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.[1] Instead the relevant criterion for organic oxidation is gain of oxygen and/or loss of hydrogen.[2] Simple functional groups can be arranged in order of increasing oxidation state. The oxidation numbers are only an approximation:[1]
| oxidation number | compounds | |
|---|---|---|
| −4 | methane | |
| −3 | alkanes | |
| −2, −1 | alkanes, alkenes, alcohols, alkyl halides, amines | |
| 0 | alkynes, geminal diols | |
| +1 | aldehydes | |
| +2 | chloroform, hydrogen cyanide, ketones | |
| +3 | carboxylic acids, amides, nitriles (alkyl cyanides) | |
| +4 | carbon dioxide, tetrachloromethane |
When methane is oxidized to carbon dioxide its oxidation number changes from −4 to +4. Classical reductions include alkene reduction to alkanes and classical oxidations include oxidation of alcohols to aldehydes. 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 on the organic compound. For example, it is usual to refer to the reduction of a ketone by lithium aluminium hydride, but not to the oxidation of lithium aluminium hydride by a ketone. Many oxidations involve removal of hydrogen atoms from the organic molecule, and reduction adds hydrogens 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.[3]
In disproportionation reactions the reactant is both oxidised and reduced in the same chemical reaction forming two separate compounds.
Asymmetric catalytic reductions and asymmetric catalytic oxidations are important in asymmetric synthesis.
Most oxidations are conducted with air or oxygen, especially in industry. These oxidation include routes to chemical compounds, remediation of pollutants, and combustion. Some commercially important oxidations are listed:
| product | scale (millions of tons/year in 1990's)[4] | |
|---|---|---|
| terephthalic acid and esters | 15 | |
| 6.5 | ||
| 0.28 | ||
| 2.2 | ||
| 4 | ||
| 2.2 |
Many reagents have been invented for organic oxidations. Organic oxidations reagents are usually classified according to the functional group attacked by the oxidant:
Often the substrate to be oxidized features more than one functional group. In such cases, selective oxidations become important.
In organic chemistry, reduction is equivalent to the addition of hydrogen atoms, usually in pairs. The reaction of unsaturated organic compounds with hydrogen gas is called hydrogenation. The reaction of saturated organic compounds with hydrogen gas is called hydrogenolysis. Hydrogenolyses necessarily cleaves C-X bonds (X = C, O, N, etc.). Reductions can also be effected by adding hydride and proton sources, the so-called heterolytic pathway. Such reactions are often effected using stoichiometric hydride reagents such as sodium borohydride or lithium aluminium hydride.