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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] 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.

More generally, the term can be applied to any desymmetrizing reaction where two molecules of one type react to give one each of two different types:[3]

This expanded definition is not limited to redox reactions, but also includes some molecular autoionization reactions, such as the self-ionization of water. In contrast, some authors use the term redistribution to refer to reactions of this type (in either direction) when only ligand exchange but no redox is involved and distinguish such processes from disproportionation and comproportionation. For example, the Schlenk equilibrium

2 RMgX → R2Mg + MgX2

is an example of a redistribution reaction.

History

The first disproportionation reaction to be studied in detail was:

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

Examples

The oxidation numbers remain constant in this acid-base reaction.

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.[14]

-------CH2–CHX + -------CH2–CHX → -------CH=CHX + -------CH2–CH2X

indicates radicals.

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:[15]

2CH3COCOOH + H2O -> CH3CH(OH)COOH + CH3COOH + 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.[16]

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.[17]

Disproportionation of sulfur intermediates

Sulfur isotopes of sediments are often measured for studying environments in the Earth's past (Paleoenvironment). Disproportionation of sulfur intermediates, being one of the processes affecting sulfur isotopes of sediments, has drawn attention from geoscientists for studying the redox conditions in the oceans in the past.

Sulfate-reducing bacteria fractionate sulfur isotopes as they take in sulfate and produce sulfide. Prior to 2010s, it was thought that sulfate reduction could fractionate sulfur isotopes up to 46 permil[18] and fractionation larger than 46 permil recorded in sediments must be due to disproportionation of sulfur intermediates in the sediment. This view has changed since the 2010s.[19] As substrates for disproportionation are limited by the product of sulfate reduction, the isotopic effect of disproportionation should be less than 16 permil in most sedimentary settings.[9]

Disproportionation can be carried out by microorganisms obligated to disproportionation or microorganisms that can carry out sulfate reduction as well. Common substrates for disproportionation include elemental sulfur, thiosulfate and sulfite.[9]

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. ^ Thamdrup, Bo; Finster, Kai; Hansen, Jens Würgler; Bak, Friedhelm (January 1993). "Bacterial Disproportionation of Elemental Sulfur Coupled to Chemical Reduction of Iron or Manganese". Applied and Environmental Microbiology. 59 (1): 101–108. doi:10.1128/aem.59.1.101-108.1993. ISSN 0099-2240. PMC 202062.
  7. ^ Habicht, Kirsten S; Canfield, Donald E; Rethmeier, J̈org (August 1998). "Sulfur isotope fractionation during bacterial reduction and disproportionation of thiosulfate and sulfite". Geochimica et Cosmochimica Acta. 62 (15): 2585–2595. doi:10.1016/s0016-7037(98)00167-7. ISSN 0016-7037.
  8. ^ Böttcher, M.E.; Thamdrup, B.; Vennemann, T.W. (May 2001). "Oxygen and sulfur isotope fractionation during anaerobic bacterial disproportionation of elemental sulfur". Geochimica et Cosmochimica Acta. 65 (10): 1601–1609. doi:10.1016/s0016-7037(00)00628-1. ISSN 0016-7037.
  9. ^ a b c Tsang, Man-Yin; Böttcher, Michael Ernst; Wortmann, Ulrich Georg (2023-08-20). "Estimating the effect of elemental sulfur disproportionation on the sulfur-isotope signatures in sediments". Chemical Geology. 632: 121533. doi:10.1016/j.chemgeo.2023.121533. ISSN 0009-2541.
  10. ^ Charlie Harding, David Arthur Johnson, Rob Janes, (2002), Elements of the P Block, Published by Royal Society of Chemistry, ISBN 0-85404-690-9
  11. ^ Non Aqueous Media.
  12. ^ 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. ISBN 978-3527306732.
  13. ^ 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.
  14. ^ Cowie, J. M. G. (1991). Polymers: Chemistry & Physics of Modern Materials (2nd ed.). Blackie. p. 58. ISBN 0-216-92980-6.
  15. ^ 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.
  16. ^ Biochemical basis of mitochondrial acetaldehyde dismutation in Saccharomyces cerevisiae
  17. ^ 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.
  18. ^ Goldhaber, M.B.; Kaplan, I.R. (April 1980). "Mechanisms of sulfur incorporation and isotope fractionation during early diagenesis in sediments of the gulf of California". Marine Chemistry. 9 (2): 95–143. doi:10.1016/0304-4203(80)90063-8. ISSN 0304-4203.
  19. ^ Sim, Min Sub; Bosak, Tanja; Ono, Shuhei (July 2011). "Large Sulfur Isotope Fractionation Does Not Require Disproportionation". Science. 333 (6038): 74–77. doi:10.1126/science.1205103. ISSN 0036-8075.