Disproportionation
Encyclopedia
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products.
For example: the UV photolysis of mercury(I) chloride
Hg2Cl2 → Hg + HgCl2 is a disproportionation since mercury (I) is both reduced to mercury (0) and oxidized to mercury (II). A similar type of reaction, but in which no element changes oxidation number, is the acid-base disproportionation reaction observed when an amphiprotic
species reacts with itself. Two common examples for conjugated bases of polyprotic acids such as bicarbonate
and dihydrogenophosphate
are respectively:
(the oxidation numbers remain constant in these acid-base reactions: O = 2-, H = 1+, C = 4+, P = 5+). This is also called autoionization.
The reverse of disproportionation, when a compound in an intermediate oxidation state is formed from compounds in lower and higher oxidation states, is called comproportionation.
This was examined using tartrate
s by Johan Gadolin
in 1788. In the Swedish version of his paper he called it 'söndring'.
, who discovered the citric acid cycle
bearing his name, confirmed the anaerobic dismutation of pyruvic acid
in lactic acid
, acetic acid
and CO2
by certain bacteria according to the global reaction:
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
in ethanol
and acetic acid
.
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 sulphur compounds in certain sulphate-reducing bacteria.
can also undergo a dismutation reaction to produce methane
and carbon dioxide
:
This disproportionation reaction is catalysed by methanogen
bacteria in their fermentative metabolism. One electron is transferred from the carbonyl
function (e– donor
) of the carboxylic group to the methyl group (e– acceptor
) of acetic acid to respectively produce CO2 and methane gas.
For example: the UV photolysis of mercury(I) chloride
Mercury(I) chloride
Mercury chloride is the chemical compound with the formula Hg2Cl2. Also known as calomel or mercurous chloride, this dense white or yellowish-white, odorless solid is the principal example of a mercury compound...
Hg2Cl2 → Hg + HgCl2 is a disproportionation since mercury (I) is both reduced to mercury (0) and oxidized to mercury (II). A similar type of reaction, but in which no element changes oxidation number, is the acid-base disproportionation reaction observed when an amphiprotic
Amphoterism
In chemistry, an amphoteric species is a molecule or ion that can react as an acid as well as a base. The word is derived from the Greek word amphoteroi meaning "both"...
species reacts with itself. Two common examples for conjugated bases of polyprotic acids such as bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...
and dihydrogenophosphate
Phosphate
A phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
are respectively:
- HCO3- + HCO3- → CO32- + H2CO3
- H2PO4- + H2PO4- → HPO42- + H3PO4
(the oxidation numbers remain constant in these acid-base reactions: O = 2-, H = 1+, C = 4+, P = 5+). This is also called autoionization.
The reverse of disproportionation, when a compound in an intermediate oxidation state is formed from compounds in lower and higher oxidation states, is called comproportionation.
History
The first disproportionation reaction to be studied in detail was:- 2 Sn2+ → Sn4+ + Sn
This was examined using tartrate
Tartrate
A tartrate is a salt or ester of the organic compound tartaric acid, a dicarboxylic acid. Its formula is O−OC-CH-CH-COO− or C4H4O62−.As food additives, tartrates are used as antioxidants, acidity regulators, and emulsifiers...
s by Johan Gadolin
Johan Gadolin
Johan Gadolin was a Finnish chemist, physicist and mineralogist. Gadolin discovered the chemical element yttrium...
in 1788. In the Swedish version of his paper he called it 'söndring'.
Examples
- ChlorineChlorineChlorine is the chemical element with atomic number 17 and symbol Cl. It is the second lightest halogen, found in the periodic table in group 17. The element forms diatomic molecules under standard conditions, called dichlorine...
gas reacts with dilute sodium hydroxide to form sodium chlorideSodium chlorideSodium chloride, also known as salt, common salt, table salt or halite, is an inorganic compound with the formula NaCl. Sodium chloride is the salt most responsible for the salinity of the ocean and of the extracellular fluid of many multicellular organisms...
, sodium chlorateSodium chlorateSodium chlorate is a chemical compound with the chemical formula . When pure, it is a white crystalline powder that is readily soluble in water. It is hygroscopic. It decomposes above 250 °C to release oxygen and leave sodium chloride...
and waterWaterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. The ionic equation for this reaction is as follows :
- 3 Cl2 + 6 OH− → 5 Cl− + ClO3− + 3 H2O
- The chlorine gas reactant is in oxidation stateOxidation stateIn chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
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 dismutation of superoxideSuperoxideA superoxide, also known by the obsolete name hyperoxide, is a compound that possesses the superoxide anion with the chemical formula O2−. The systematic name of the anion is dioxide. It is important as the product of the one-electron reduction of dioxygen O2, which occurs widely in nature...
free radical to hydrogen peroxideHydrogen peroxideHydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
and oxygenOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, catalysed in living systems by the enzymeEnzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
superoxide dismutaseSuperoxide dismutaseSuperoxide dismutases are a class of enzymes that catalyze the dismutation of superoxide into oxygen and hydrogen peroxide. As such, they are an important antioxidant defense in nearly all cells exposed to oxygen...
:
- 2 O2− + 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 reactionCannizzaro reactionThe Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position...
, an aldehydeAldehydeAn aldehyde is an organic compound containing a formyl group. This functional group, with the structure R-CHO, consists of a carbonyl center bonded to hydrogen and an R group....
is converted into an alcoholAlcoholIn chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
and a carboxylic acidCarboxylic acidCarboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
. In the related Tishchenko reactionTishchenko reactionThe Tishchenko reaction is a chemical reaction that involves disproportionation of an aldehyde lacking a hydrogen atom in the alpha position in the presence of an alkoxide. The reaction product is an ester. Catalysts are aluminium alkoxides or sodium alkoxides...
, the organic redox reactionOrganic redox reactionOrganic reductions or organic oxidations or organic redox reactions are redox reactions that take place with organic compounds. In organic chemistry oxidations and reductions are different from ordinary redox reactions because many reactions carry the name but do not actually involve electron...
product is the corresponding esterEsterEsters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...
. In the Kornblum–DeLaMare rearrangementKornblum–DeLaMare rearrangementThe Kornblum–DeLaMare rearrangement is a rearrangement reaction in organic chemistry in which a primary or secondary organic peroxide is converted to the corresponding ketone and alcohol under base catalysis...
, a peroxide is converted to a ketoneKetoneIn organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...
and an alcohol.
- The disproportionation of hydrogen peroxideHydrogen peroxideHydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
into water and oxygen catalysed by either potassium iodidePotassium iodidePotassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with...
or the enzyme catalaseCatalaseCatalase is a common enzyme found in nearly all living organisms that are exposed to oxygen, where it catalyzes the decomposition of hydrogen peroxide to water and oxygen...
:
- 2 H2O2 → 2 H2O + O2
- The Boudouard reactionBoudouard reactionThe Boudouard reaction is the redox reaction of a chemical equilibrium mixture of carbon monoxide and carbon dioxide at a given temperature. It is the disproportionation of carbon monoxide into carbon dioxide and graphite or its reverse:...
is for example used in the HiPco method for producing carbon nanotubesCarbon nanotubeCarbon nanotubes are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than for any other material...
, high pressure carbon monoxideCarbon monoxideCarbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
disproportionates when catalysed on the surface of an iron particle:
- 2 CO → C + CO2
Biochemistry
In 1937, Hans Adolf KrebsHans Adolf Krebs
Sir Hans Adolf Krebs was a German-born British physician and biochemist. Krebs is best known for his identification of two important metabolic cycles: the urea cycle and the citric acid cycle...
, who discovered the citric acid cycle
Citric acid cycle
The citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...
bearing his name, confirmed the anaerobic dismutation of pyruvic acid
Pyruvic acid
Pyruvic acid is an organic acid, a ketone, as well as the simplest of the alpha-keto acids. The carboxylate ion of pyruvic acid, CH3COCOO−, is known as pyruvate, and is a key intersection in several metabolic pathways....
in lactic acid
Lactic acid
Lactic acid, also known as milk acid, is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula C3H6O3...
, acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
and CO2
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
by certain bacteria according to the global reaction:
- 2 pyruvic acid + H2O → lactic acid + acetic acid + 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
Fermentation (biochemistry)
Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
reactions. Fermentation reactions can also be considered as disproportionation or dismutation biochemical reactions. Indeed, the donor
Electron donor
An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....
and acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
of electrons in the redox reactions supplying the chemical energy
Chemical energy
Chemical energy is the potential of a chemical substance to undergo a transformation through a chemical reaction or, to transform other chemical substances...
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
Acetaldehyde
Acetaldehyde is an organic chemical compound with the formula CH3CHO or MeCHO. It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale industrially. Acetaldehyde occurs naturally in coffee, bread, and ripe fruit, and is produced by plants as part...
in ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
and acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
.
While in respiration
Respiration
Respiration may refer to:Biology* Respiratory system, the anatomical system of an organism used for respiration* Cellular respiration, the process in which nutrients are converted into useful energy in a cell...
electrons are transferred from substrate
Substrate (biochemistry)
In biochemistry, a substrate is a molecule upon which an enzyme acts. Enzymes catalyze chemical reactions involving the substrate. In the case of a single substrate, the substrate binds with the enzyme active site, and an enzyme-substrate complex is formed. The substrate is transformed into one or...
(electron donor
Electron donor
An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....
) to an electron acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
, 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
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
of the substrate. Most of the fermentative substrates are organic molecules. However, a rare type of fermentation may also involve the disproportionation of inorganic sulphur compounds in certain sulphate-reducing bacteria.
Methane gas production in fermentation
Acetic acidAcetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
can also undergo a dismutation reaction to produce methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
:
- CH3COO– + H+ → CH4 + CO2 ΔG° = -36 kJ/reaction
This disproportionation reaction is catalysed by methanogen
Methanogen
Methanogens are microorganisms that produce methane as a metabolic byproduct in anoxic conditions. They are classified as archaea, a group quite distinct from bacteria...
bacteria in their fermentative metabolism. One electron is transferred from the carbonyl
Carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....
function (e– donor
Electron donor
An electron donor is a chemical entity that donates electrons to another compound. It is a reducing agent that, by virtue of its donating electrons, is itself oxidized in the process....
) of the carboxylic group to the methyl group (e– acceptor
Electron acceptor
An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process....
) of acetic acid to respectively produce CO2 and methane gas.
See also
- DismutaseDismutaseA dismutase is an enzyme that catalyzes a dismutation reaction....
- OxidoreductaseOxidoreductaseIn biochemistry, an oxidoreductase is an enzyme that catalyzes the transfer of electrons from one molecule to another...
- FermentationFermentationFermentation may refer to:* Fermentation , the use of fermentation in food preparation* Fermentation , a metabolic process whereby electrons released from nutrients are ultimately transferred to molecules obtained from the breakdown of those same nutrients* Fermentation , the process of...
- Fermentation (biochemistry)Fermentation (biochemistry)Fermentation is the process of extracting energy from the oxidation of organic compounds, such as carbohydrates, using an endogenous electron acceptor, which is usually an organic compound. In contrast, respiration is where electrons are donated to an exogenous electron acceptor, such as oxygen,...
- Krebs cycle: citric acid cycleCitric acid cycleThe citric acid cycle — also known as the tricarboxylic acid cycle , the Krebs cycle, or the Szent-Györgyi-Krebs cycle — is a series of chemical reactions which is used by all aerobic living organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats and...