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
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]
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.
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:
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:
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]
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.
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]
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.
^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.
ISBN0-7167-4878-9.
^Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001.
ISBN0-12-352651-5.
^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.
^Cowie, J. M. G. (1991). Polymers: Chemistry & Physics of Modern Materials (2nd ed.). Blackie. p. 58.
ISBN0-216-92980-6.