The Stepwise Oxidation of Ethane

Each of the seven bonds in the ethane molecule contains a pair of electrons which can be attacked by an oxidizing agent — in this illustration, oxygen. Six of the bonds are carbon-hydrogen bonds, and one is a carbon-carbon bond. As oxygen attacks each bond, it inserts itself between the two atoms, grabbing onto the electrons tightly in the process. The carbon-oxygen and hydrogen-oxygen bonds are therefore fully oxidized. This reduces the number of available reduced bonds by one (as indicated by the number next to each molecular structure).

When 2 hydroxy groups (OHs) bond to the same carbon, a dehydration reaction occurs which results in the loss of a water molecule and a formation of a carbon-oxygen double bond. This happens when ethanol is oxidized to acetaldehyde, when ethylene glycol is oxidized to glycoaldehyde, when glycoaldehyde is oxidized to glyoxal, and when glycolic acid is oxidized to glyoxylic acid.

There are two pathways for the second, third and fourth oxidations because of the two possible oxidation sites. By the fifth oxidation, there is only one possible reaction product and the two paths converge back to one.

The seventh and last oxidation attacks the carbon-carbon bond, splitting the molecule in half. In fact, the carbon-carbon bond could be split earlier in the process resulting in a plethora of single-carbon fragments (methanol, formaldehyde, and formic acid). Because of the complexity involved, these reactions are not illustrated.

In strictly chemical reactions (non-enzymatic), these oxidations occur through high-energy free-radical intermediates (not illustrated) which are somewhat random. In biological systems, oxidation reactions are carefully controlled by enzymes which precisely direct the oxidation to a specific site.

The reaction product from a free-radical oxidation also depends upon the specific oxidant or free radical involved. Oxygen is typically found as O2, but it may also be found as ozone (O3), hydroxyl radical (OH.), superoxide anion (O2-), or singlet oxygen (O=O, or O+O-). Each form of oxygen attacks in its individual manner and results in different by-products. Free radicals selectively attack single-electron targets, and singlet-state ozone and ozygen selectively attack two-electron targets (i.e., double bonds). ——SWF

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