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Subject - Organic Chemistry
In general, the term disproportionation describes a reaction in which by decomposition or rearrangement of identical reaction partners different compounds are generated.
Frequently, disproportionations are observed in redox reactions. In these reactions, one and the same starting material is oxidized and reduced at the same time and the oxidation numbers of several atoms of the same type change in different directions.
For example, chlorine (oxidation number zero) in sodium hydroxide disproportionates to chloride (-1) and hypochlorite (+1).
+ 2 → + +
TheCannizzaro reaction, too, is a disproportionation reaction. Here, two aldehyde molecules yield one carboxylic acid molecule by oxidation and one alcohol molecule by reduction.
2 → +
Especially frequent are disproportionations of hydrocarbon radicals.
2 → +
The disproportionation of two ethyl radicals into ethylene and ethane represents an example of this type of reaction. In this case, the formal redox reaction is not considered because oxidation stages are only rearranged and no new ones are formed. The important part is the transfer of hydrogen atoms (in rare cases other atom groups) generating one saturated and one unsaturated molecule from two radical molecules. Another example of this type of reaction is the disproportionation of two polymeric radicals (in this case the R groups represent large chains or substituents). Both unpaired electrons combine to form a π-bonding pair with simultaneous transfer of a hydrogen atom thus terminating the chain reaction. At this point, chain growth competes with disproportionation. In the former case, both molecules combine to yield one having the size of both combined, while in the latter two molecules each of the size of the radicals are generated.
The opposite to disproportionation generally is called comproportionation or synproportionation.
Recommended Learning Units
Cannizzaro Reaction20 min.
ChemistryOrganic ChemistryReduction Reactions
This learning unit introduces the disproportionation of aldehydes known as the Cannizzaro reaction. The crossed Cannizzaro reaction and the industrial applications of this reaction are also explained.