19.1: Oxidation and Reduction in Organic Chemistry
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In ionic and free radical reactions, oxidation and reduction are defined as processes by which an element undergoes a net loss or gain of electrons, respectively. The concept as applied to organic covalent compounds, where elements share electrons rather than losing or gaining them is the same, but it’s frequently simplified and narrowed down to make it easier to recognize these processes. Therefore it must be kept in mind that, while the following definition is grossly simplified, it serves the goal of quickly identifying oxidation and reduction processes in many organic reactions.
In reference to organic molecules, oxidation is a process by which a carbon atom gains bonds to more electronegative elements, most commonly oxygen. Reduction is a process by which a carbon atom gains bonds to less electronegative elements, most commonly hydrogen.
The following chart summarizes these concepts when applied to organic transformations. [ox] stands for oxidation, and [red] stands for reduction.
Some points must be noted. First, a gain or loss of bonds means simply more or less bonds. The double bond counts as two bonds and the triple bond counts as three bonds. Thus in the carbonyl group (C=O) carbon is considered to have two bonds to oxygen. Therefore, this carbon has a higher oxidation state than the alcohol carbon, which has only one bond to oxygen. In routine terminology, it is said that the aldehyde is a more highly oxidized functional group than the alcohol. Oxidation reactions are therefore those in which the central carbon of a functional group is transformed into a more highly oxidized form, and reduction reactions are those in which the central carbon is transformed into a more highly reduced form.
Second, there can be several functional groups where the central carbon has the same oxidation state. For example, the carbons bonded to oxygen in alcohols and ethers have the same oxidation state. Likewise for aldehydes and their hydrated forms, and for carboxylic acids and their derivatives. However, most references to oxidation and reduction reactions in organic chemistry textbooks involve the functional groups presented in the chart above.
The chart presented before shows the oxidation and reduction states for a molecule that contains only one carbon. But most organic compounds contain more than one carbon. The maximum oxidation state that a particular carbon can attain depends on how many other carbons it must remain attached to. For example, a molecule with two carbon atoms could not be oxidized all the way to carbon dioxide because the carbon atom in CO2 must have four bonds to oxygen, leaving no room for bonds to other carbons. The maximum oxidation state that a carbon can attain decreases gradually as the number of bonds to other carbons increases. Thus, the maximum oxidation state possible for a carbon that’s bonded to one other carbon is the carboxylic acid stage, and so on. The following chart illustrates this idea.
ONE SINGLE CARBON
CARBON BONDED TO ONE OTHER CARBON
CARBON BONDED TO TWO OTHER CARBONS
CARBON BONDED TO THREE OTHER CARBONS