Autoxidation

Autoxidation is a spontaneous oxidation of substrates by molecular oxygen (O2) that proceeds through free-radical chain reactions under ambient or mildly elevated conditions. It typically requires no strong external oxidant and can occur at room temperature when reactive radicals are generated by heat, light, impurities, or trace metal catalysts. Autoxidation is a general mechanism applicable to hydrocarbons, ethers, lipids, and many polymers.

The mechanism is classically described in three stages: initiation, propagation, and termination. Initiation creates a carbon-centered radical (R•) from the substrate or from impurities. In propagation, the radical quickly reacts with O2 to form a peroxy radical (ROO•). The peroxy radical abstracts a hydrogen atom from another substrate molecule (RH), yielding a hydroperoxide (ROOH) and regenerating the carbon-centered radical (R•). This cycle propagates and leads to accumulation of hydroperoxides and various secondary products. Termination occurs when two radicals combine to form a stable, non-radical product, or when radical concentrations drop.

Common products of autoxidation include hydroperoxides as primary oxidation products and, after further decomposition, carbonyl compounds, alcohols, aldehydes, and ketones. In fats and oils, autoxidation drives rancidity and lipid peroxidation; in polymers, it contributes to aging, embrittlement, and discolored products. The rate of autoxidation is influenced by oxygen availability, temperature, light, and the presence of metal ions or catalysts. Inhibitors such as antioxidants (for example, phenolic stabilizers) or metal chelators can slow or prevent autoxidation by scavenging radicals or sequestering catalysts.

Autoxidation is a central concept in food chemistry, polymer science, and materials aging, reflecting how exposure to air can gradually alter the chemical makeup and properties of diverse substances.