20.16: Radical Autoxidation
The oxidation of an organic compound in the presence of air or oxygen is called autoxidation. For example, cumene reacts with oxygen to form hydroperoxide. Autoxidation involves initiation, propagation, and termination steps. Many organic compounds are susceptible to autoxidation—especially ethers in the presence of oxygen, which form hydroperoxides. Even though this reaction is slow, old ether bottles contain small amounts of peroxide, which leads to laboratory explosions during ether distillation, making them dangerous to use. Naturally occurring fats and oil, like vegetable oil, can also undergo autoxidation. These oils are generally a mixture of triglycerides, which consist of three long hydrocarbon chains with double bonds.
Typically, the allylic position of triglycerides gets oxidized to form hydroperoxides that are responsible for the foul smell of foods containing unsaturated oils. Consequently, foods with unsaturated oils have a short lifetime unless radical inhibitors are used to inhibit the radical formation. These radical inhibitors, such as BHT and BHA, are used as food preservatives. Both BHT and BHA react with radicals to form resonance stabilized radicals. Additionally, the tert-butyl groups of these compounds sterically hinder the radical center and decrease the reactivity of the radicals. Thus, these radical inhibitors are called antioxidants, as they scavenge and destroy radicals.
One can also find several natural antioxidants that help prevent the oxidation of cell membranes and biologically important compounds. For example, vitamin E and vitamin C are natural antioxidants. These compounds react with the reactive radicals to form less reactive and stabilized radicals.