Oxidation of drying oils containing non-conjugated and conjugated double bonds catalyzed by a cobalt catalyst

The oxidation of drying oil containing non-conjugated (linseed oil) and conjugated (tung oil) double bonds catalyzed by Co(II)-2-ethylhexanoate (Co-EH) is investigated. There is distinctive difference in the oxidation mechanism between the two model oils. For the non-conjugated linseed oil, H-abstraction occurs via the most susceptible double allylic H atom. A large amount of oxygen is taken up, leading to the formation of hydroperoxides; the hydroperoxides are then decomposed into alkoxy and peroxy free radicals, followed by the formation of cross-links and byproducts. In contrast, for the mainly conjugated tung oil, the oxidation has to start with H-abstraction from the monoallylic position. In comparison with linseed oil, a lower amount of oxygen is consumed, and much less hydroperoxides and byproducts are formed. It indicates that, once free radicals are formed, they tend to directly add to a conjugated double bond, instead of abstracting a monoallylic H atom. A tentative explanation is given in terms of bond dissociation energy of related bonds.