free radical
[¦frē ′rad·ə·kəl]Free Radical
a kinetically independent species characterized by the presence of an unpaired electron. For example, among the inorganic free radicals with one electron in the outer shell are atoms of hydrogen (H˙), alkali metals (Na˙ K˙), and halogens (CI˙, Br˙, F˙, I˙), as well as molecules of nitrogen oxide, NO, and dioxide, ˙NO2 (the dot denotes an unpaired electron). Free radicals are most widely distributed in organic chemistry, where they are classified as short-lived (reactive) or long-lived (stable). Short-lived alkyl (R˙) and aryl (Ar˙) radicals with a lifetime of less than 0.1 sec are formed during homolytic scission of various chemical bonds.
The alkyl radicals methyl (ĊH3) and ethyl (CH3ĊH2) were discovered in 1929 by F. Paneth upon thermal dissociation of tetramethyl and tetraethyl lead in the gaseous phase. Shortlived free radicals are characterized by the reactions of recombination (1), combination (2), and disproportionation (3), which take place at very high rates:
(1) CH3CH2ĊH2 + CH3CH2ĊH2 = CH3(CH2)4CH3
(2) CH3CH2ĊH2 + R = CH3CH2CH2Ṙ
(3) CH3CH2ĊH2 + CH3CH2ĊH2
= CH3CH2CH3 + CH3CH=CH2
C. Hinshelwood and N. N. Semenov pointed out the important role of short-lived free radicals in chain reactions, whose mechanism includes the abovementioned types of reactions.
A substantial number of free radicals belong to the long-lived, or stable, category. The lifetime of these radicals ranges from several minutes to several months or even years, depending on conditions (for example, the presence or absence of moisture and atmospheric oxygen). The main reasons for the higher stability of these free radicals are the partial loss of activity by an unpaired electron as a result of its interaction with many atoms in the molecule (the “delocalization” of an unpaired electron) and the poor accessibility of the atom carrying an unpaired electron because of its screening by neighboring atoms.
The first stable free radical, triphenylmethyl, (C6H5)3Ċ, was obtained in 1900 by the American chemist M. Gomberg by reacting silver with triphenylmethyl bromide. The stability of this radical is associated with the delocalization of an unpaired electron with respect to all the atoms, which may be formally explained by resonance between the possible electron structures:
A large number of triarylmethyl free radicals are known. Among radicals that are stable because of steric phenomena are the oxidation products of substituted phenols, called phenoxyl free radicals—for example, tri-tert-butylphenoxyl (1). Other examples of long-lived free radicals are α, α-diphenyl-β-picrylhydrazyl (II), iminoxyl free radicals such as tetramethyl piperidinoxyl (III), and bis-trifluoromethyl nitroxyl (IV):
The oxidation or reduction of neutral molecules causes the formation of charged free radicals—cation radicals (for example, during oxidation of aromatic hydrocarbons with oxygen) or anion radicals (during reduction of aromatic hydrocarbons by alkali metals):
A separate group of anion radicals, which was discovered in 1932 by the German chemist L. Michaelis, is formed by the products of single-electron quinone reduction, or semiquinones, such as benzosemiquinone:
Free radicals containing two unpaired electrons that do not interact are called biradicals, or diradicals. An example of a biradical is Schlenk’s hydrocarbon:
The oxygen molecule belongs to the inorganic biradicals. Polyradicals which contain more than two unpaired electrons, also exist.
Various physicochemical methods, such as electron spectroscopy, mass spectroscopy, electrochemical methods, and nuclear magnetic resonance, are used to study free radicals. The most effective method is electron paramagnetic resonance (EPR), which may also be used to examine short-lived free radicals. EPR provides unique data on the physical nature of the unpaired electron and on the electron’s behavior in the molecule; these data are extremely valuable in carrying out calculations in quantum chemistry.
Short-lived free radicals are intermediate particles in many organic reactions (radical halogenation, sulfochlorination, metallization, the dissociation of organic peroxides, and the Wit-tig, Kolbe, and Konovalov reactions), and also in reactions that proceed under the action of ionizing radiation. Long-lived free radicals serve as stabilizers for readily oxidizing compounds and as “traps” for short-lived radicals; they are also used in numerous kinetic studies. The investigation of cation and anion radicals yields valuable information on the nature of ion interaction in solutions. Free radicals play a major role in oxidation-reduction, photochemical, and catalytic processes, as well as in important industrial processes, such as polymerization, telomerization, pyrolysis, cracking, combustion, explosion, and heterogeneous catalysis.
REFERENCES
Walling, C. Svobodnye radikaly v rastvore. Moscow, 1960. (Translated from English.)Semenov, N. N. O nekotorykh problemakh khimicheskoi kinetiki i reaktsionnoi sposobnosti, 2nd ed. Moscow, 1958.
Buchachenko, A. L., and A. M. Vasserman. Stabil’nye radikaly: Elektronnoe stroenie, reaktsionnaia sposobnost’ i primenenie. Moscow, 1973.
Free radicals may also be produced in organisms under the action of various physical and chemical factors. In particular, the effect of radiation is associated with formation of free radicals during radiolysis of water contained in the cells (the radicals ˙OH and HO˙2), and also upon action of radiation on the molecules of organic substances and cell biopolymers. Iminoxyl free radicals are widely used in biochemical research to clarify the configuration of protein molecules (the spin labeling and paramagnetic probe methods) and the functional properties of biological membranes.
REFERENCES
Kozlov, Iu. P. “Svobodnoradikal’nye protsessy v biologicheskikh sistemakh.” In Biofizika. Moscow, 1968.Ingram, D. Elektronnyi paramagnetkhnyi rezonans v biologii. Moscow, 1972. (Translatedfrom English.)
IU. P. KOZLOV