|MadSci Network: Chemistry|
It's not a very easy answer. There is an excellent discussion by Prof. Robert Salomon at Case-Western, which you can find at http://www.case.edu/artsci/chem/faculty/salomon/Mechanisms.htm, which goes into detail of several proposed mechanisms. It might be a little bit complex, but then, the mechanism is not a simple one.
In short, linoleic acid, like most fatty acids, is subject to oxidation by oxygen in the air. The body also metabolizes linoleic acid by an oxidation process (that's the "enzymatic pathway" shown midway down Prof. Salomon's page). In all cases, it is a double bond that is being oxidized. Depending on the level of oxidation, the double bond can simply open to a single bond, leaving a hydroxide radical clinging to one side of it, or it can be completely split, leaving an oxygen behind at the two ends.
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