Disproportionation


Disproportionation

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:

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.

Contents

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'.[1][2]

Examples

3Cl2 + 6OH → 5Cl + ClO3 + 3H2O
The chlorine gas 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.
2O2 + 2H+ → H2O2 + O2
The oxidation state of oxygen is -1/2 in the superoxide free radical anion, -1 in hydrogen peroxide and 0 in dioxygen.
2H2O2 → 2H2O + O2
2CO → C + CO2

Biochemistry

In 1937, Hans Adolf Krebs, 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:[4]

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 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.[5]

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.[6]

See also

References

  1. ^ Gadolin Johan (1788) K. Sv. Vet. Acad. Handl. 1788, 186-197.
  2. ^ Gadolin Johan (1790) Crells Chem. Annalen 1790, I, 260-273.
  3. ^ Charlie Harding, David Arthur Johnson, Rob Janes, (2002), Elements of the P Block, Published by Royal Society of Chemistry, ISBN 0854046909
  4. ^ Krebs, H.A. (1937). "LXXXVIII - Dismutation of pyruvic acid in gonoccus and staphylococcus". Biochem J. 31 (4): 661–671. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1266985/pdf/biochemj01050-0180.pdf. 
  5. ^ [http://jb.asm.org/cgi/reprint/173/21/7012.pdf | Biochemical basis of mitochondrial acetaldehyde dismutation in Saccharomyces cerevisiae]
  6. ^ A novel type of energy metabolism involving fermentation of inorganic sulphur compounds.

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  • disproportionation — disproporcionavimasis statusas T sritis chemija apibrėžtis Atomų arba grupių persiskirstymas tarp dviejų vienodų molekulių, radikalų, grupių. atitikmenys: angl. dismutation; disproportionation rus. дисмутация; диспропорционирование ryšiai:… …   Chemijos terminų aiškinamasis žodynas

  • disproportionation — disproporcionavimasis statusas T sritis chemija apibrėžtis Elemento oksidacijos laipsnio padidėjimas ir sumažėjimas vienodų molekulių reakcijos metu. atitikmenys: angl. dismutation; disproportionation rus. дисмутация; диспропорционирование ryšiai …   Chemijos terminų aiškinamasis žodynas

  • disproportionation — |disprəˌpōrshə|nāshən, pȯr noun : the transformation of a substance into two or more dissimilar substances usually by a process involving simultaneous oxidation and reduction : dismutation disproportionation of hydrogen peroxide to water and… …   Useful english dictionary

  • disproportionation — noun Date: circa 1929 the transformation of a substance into two or more dissimilar substances usually by simultaneous oxidation and reduction • disproportionate intransitive verb …   New Collegiate Dictionary

  • disproportionation — /dis preuh pawr sheuh nay sheuhn, pohr /, n. Chem. the simultaneous oxidation and reduction of a substance reacting with itself, thereby forming two dissimilar molecules, as 2C2H4 > C2H6+C2H2. [1925 30; DISPROPORTION + ATION] * * * …   Universalium

  • disproportionation — noun a) A form of redox reaction in which part of a reactant is oxidized and part of it is reduced simultaneously b) A reaction wherein a single reactant generates two chemical species which are different from each other and the initial reactant …   Wiktionary

  • disproportionation — noun Chemistry a reaction in which a substance is simultaneously oxidized and reduced, giving two different products …   English new terms dictionary

  • disproportionation — dis·pro·por·tion·a·tion …   English syllables

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