Disproportionation

In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states.[1][2] More generally, the term can be applied to any desymmetrizing reaction of the following type: 2 A → A' + A", regardless of whether it is a redox or some other type of process.[3]

For other uses, see Proportionality.

Examples

Mercury(I) chloride disproportionates upon UV-irradiation:

Hg2Cl2 → Hg + HgCl2

Phosphorous acid disproportionates upon heating to give phosphoric acid and phosphine:

4 H
3PO
3 → 3 H3PO4 + PH3

As mentioned above, desymmetrizing reactions are sometimes referred to as disproportionation, as illustrated by the thermal degradation of bicarbonate:

2 HCO−
3 → CO2−
3 + H2CO3

The oxidation numbers remain constant in this acid-base reaction. This process is also called autoionization.

Another variant on disproportionation is radical disproportionation, in which two radicals form an alkane and alkene.

Reverse reaction

The reverse of disproportionation, such as when a compound in an intermediate oxidation state is formed from precursors of lower and higher oxidation states, is called comproportionation, also known as synproportionation.

History

The first disproportionation reaction to be studied in detail was:

2 Sn2+ → Sn4+ + Sn

This was examined using tartrates by Johan Gadolin in 1788. In the Swedish version of his paper he called it 'söndring'.[4][5]

Further examples
3 Cl2 + 6 OH− → 5 Cl− + ClO3− + 3 H2O
  • The chlorine gas reactant is in oxidation state 0. In the products, the chlorine in the Cl− ion has an oxidation number of −1, having been reduced, whereas the oxidation number of the chlorine in the ClO3− ion is +5, indicating that it has been oxidized.
3 BrF → BrF3 + Br2
2 O2− + 2 H+ → H2O2 + O2
The oxidation state of oxygen is −1/2 in the superoxide free radical anion, −1 in hydrogen peroxide and 0 in dioxygen.
2 H2O2 → 2 H2O + O2
  • In the Boudouard reaction, carbon monoxide disproportionates to carbon and carbon dioxide. The reaction is for example used in the HiPco method for producing carbon nanotubes, high-pressure carbon monoxide disproportionates when catalysed on the surface of an iron particle:
2 CO → C + CO2
2 NO2 + H2O → HNO3 + HNO2
2 S
2O2−
4 + H
2O → S
2O2−
3 + 2 HSO−
3
3 Na
2S
2O
4 + 6 NaOH → 5 Na
2SO
3 + Na
2S + 3 H
2O
2 MnO
2 + 3 SO
2 → MnS
2O
6 + MnSO
4[9]

Biochemistry

In 1937, Hans Adolf Krebs, who discovered the citric acid cycle bearing his name, confirmed the anaerobic dismutation of pyruvic acid into lactic acid, acetic acid and CO2 by certain bacteria according to the global reaction:[10]

2 pyruvic acid + H2O → lactic acid + acetic acid + CO2

The dismutation of pyruvic acid in other small organic molecules (ethanol + CO2, or lactate and acetate, depending on the environmental conditions) is also an important step in fermentation reactions. Fermentation reactions can also be considered as disproportionation or dismutation biochemical reactions. Indeed, the donor and acceptor of electrons in the redox reactions supplying the chemical energy in these complex biochemical systems are the same organic molecules simultaneously acting as reductant or oxidant.

Another example of biochemical dismutation reaction is the disproportionation of acetaldehyde into ethanol and acetic acid.[11]

While in respiration electrons are transferred from substrate (electron donor) to an electron acceptor, in fermentation part of the substrate molecule itself accepts the electrons. Fermentation is therefore a type of disproportionation, and does not involve an overall change in oxidation state of the substrate. Most of the fermentative substrates are organic molecules. However, a rare type of fermentation may also involve the disproportionation of inorganic sulfur compounds in certain sulfate-reducing bacteria.[12]

See also

References
  1. Shriver, D. F.; Atkins, P. W.; Overton, T. L.; Rourke, J. P.; Weller, M. T.; Armstrong, F. A. “Inorganic Chemistry” W. H. Freeman, New York, 2006. ISBN 0-7167-4878-9.
  2. Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  3. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version:  (2006–) "disproportionation". doi:10.1351/goldbook.D01799
  4. Gadolin Johan (1788) K. Sv. Vet. Acad. Handl. 1788, 186-197.
  5. Gadolin Johan (1790) Crells Chem. Annalen 1790, I, 260-273.
  6. Charlie Harding, David Arthur Johnson, Rob Janes, (2002), Elements of the P Block, Published by Royal Society of Chemistry, ISBN 0-85404-690-9
  7. Non Aqueous Media.
  9. J. Meyer and W. Schramm, Z. Anorg. Chem., 132 (1923) 226. Cited in: A Comprehensive Treatise on Theoretical and Inorganic Chemistry, by J.W. Meller, John Wiley and Sons, New York, Vol. XII, p. 225.
  10. Krebs, H.A. (1937). "LXXXVIII - Dismutation of pyruvic acid in gonoccus and staphylococcus". Biochem. J. 31 (4): 661–671. doi:10.1042/bj0310661. PMC 1266985. PMID 16746383.
  11. Biochemical basis of mitochondrial acetaldehyde dismutation in Saccharomyces cerevisiae
  12. Bak, Friedhelm; Cypionka, Heribert (1987). "A novel type of energy metabolism involving fermentation of inorganic sulphur compounds". Nature. 326 (6116): 891–892. Bibcode:1987Natur.326..891B. doi:10.1038/326891a0. PMID 22468292. S2CID 27142031.
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