Disproportionation
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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]

Examples

Hg2Cl2 -> Hg + HgCl2
4 -> 3 H3PO4 + PH3
  • Desymmetrizing reactions are sometimes referred to as disproportionation, as illustrated by the thermal degradation of bicarbonate:
2 -> + H2CO3
The oxidation numbers remain constant in this acid-base reaction. This process is also called autoionization.


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

Polymer chemistry

In free-radical chain-growth polymerization, chain termination can occur by a disproportionation step in which a hydrogen atom is transferred from one growing chain molecule to another which produces two dead (non-growing) chains.[10]

-------CH2-C°HX + -------CH2-C°HX -> -------CH=CHX + -------CH2-CH2X

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:[11]

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.[12]

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.[13]

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.
  8. ^ a b José Jiménez Barberá; Adolf Metzger; Manfred Wolf (2000). "Sulfites, Thiosulfates, and Dithionites". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a25_477.
  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. ^ Cowie, J. M. G. (1991). Polymers: Chemistry & Physics of Modern Materials (2nd ed.). Blackie. p. 58. ISBN 0-216-92980-6.
  11. ^ 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.
  12. ^ Biochemical basis of mitochondrial acetaldehyde dismutation in Saccharomyces cerevisiae
  13. ^ 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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Disproportionation
 



 



 
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