View source for Disproportionation

{{Short description|Redox reaction whose products have higher and lower oxidation states than the reactant}}
{{Other uses|Proportionality (disambiguation){{!}}Proportionality}}
{{More citations needed|date=June 2022}}

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 state.<ref>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}}.</ref><ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. {{ISBN|0-12-352651-5}}.</ref> 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 ''symproportionation''.

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:<ref>{{GoldBookRef |title=disproportionation |file=D01799 }}</ref>
:{{chem2|2 A -> A' + A"}}
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 (chemistry)|''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.<br />For example, the [[Schlenk equilibrium]]

:{{chem2|2 RMgX -> R2Mg + MgX2}}

is an example of a redistribution reaction.

== History ==
The first disproportionation reaction to be studied in detail was:

:{{chem2|2 Sn^{2+} -> Sn^{4+} + Sn}}

This was examined using [[tartrate]]s by [[Johan Gadolin]] in 1788. In the Swedish version of his paper he called it {{lang|sv|söndring}}.<ref>Gadolin Johan (1788) ''K. Sv. Vet. Acad. Handl.'' 1788, 186-197.</ref><ref>Gadolin Johan (1790) ''Crells Chem. Annalen'' 1790, I, 260-273.''</ref>

== Examples ==
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* [[Mercury(I) chloride]] disproportionates upon UV-irradiation:{{Clarify|reason=At which temperatures?|date=June 2022}}
::{{chem2|Hg2Cl2 -> HgCl2 + Hg}}

* [[Phosphorous acid]] disproportionates upon heating to 200°C to give [[phosphoric acid]] and [[phosphine]]:
::{{chem2|4 H3PO3 -> 3 H3PO4 + PH3}}

* Desymmetrizing reactions are sometimes referred to as disproportionation, as illustrated by the thermal degradation of bicarbonate:
::{{chem2|2 HCO3- -> CO3(2-) + H2CO3}}
:The oxidation numbers remain constant in this acid-base reaction.

* Another variant on disproportionation is [[radical disproportionation]], in which two radicals form an alkene and an alkane.
::<chem>{2CH3-\underset{^\bullet}CH2 -> {H2C=CH2} + H3C-CH3}</chem>

* Disproportionation of sulfur intermediates by microorganisms is widely observed in sediments.<ref>{{Cite journal |last1=Thamdrup |first1=Bo |last2=Finster |first2=Kai |last3=Hansen |first3=Jens Würgler |last4=Bak |first4=Friedhelm |date=January 1993 |title=Bacterial Disproportionation of Elemental Sulfur Coupled to Chemical Reduction of Iron or Manganese |url=http://dx.doi.org/10.1128/aem.59.1.101-108.1993 |journal=Applied and Environmental Microbiology |volume=59 |issue=1 |pages=101–108 |doi=10.1128/aem.59.1.101-108.1993 |pmid=16348835 |issn=0099-2240|pmc=202062 }}</ref><ref>{{Cite journal |last1=Habicht |first1=Kirsten S |last2=Canfield |first2=Donald E |last3=Rethmeier |first3=J̈org |date=August 1998 |title=Sulfur isotope fractionation during bacterial reduction and disproportionation of thiosulfate and sulfite |url=http://dx.doi.org/10.1016/s0016-7037(98)00167-7 |journal=Geochimica et Cosmochimica Acta |volume=62 |issue=15 |pages=2585–2595 |doi=10.1016/s0016-7037(98)00167-7 |issn=0016-7037}}</ref><ref>{{Cite journal |last1=Böttcher |first1=M.E. |last2=Thamdrup |first2=B. |last3=Vennemann |first3=T.W. |date=May 2001 |title=Oxygen and sulfur isotope fractionation during anaerobic bacterial disproportionation of elemental sulfur |url=http://dx.doi.org/10.1016/s0016-7037(00)00628-1 |journal=Geochimica et Cosmochimica Acta |volume=65 |issue=10 |pages=1601–1609 |doi=10.1016/s0016-7037(00)00628-1 |issn=0016-7037}}</ref><ref name=":0">{{Cite journal |last1=Tsang |first1=Man-Yin |last2=Böttcher |first2=Michael Ernst |last3=Wortmann |first3=Ulrich Georg |date=2023-08-20 |title=Estimating the effect of elemental sulfur disproportionation on the sulfur-isotope signatures in sediments |url=https://www.sciencedirect.com/science/article/pii/S0009254123002334 |journal=Chemical Geology |language=en |volume=632 |pages=121533 |doi=10.1016/j.chemgeo.2023.121533 |issn=0009-2541}}</ref>
::{{chem2|4 S^{0} + 4 H2O -> 3 H2S + SO4(2-) + 2 H+}}
::{{chem2|3 S^{0} + 2 FeOOH -> SO4(2-) + 2 FeS + 2 H+}}
::{{chem2|4 SO3(2-) + 2 H+ -> H2S + SO4(2-)}}

* [[Chlorine]] gas reacts with concentrated [[sodium hydroxide]] to form [[sodium chloride]], [[sodium chlorate]] and [[water]]. The ionic equation for this reaction is as follows:<ref>Charlie Harding, David Arthur Johnson, Rob Janes, (2002), ''Elements of the P Block'', Published by Royal Society of Chemistry, {{ISBN|0-85404-690-9}}</ref>
::{{chem2|3 Cl2 + 6 OH- -> 5 Cl- + ClO3- + 3 H2O}}
:The chlorine reactant is in [[oxidation state]] 0. In the products, the chlorine in the Cl<sup>−</sup> ion has an oxidation number of −1, having been reduced, whereas the oxidation number of the chlorine in the {{chem2|ClO3(-)}} ion is +5, indicating that it has been oxidized.

* Decomposition of numerous [[Interhalogen#Types of interhalogens|interhalogen compounds]] involve disproportionation. [[Bromine fluoride]] undergoes a disproportionation reaction to form [[bromine trifluoride]] and [[bromine]] in non-aqueous media:<ref>{{Cite book|title=Book: Non-Aqueous Media, exact reference of this book is lacking: need to be completed!}}</ref>{{Citation needed|date=March 2024}}
::{{chem2|3 BrF -> BrF3 + Br2}}

* The dismutation of [[superoxide]] [[free radical]] to [[hydrogen peroxide]] and [[oxygen]], catalysed in living systems by the [[enzyme]] [[superoxide dismutase]]:
::{{chem2|2 O2- + 2 H+ -> H2O2 + O2}}
:The oxidation state of oxygen is −{{frac|1|2}} in the superoxide free radical anion, −1 in hydrogen peroxide and 0 in dioxygen.

* In the [[Cannizzaro reaction]], an [[aldehyde]] is converted into an [[Alcohol (chemistry)|alcohol]] and a [[carboxylic acid]]. In the related [[Tishchenko reaction]], the [[organic redox reaction]] product is the corresponding [[ester]]. In the [[Kornblum–DeLaMare rearrangement]], a peroxide is converted to a [[ketone]] and an alcohol.

* The disproportionation of [[hydrogen peroxide]] into water and oxygen catalysed by either [[potassium iodide]] or the enzyme [[catalase]]:
::{{chem2|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 Nanotube|carbon nanotubes]]; high-pressure [[carbon monoxide]] disproportionates when catalysed on the surface of an iron particle:
::{{chem2|2 CO -> C + CO2}}

* [[Nitrogen]] has oxidation state +4 in [[nitrogen dioxide]], but when this compound reacts with water, it forms both [[nitric acid]] and [[nitrous acid]], where nitrogen has oxidation states +5 and +3 respectively:
::{{chem2|2 NO2 + H2O -> HNO3 + HNO2}}

* In [[hydrazoic acid]] and [[sodium azide]], each of the 3 nitrogen atoms of these very energetic linear polyatomic species has an oxidation state of −{{frac|1|3}}. These unstable and highly [[Toxicity|toxic]] compounds will disproportionate in [[aqueous solution]] to form gaseous nitrogen ({{chem2|N2}}) and [[ammonium ion]]s, or [[ammonia]], depending on [[pH]] conditions, as it can be conveniently verified by means of the [[Frost diagram]] for nitrogen:<ref name="Hendrix2019">{{cite journal | last1=Hendrix | first1=Katrien | last2=Bleyen | first2=Nele | last3=Mennecart | first3=Thierry | last4=Bruggeman | first4=Christophe | last5=Valcke | first5=Elie | title=Sodium azide used as microbial inhibitor caused unwanted by-products in anaerobic geochemical studies | journal=Applied Geochemistry | volume=107 | year=2019 | issn=0883-2927 | doi=10.1016/j.apgeochem.2019.05.014 | pages=120–130}}</ref>

:Under acidic conditions, hydrazoic acid disproportionates as: 
::{{chem2|3 HN3 + H+ -> 4 N2 + NH4+}}

:Under neutral, or basic, conditions, the azide [[anion]] disproportionates as:
::{{chem2|3 N3- + 3 H2O -> 4 N2 + NH3 + 3 OH-}}

* [[Dithionite]] undergoes acid hydrolysis to [[thiosulfate]] and [[bisulfite]]:<ref name=Ullmann>{{Ullmann |author1=José Jiménez Barberá |author2=Adolf Metzger |author3=Manfred Wolf |title=Sulfites, Thiosulfates, and Dithionites ||year=2000 |doi=10.1002/14356007.a25_477 |isbn=978-3527306732 }}</ref>
::{{chem2|2 S2O4(2-) + H2O -> S2O3(2-) + 2 HSO3-}}

* Dithionite also undergoes alkaline hydrolysis to [[sulfite]] and [[sulfide]]:<ref name=Ullmann/>
::{{chem2|3 Na2S2O4 + 6 NaOH -> 5 Na2SO3 + Na2S + 3 H2O}}

* [[Dithionate]] is prepared on a larger scale by oxidizing a cooled aqueous solution of [[sulfur dioxide]] with [[manganese dioxide]]:<ref>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.</ref> {{Citation needed|date=August 2020}}
::{{chem2|2 MnO2 + 3 SO2 -> MnS2O6 + MnSO4}}
<!-- * Disproportionation of [[trichlorosilane]] is the preferred route to prepare [[dichlorosilane]].<ref name="Vor2">Vorotyntsev, V., Mochalov, G., Kolotilova, M., Kinetics of Dichlorosilane Separation from a Mixture of Chlorosilanes by Distillation Using a Regular Packing, Theoretical Foundations of Chemical Engineering, 38(4), 355-359</ref> [[Silicon tetrachloride]] appears to be the byproduct.
::{{chem2|2 SiHCl3 <-> SiCl4 + SiH2Cl2}}
::Notice that this reaction can be viewed as disproportionation only because, in some formal senses, SiHCl<sub>3</sub>, SiCl<sub>4</sub>, and SiH<sub>2</sub>Cl<sub>2</sub> are classifiable as Si(II), Si(IV), and Si(0), respectively. -->
<!-- (many practicing chemists would see those ox state assignments are silly). -->

== 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.<ref name=Cowie>{{cite book |last1=Cowie |first1=J. M. G. |title=Polymers: Chemistry & Physics of Modern Materials |date=1991 |publisher=Blackie |isbn=0-216-92980-6 |page=58 |edition=2nd}}</ref>
:: Chain—CH<sub>2</sub>–CHX<sup>•</sup>  +  Chain—CH<sub>2</sub>–CHX<sup>•</sup>  →  Chain—CH=CHX  +  Chain—CH<sub>2</sub>–CH<sub>2</sub>X

in which, Chain— represents the already formed polymer chain, and <sup>•</sup> indicates a reactive [[Radical (chemistry)|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 [[Carbon dioxide|CO<sub>2</sub>]] by certain bacteria according to the global reaction:<ref>{{cite journal | last = Krebs | first = H.A. | year = 1937 |title = LXXXVIII - Dismutation of pyruvic acid in gonoccus and staphylococcus | journal = Biochem. J. | volume = 31 | issue = 4 | pages = 661–671 | pmc = 1266985 | pmid=16746383| doi = 10.1042/bj0310661 }}</ref>

:{{chem2|2 CH3COCOOH + H2O -> CH3CH(OH)COOH + CH3COOH + CO2}}

The dismutation of pyruvic acid in other small organic molecules (ethanol + CO<sub>2</sub>, or lactate and acetate, depending on the environmental conditions) is also an important step in [[Fermentation (biochemistry)|fermentation]] reactions. Fermentation reactions can also be considered as disproportionation or dismutation [[biochemical reaction]]s. Indeed, the [[electron donor|donor]] and [[electron acceptor|acceptor]] of electrons in the [[redox reaction]]s 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]].<ref>[http://jb.asm.org/cgi/reprint/173/21/7012.pdf  Biochemical basis of mitochondrial acetaldehyde dismutation in Saccharomyces cerevisiae]</ref>

While in [[Cellular respiration|respiration]] electrons are transferred from [[substrate (biochemistry)|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]].<ref>{{Cite journal |doi = 10.1038/326891a0|pmid = 22468292|title = A novel type of energy metabolism involving fermentation of inorganic sulphur compounds|journal = Nature|volume = 326|issue = 6116|pages = 891–892|year = 1987|last1 = Bak|first1 = Friedhelm|last2 = Cypionka|first2 = Heribert|bibcode = 1987Natur.326..891B|s2cid = 27142031}}</ref>

== Disproportionation of sulfur intermediates ==
[[Sulfur isotope biogeochemistry|Sulfur isotopes]] of sediments are often measured for studying environments in the Earth's past ([[Paleoecology|paleoenvironment]]). Disproportionation of sulfur intermediates, being one of the processes affecting sulfur isotopes of sediments, has drawn attention from [[Geologist|geoscientists]] for studying the [[redox]] conditions in the oceans in the past.

[[Sulfate-reducing microorganism|Sulfate-reducing bacteria]] fractionate [[Sulfur isotope biogeochemistry|sulfur isotopes]] as they take in [[sulfate]] and produce [[sulfide]]. Prior to 2010s, it was thought that sulfate reduction could fractionate [[Sulfur isotope biogeochemistry|sulfur isotopes]] up to 46&nbsp;‰<ref>{{Cite journal |last1=Goldhaber |first1=M.B. |last2=Kaplan |first2=I.R. |date=April 1980 |title=Mechanisms of sulfur incorporation and isotope fractionation during early diagenesis in sediments of the gulf of California |url=http://dx.doi.org/10.1016/0304-4203(80)90063-8 |journal=Marine Chemistry |volume=9 |issue=2 |pages=95–143 |doi=10.1016/0304-4203(80)90063-8 |issn=0304-4203}}</ref> and fractionation larger than 46&nbsp;‰ recorded in sediments must be due to disproportionation of sulfur intermediates in the sediment. This view has changed since the 2010s.<ref>{{Cite journal |last1=Sim |first1=Min Sub |last2=Bosak |first2=Tanja |last3=Ono |first3=Shuhei |date=July 2011 |title=Large Sulfur Isotope Fractionation Does Not Require Disproportionation |url=http://dx.doi.org/10.1126/science.1205103 |journal=Science |volume=333 |issue=6038 |pages=74–77 |doi=10.1126/science.1205103 |pmid=21719675 |issn=0036-8075}}</ref> As substrates for disproportionation are limited by the product of [[Sulfate-reducing microorganism|sulfate reduction]], the isotopic effect of disproportionation should be less than 16&nbsp;‰ in most sedimentary settings.<ref name=":0" />

Disproportionation can be carried out by microorganisms obligated to disproportionation or microorganisms that can carry out [[Sulfate-reducing microorganism|sulfate reduction]] as well. Common substrates for disproportionation include elemental [[sulfur]] ({{Chem2|S8}}), [[thiosulfate]] ({{Chem2|S2O3(2-)}}) and [[sulfite]] ({{Chem2|SO3(2-)}}).<ref name=":0" />

== Claus reaction: a comproportionation reaction ==
{{Main article|Claus process}}
The [[Claus reaction]] is an example of [[comproportionation]] reaction (the inverse of disproportionation) involving [[hydrogen sulfide]] ({{chem2|H2S}}) and [[sulfur dioxide]] ({{chem2|SO2}}) to produce [[Allotropes of sulfur|elemental sulfur]] and [[water]] as follows:

:{{chem2|2 H2S + SO2 -> 3 S + 2 H2O}}

The Claus reaction is one of the chemical reactions involved in the [[Claus process]] used for the [[desulfurization]] of [[gas]]es in the [[oil refinery]] plants and leading to the formation of [[solid]] elemental sulfur ({{chem2|S8}}), which is easier to store, transport, reuse when possible, and dispose of.

== See also ==
* [[Dismutase]]
* [[Oxidoreductase]]
* [[Fermentation (biochemistry)]]

== References ==
{{Reflist}}

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[[Category:Chemical reactions]]
[[Category:Chemical processes]]
[[Category:Organic reactions]]
[[Category:Biochemistry]]