freeradical

Free radical, or freeradical, is a chemical species that contains one or more unpaired electrons, making it highly reactive. Most free radicals are neutral; charged radicals also exist. The unpaired electron resides in an orbital and often leads to an incomplete valence shell, driving rapid chemical reactions with other molecules or with themselves. Free radicals are typically short-lived in solution unless their reactivity is reduced by resonance stabilization, steric protection, or distribution of the unpaired electron over a conjugated system.

Free radicals are formed by homolytic bond cleavage, redox reactions, photolysis, or radiolysis. Common routes include thermal decomposition of initiators, oxidative processes, and atmospheric or combustion chemistry. Stability varies widely; allylic or benzylic radicals are relatively more persistent due to resonance.

Radical reactions are central to polymerization, combustion, and various organic syntheses. In biology, reactive oxygen and nitrogen species act as signaling molecules at low concentrations but can cause oxidative damage at high levels. Free radicals contribute to aging, inflammation, and DNA or lipid damage; cells counter them with antioxidants and repair mechanisms.

Detection and study of free radicals employ electron spin resonance (ESR or EPR) spectroscopy, often with spin-trapping techniques. In industry, radicals are exploited as polymerization initiators (for example, benzoyl peroxide) or in controlled radical polymerizations. Environmental and atmospheric chemistry emphasizes radicals such as hydroxyl (OH) and peroxy (RO2) species that regulate pollutant lifetimes.