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^- → CO₃^2- + H₂CO₃

H₂PO₄^- + H₂PO₄^- → HPO₄^2- + 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 tartrates by Johan Gadolin in 1788. In the Swedish version of his paper he called it 'söndring'.^[1][2]

Examples

• Chlorine gas reacts with dilute sodium hydroxide to form sodium chloride, sodium chlorate and water. The ionic equation for this reaction is as follows ^[3]:

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.

• The dismutation of superoxide free radical to hydrogen peroxide and oxygen, catalysed in living systems by the enzyme superoxide dismutase:

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.

• 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:

2H₂O₂ → 2H₂O + O₂

• The Boudouard 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:

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

• Dismutase
• Oxidoreductase
• Fermentation
• Fermentation (biochemistry)
• Krebs cycle: citric acid cycle

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.