Oxyl

Oxyl is a term used in chemistry to denote oxygen-centered radicals—species in which the unpaired electron resides on an oxygen atom rather than on carbon or another element. The prototypical oxyl radical is the alkoxyl radical RO•, formed by homolytic cleavage of an O–H or O–R bond. The oxyl designation also applies to metal-oxo or metal-oxyl species, where the unpaired electron is largely localized on the oxygen atom bound to a metal center, M–O•, and to other oxygen-centered radicals such as hydroxyl-type species considered in a broad oxyl context.

Nomenclature and usage: The label oxyl emphasizes the oxygen-centered radical character. In practice, alkoxyl radicals are typically called alkoxyl or RO•; hydroxyl radicals HO• are sometimes described as oxyl in broad discussions, but are normally named hydroxyl radicals. The term oxyl appears more often in physical organic and organometallic radical chemistry, where the focus is on the nature and reactivity of the oxygen-centered electron deficiency.

Formation: Oxyl radicals form under photolysis or thermal homolysis of O–H or O–R bonds, during the oxidation of alcohols, ethers, or during combustion and atmospheric oxidation processes. They are typically short-lived, highly reactive species that can abstract hydrogen, undergo beta-scission, or recombine with other radicals.

Relevance: Oxyl radicals drive many oxidative processes, including lipid peroxidation, polymer degradation, and atmospheric oxidation chains. Their reactivity is a central topic in radical chemistry, spectroscopy (notably EPR), and mechanistic studies of oxidation.