Oxidation of a Monoenoic Acid:

Using oleic acid as an example, a hydrogen could be removed from either C-8 or C-10, as these positions are located alpha to the double bond. Abstraction from carbon 8 results in the two radicals A and B which are positional isomers of each other stabilized by resonance:

Or abstraction from carbon 11 can occur, resulting in the two radicals C and D:

Oxygen can be added to each radical to form peroxy radicals at C-8, C-9, C-10 or C-11. Addition to the 8 and 10 positions yield the peroxy radicals shown below:

These radicals may abstract hydrogens from other molecules to yield the hydroperoxides shown below:

The addition oxygen at the 11 and 9 positions with subsequent addition of abstracted hydrogen molecules results in the peroxy radicals and hydroperoxides shown below:

The situation with a dienoic acid is a little different. While there are more positions a to a double bond, there is one position that is a to two double bonds. This position is very reactive. For linoleic acid, carbon 11 is a to two double bonds and will be removed to yield:

There are two possible resonant structures that can result from this radical. The radical may shift to carbon 14 with the double bond reforming between carbons 11 and 12. The radical may also shift to carbon 9 with the double bond forming between carbons 10 and 11. Both of these cases result in conjugated structures that are at lower energies than are the non conjugated structures they were derived from. For this reason, the oxidation of linoleic acid yields approximately equal amounts of the C 13 and C 9 radical with only traces of the original C 11 radical present. The resonant structures formed are shown below:

Addition of oxygen and abstraction of hydrogen from other molecules would yield equal amounts of 13-hydroperoxido- 8, 11 octadecadienoic acid and 9- hydroperoxido 10,12 -octadecadienoic acid. The 11 - hydroperoxido- 9, 12 -octadecadienoic acid is essentially nonexistent.

Once formed, hydroperoxides may break down through a number of mechanisms. A common breakdown scheme is called dismutation. In this reaction a hydroperoxide reacts with another molecule or radical to form two new compounds.

This reaction scheme is capable of generating aldehydes, ketones, alcohols and hydrocarbons. Many of the volatile compounds formed during lipid oxidation originate through similar dismutations.

Hydroperoxides are not stable compounds and given time, they will break down. A typical mechanism, as shown below, results in the formation of two radicals from a single hydroperoxide molecule.

Both of these new radicals can initiate further oxidation. Some metals can speed up this reaction. For example:

Note that both ions and free radicals were formed. The net reaction is:

Copper was the catalyst. Copper did not initiate the reaction, but once the hydroperoxides were formed, it sped up their breakdown.