Disproportionation

Disproportionation is a specific type of redox reaction in which a species is simultaneously reduced and oxidised to form two different products.

Overview

For example, the UV photolysis of mercury(I) chloride Hg₂Cl₂ → Hg + HgCl₂ is a disproportionation. Mercury(I) is a diatomic dication Hg²⁺
₂. In this reaction, the chemical bond in the molecular ion is broken, and one mercury atom is reduced to mercury(0), and the other is 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:

2 HCO⁻
₃ → CO²⁻
₃ + H₂CO₃

2 H
₂PO⁻
₄ → HPO²⁻
₄ + H₃PO₄

The oxidation numbers remain constant in these acid-base reactions: O = −2, H = +1, C = +4, P = +5. This is also called autoionization.

Another variant on disproportionation is radical disproportionation, in which two radicals form an alkane and alkene.

Reverse reaction

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, also known as symproportionation.

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

• 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 Cl₂ + 6 OH⁻ → 5 Cl⁻ + ClO₃⁻ + 3 H₂O

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:

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 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

• 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:

2 CO → C + CO₂

• Nitrogen has oxidation state +IV in nitrogen dioxide, but when this compound reacts with water, it forms both nitric acid and nitrous acid, where nitrogen has oxidation states +V and +III respectively:

• 2 NO₂ + H₂O → HNO₃ + HNO₂

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:

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 into 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 sulfur compounds in certain sulfate-reducing bacteria.

See also

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