ejic.201100767.
2. Xin Xu and William A. Goddard III. Peroxonechemistry: Formation of H₂O₃ and ring-(HO₂)(HO₃) from O₃/H₂O₂
3. Martins-Costa, Marilia; Anglada, Josep M.; Ruiz-Lopez, Manuel F. "Structure, stability, and dynamics of hydrogen polyoxides". International Journal of Quantum Chemistry. 111: 1543–1554. doi:10.1002/qua.22695.