Organic redox reaction
Overview
Organic 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 transfer in the electrochemical sense of the word.
Following the rules for determining the oxidation number for an individual carbon atom leads to
- oxidation number -4 for alkanes,
- oxidation number -2 for alkenes, alcohols, alkyl halides, amines,
- oxidation number 0 for alkynes, ketones, aldehydes, geminal diols,
- oxidation number +2 for carboxylic acids, amides, chloroform and
- oxidation number +4 for carbon dioxide, tetrachloromethane.
Methane is oxidized to carbon dioxide because the oxidation number changes from -4 to +4. Classical reductions include alkene reduction to alkanes and classical oxidations include oxidation of alcohols to aldehydes with manganese dioxide. In oxidations electrons are removed and the electron density of a molecule is reduced. In reductions electron density increases when electrons are added to the molecule. This terminology is always centered around the organic compound. So a ketone is always reduced by Lithium aluminium hydride but it is bad form to have lithium aliminium hydride oxidized by a ketone. Many oxidations involve removal of protons from the organic molecule and the reverse reduction adds protons to an organic molecule.
Many reactions classified as reductions also appear in other classes. For instance conversion of the ketone to an alcohol by Lithium aluminium hydride can be considered a reduction but the hydride is also a good nucleophile in nucleophilic substitution. Many redox reactions in organic chemistry have coupling reaction reaction mechanism involving free radical intermediates. True organic redox chemistry can be found in electrochemical organic synthesis or electrosynthesis. Examples of organic reactions that can take place in an electrochemical cell are the Kolbe electrolysis [1]
In disproportionation reactions the reactant is both oxidised and reduced in the same chemical reaction forming 2 separate compounds.
Asymmetric catalytic reductions and asymmetric catalytic oxidations are important in asymmetric synthesis.
Organic reductions
Several reaction mechanisms exist for organic reductions:
- Direct electron transfer in one-electron reduction with the Birch reduction as example
- Hydride transfer in reductions with for example Lithium aluminium hydride or a hydride shift as in the Meerwein-Ponndorf-Verley reduction
- Hydrogen reductions with a catalyst such as the Lindlar catalyst or the Adkins catalyst or in specific reductions such as the Rosenmund reduction.
- Disproportionation reaction such as the Cannizzaro reaction
Reductions that do not fit in any reduction reaction mechanism and in which just the change in oxidation state is reflected include the Wolff-Kishner reaction.
Organic oxidations
Several reaction mechanisms exist for organic oxidations:
- Single electron transfer
- Oxidations through ester intermediates with chromic acid or manganese dioxide
- Hydrogen atom transfer as in Free radical halogenation
- Oxidation with oxygen (combustion)
- Oxidation involving ozone in ozonolysis and peroxides
- Oxidations involving an elimination reaction mechanism such as the Swern oxidation, the Kornblum oxidation and with reagents such as IBX acid and Dess-Martin periodinane.
- oxidation by nitroso radicals Fremy's salt or TEMPO
References