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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Disproportionation | 1/2 | https://en.wikipedia.org/wiki/Disproportionation | reference | science, encyclopedia | 2026-05-05T10:47:17.360434+00:00 | kb-cron |
In chemistry, disproportionation, sometimes called dismutation (the French word), 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. 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:
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 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:
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.
== Examples == Mercury(I) chloride disproportionates upon UV-irradiation: Hg2Cl2 → HgCl2 + Hg Phosphorous acid disproportionates upon heating to 200°C to give phosphoric acid and phosphine: 4 H3PO3 → 3 H3PO4 + PH3 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. Another variant on disproportionation is radical disproportionation, in which two radicals form an alkene and an alkane.
2
CH
3
−
C
∙
H
2
⟶
H
2
C
=
CH
2
+
H
3
C
−
CH
3
{\displaystyle {\ce {2CH3-{\underset {^{\bullet }}{C}}H2->{H2C=CH2}+H3C-CH3}}}
Disproportionation of sulfur intermediates by microorganisms is widely observed in sediments. 4 S0 + 4 H2O → 3 H2S + SO2−4 + 2 H+ 3 S0 + 2 FeOOH → SO2−4 + 2 FeS + 2 H+ 4 SO2−3 + 2 H+ → H2S + SO2−4 Chlorine gas reacts with concentrated sodium hydroxide to form sodium chloride, sodium chlorate and water. The ionic equation for this reaction is as follows: 3 Cl2 + 6 OH− → 5 Cl− + ClO−3 + 3 H2O 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 ClO−3 ion is +5, indicating that it has been oxidized. Decomposition of numerous interhalogen compounds involve disproportionation. Bromine fluoride undergoes a disproportionation reaction to form bromine trifluoride and bromine in non-aqueous media: 3 BrF → BrF3 + Br2 The dismutation of superoxide free radical to hydrogen peroxide and oxygen, catalysed in living systems by the enzyme superoxide dismutase: 2 O−2 + 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. In the Cannizzaro reaction, an aldehyde is converted into an 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: 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 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: 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 −1⁄3. These unstable and highly toxic compounds will disproportionate in aqueous solution to form gaseous nitrogen (N2) and ammonium ions, or ammonia, depending on pH conditions, as it can be conveniently verified by means of the Frost diagram for nitrogen: Under acidic conditions, hydrazoic acid disproportionates as: 3 HN3 + H+ → 4 N2 + NH+4 Under neutral, or basic, conditions, the azide anion disproportionates as: 3 N−3 + 3 H2O → 4 N2 + NH3 + 3 OH− Dithionite undergoes acid hydrolysis to thiosulfate and bisulfite: 2 S2O2−4 + H2O → S2O2−3 + 2 HSO−3 Dithionite also undergoes alkaline hydrolysis to sulfite and sulfide: 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: 2 MnO2 + 3 SO2 → MnS2O6 + MnSO4
== 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.
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: