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. Simple functional groups can be arranged in order of increasing oxidation state. The oxidation numbers are only an approximation: 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. 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 the reverse 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. 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.