Disproportionation

Source: Wikipedia, the free encyclopedia.

In

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

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

  • Desymmetrizing reactions are sometimes referred to as disproportionation, as illustrated by the thermal degradation of bicarbonate:
The oxidation numbers remain constant in this acid-base reaction.
  • Disproportionation of sulfur intermediates by microorganisms are widely observed in sediments.[6][7][8][9]
The chlorine 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.
The oxidation state of oxygen is −1/2 in the superoxide free radical anion, −1 in hydrogen peroxide and 0 in dioxygen.

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 one, which produces two dead (non-growing) chains.[14]

Chain—CH2–CHX + Chain—CH2–CHX → Chain—CH=CHX + Chain—CH2–CH2X

in which, Chain— represents the already formed polymer chain, and indicates a reactive free radical.

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]

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

oxidant
.

Another example of biochemical dismutation reaction is the disproportionation of acetaldehyde into ethanol and acetic acid.[16]

While in

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]

Claus reaction: a comproportionation reaction

The

Claus reaction is an example of comproportionation reaction (the inverse of disproportionation) involving hydrogen sulfide (H2S) and sulfur dioxide (SO2) to produce elemental sulfur and water
as follows:

The Claus reaction is one of the chemical reactions involved in the Claus process used for the desulfurization of gases in the oil refinery plants and leading to the formation of solid elemental sulfur, more easy to store, transport and dispose off.

See also

References

  1. .
  2. .
  3. ^ Gadolin Johan (1788) K. Sv. Vet. Acad. Handl. 1788, 186-197.
  4. ^ Gadolin Johan (1790) Crells Chem. Annalen 1790, I, 260-273.
  5. PMC 202062
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  7. .
  8. ^ .
  9. ^ Book: Non-Aqueous Media, exact reference of this book is lacking: need to be completed!.
  10. ^ .
  11. ^ 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.
  12. .
  13. .
  14. ^ Biochemical basis of mitochondrial acetaldehyde dismutation in Saccharomyces cerevisiae
  15. S2CID 27142031
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  17. .