20.14:
Radical Autoxidation
Oxidation of organic compounds in the presence of air or atmospheric oxygen is termed autoxidation.
It is a free radical chain reaction consisting of initiation, propagation, and termination steps.
In the initiation step, an initiator abstracts hydrogen from the organic compound to form a carbon-centered radical.
The radical then enters the first propagation step, where it couples with the molecular oxygen to form a peroxy radical.
In the second propagation step, the peroxy radical abstracts hydrogen from another organic compound molecule forming the product peroxide and regenerating the carbon-centered radical.
The regenerated radical further propagates the chain reaction.
Finally, in the termination step, two carbon-centered radicals couple together to form a non-radical product.
Generally, the autoxidation process is initiated by light. In the absence of light, the process is very slow. For this reason, most organic chemicals are stored in dark bottles.
In particular, compounds with allylic and benzylic hydrogens are more susceptible to autoxidation as the free radicals formed in the initiation step are resonance stabilized.
20.14:
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.
