Radical Autoxidation

Radical autoxidation consists of initiation, propagation, and termination steps. In initiation, an initiator abstracts hydrogen from an organic compound to form a carbon-centered radical. During propagation, the radical couples with molecular oxygen to form a peroxy radical, which then abstracts hydrogen from another molecule, regenerating the carbon-centered radical and forming a peroxide product. Finally, termination occurs when two carbon-centered radicals couple together to form a non-radical product.

Light generally initiates the autoxidation process in organic compounds. In the absence of light, autoxidation proceeds very slowly. Therefore, most organic chemicals are stored in dark bottles to minimize light exposure and prevent unwanted oxidation reactions that could degrade the compounds during storage.

Compounds with allylic and benzylic hydrogens are highly susceptible to autoxidation. The free radicals formed during the initiation step at these positions are resonance stabilized, making them more stable and reactive. This stabilization allows the chain reaction to proceed more readily, increasing the rate of autoxidation compared to other organic compounds.

Antioxidants like BHT and BHA are radical inhibitors that scavenge and destroy radicals by reacting with them to form resonance-stabilized radicals. The tert-butyl groups of these compounds sterically hinder the radical center, decreasing reactivity. Natural antioxidants such as vitamin E and vitamin C work similarly, converting reactive radicals into less reactive, stabilized forms to prevent oxidation of foods and biological compounds.

Ethers are susceptible to autoxidation in the presence of oxygen, forming hydroperoxides over time. Even though this reaction is slow, old ether bottles accumulate small amounts of peroxide. During ether distillation, these peroxides can decompose explosively when heated, making old ether bottles hazardous to use in the laboratory.

Naturally occurring fats and oils, like vegetable oil, contain triglycerides with multiple hydrocarbon chains and double bonds. Autoxidation occurs at the allylic positions of these triglycerides, forming hydroperoxides responsible for the foul smell of oxidized foods. Without radical inhibitors as food preservatives, foods containing unsaturated oils have a short lifetime due to this oxidative degradation.

In the propagation phase of autoxidation, the peroxy radical is formed when a carbon-centered radical couples with molecular oxygen. The peroxy radical then abstracts hydrogen from another organic compound molecule, producing the peroxide product and regenerating a carbon-centered radical. This regenerated radical continues the chain reaction, allowing autoxidation to propagate efficiently.