The term oxidation originally meant a reaction in which oxygen combines chemically with another substance, but its usage has long been broadened to include any reaction in which electrons are transferred. Oxidation and reduction always occur simultaneously (redox reactions), and the substance that gains electrons is termed the oxidizing agent. For example, cupric ion is the oxidizing agent in the folowing reaction: Fe (metal) + Cu2+ → Fe2+ + Cu (metal). Here, two electrons (negative charges) are transferred from the iron atom to the copper atom; thus, the iron becomes positively charged (is oxidized) by loss of two electrons, while the copper receives the two electrons and becomes neutral (is reduced). Electrons may also be displaced within the molecule without being completely transferred from it. Such partial loss of electrons likewise constitutes oxidation in its broader sense and leads to the application of the term to a large number of processes that at first sight might not be considered oxidations. Reaction of a hydrocarbon with a halogen, e.g., CH4 + 2Cl → CH3Cl + HCl, involves partial oxidation of the methane; halogen addition to a double bond is regarded as an oxidation.Dehydrogenation is also a form of oxidation, when two hydrogen atoms, each having one electron, are removed from a hydrogen-containing organic compound by a catalytic reaction with air or oxygen, as in oxidation of alcohols to aldehydes. See Dehydrogenation.
