Radical addition of oxygen autoxidation reactions

Isolated double and triple bonds are reduced readily, whereas conjugated alkenes and aromatic systems are difficult to hydrogenate. Carbonyl double bonds react only very slowly, if at all, so it is possible to achieve selective reduction of C=C double bonds in the presence of aromatic and carbonyl functions. 

Alkynes may also be hydrogenated, initially to alkenes, and then further to alkanes. By suitable modification of the catalyst, it has proved possible to stop the reaction at the intermediate alkene. Typically, platinum or palladium catalysts partially deactivated (poisoned) with lead salts are fonnd to be suitable for reduction of alkynes to alkenes. Again, syn addition is observed. 

The slow spontaneous oxidation of compounds in the presence of oxygen is termed autoxidation (autooxidation). This radical process is responsible for a variety of transformations, such as the drying of paints and varnishes, the development of rancidity in foodstuff fats and oils, the perishing of rabber, air oxidation of aldehydes to acids, and the formation of peroxides in ethers. 

The processes that occur when cyclohexene reacts with oxygen in the presence of an initiator to give the allylic hydroperoxide exemplify this nicely. 

the radical from the initiation reaction abstracts hydrogen from the allylic position of cyclohexene, as we have seen previously, to give the resonance-stabilized radical (see Section 9.2). 

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