Radikaller

Radikaller (radicals) are chemical species that contain one or more unpaired electrons. The presence of an unpaired electron makes radicals highly reactive and often short-lived, so they typically exist only transiently in chemical systems. They can be neutral or carry a charge as radical cations or radical anions. They participate in a variety of processes, including chain reactions in organic synthesis and oxidation-reduction chemistry in the atmosphere and biology.

Organic radicals include alkyl radicals such as the methyl radical CH3•, aryl radicals, and acyl radicals. Inorganic radicals include the hydroxyl radical OH•, chlorine radical Cl•, and nitrogen dioxide radical NO2•; the superoxide radical O2•− is a radical anion. Some radicals are stabilized by delocalization or steric hindrance and can be observed or isolated under special conditions; persistent nitroxide radicals are common examples used as spin labels and mediators.

Radicals form by homolytic bond cleavage, photolysis, or redox reactions that produce species with unpaired electrons. Once formed, they participate in chain reactions comprising initiation, propagation, and termination steps. In polymer chemistry, radical polymerization uses initiators such as azo compounds or peroxides to generate growing chains. In atmospheric and combustion chemistry, radicals such as OH•, HO2•, and NO3• drive the oxidation of hydrocarbons and pollutants.

Analytical methods for radicals include electron spin resonance (ESR or EPR) spectroscopy, which detects unpaired electrons. In biology and medicine, reactive oxygen species, a subset of radicals, can cause oxidative damage but also participate in signaling and defense. Antioxidants act as radical scavengers, limiting damage by neutralizing reactive radicals.