In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired valence electron.IUPAC Gold Book radical (free radical) PDF With some exceptions, these unpaired electrons make radicals highly chemically reactive.
Radicals are important in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes.
Organic radicals Formation Thermolysis Thermolysis refers to generation of radicals by thermally induced fragmentation of labile precursors. Diacyl peroxides degrade by homolysis of the O-O bond to give carboxyl radicals that eliminate carbon dioxide:
Some diimides lose dinitrogen, releasing a pair of radicals. A useful example is azobisisobutyronitrile.
By redox
Radical anions form by one-electron redox reactions (reductions) of organic compounds. Although the transformation is simple in concept, few organic compounds reduce at potentials accessible in the laboratory. Stability is conferred on the radical anion when the charge can be delocalized. Examples of observable radical anions are alkali metal naphthalenides, anthracenides, and ketyls.Robert T. Morrison, Robert N. Boyd, and Robert K. Boyd, Organic Chemistry, 6th edition (Benjamin Cummings), 1992,
One-electron oxidation of organic compounds gives radical (S=1/2) derivatives. Although oxidation is simple in concept, few organic compounds oxidize at potentials accessible in the laboratory. The formation of radical cations is important in mass spectrometry, which employs high voltages to strip electrons from gaseous samples. On a preparative scale, only organic compounds with multiple electron donating substituents can be oxidized to give observable salts. Tetrathiafulvalene is one such example.
By H-atom abstraction The abstraction of an H atom from organic compounds generates radicals. Although the transformation is simple conceptually, C−H bonds are typically very strong and resist cleavage (hence the considerable stability of hydrocarbons). As reflected in tables of bond dissociation energies, C−H bonds require more than 100 kcal to break homolytically. Compounds bearing carbon–hydrogen bonds react with radicals in the order primary < secondary < tertiary < benzyl < allyl, reflecting the order in C–H bond dissociation energy Allylic (and benzylic) C−H bonds and especially doubly allylic (and doubly benzylic) C−H bonds are weaker and susceptible to H-atom abstraction. Similarly labile are C−H bonds adjacent to ethers and amines, whose lone pairs of electrons stabilize the resulting carbon-centered radicals by delocalization. The ability of adjacent lone pairs to stabilize radicals is called the α effect.
As is often cited, the C−H bond in triphenylmethyl (trityl) derivatives is weakened. In this case the radical is stabilized (and the C−H bond weakened) by resonance effects. Another stabilizing effect specific to radicals is the captodative effect.Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry. p. 573,
Bonding and characterization In the terminology of molecular orbital theory, a radical features a so-called "singly-occupied molecular orbital" or SOMO. In the absence of steric bulk around the atom bearing the SOMO, the SOMO tends to form a bond by dimerization of the radical. Suppressing this tendency and hence stabilizing the radical, is a major focus of radical chemistry, both steric and electronic effect apply.
Since organic radicals are electron-deficient molecules, they tend to be electrophilic. Consequently, organic radicals are stabilized by electron-donating groups. A variety of factors stabilize radicals, e.g., the "capto-dative effect"., p. 651. , p. 256 writes: "There is some evidence in favor of the captodative effect, some of it from ESR studies. However, there is also experimental and theoretical evidence against it," with extensive citations on both sides.
The technique electron-spin resonance provides insights into the bonding of radicals. It employs microwave energies to flip the unpaired spin. The energy of the spin flip is usually near g = 2, similar to that of a naked electron, i.e. in organic radicals the odd electron is only slightly affected by its environment. The nuclear magnetic properties of the atom carrying the unpaired spin and its directly bonded neighbors are also manifested in the spectrum.
Reactions Recombination For most organic radicals in solution, the dominant reaction is simply their dimerization, also called recombination:Advanced Organic Chemistry F.A. Carey R.J. Sundberg
Because of the strength of the C-C bond, these reactions are irreversible.
Addition to alkenes
In free-radical additions, a radical adds to an alkene. This addition generates a new radical, which can add to yet another alkene, etc. This behavior underpins radical polymerization, technology that produces many plastics. When they add to unsaturated substrates, e.g. alkenes, a chain reactions ensue, leading to polymerization. The overall process can usually be divided into three distinct processes. These are initiation, propagation, and termination. Initiation reactions are those that result in a net increase in the number of radicals. They may involve the formation of radicals from stable species as in Reaction 1 above or they may involve reactions of radicals with stable species to form more radicals. Propagation reactions are those reactions involving radicals in which the total number of radicals remains the same. Termination reactions are those reactions resulting in a net decrease in the number of radicals. Typically two radicals combine to form a more stable species, for example:
Other reactions Free radicals engage in many other reactions: radical-nucleophilic aromatic substitution, radicals add to an aryl halide. The halide is displaced concomitant with the addition of an electron a special case of nucleophilic aromatic substitution. Intramolecular free radical reactions (substitution or addition) such as the Hofmann–Löffler reaction or the Barton reaction Fragmentation reactions include the Norrish reaction, the Hunsdiecker reaction Carbon–carbon coupling reactions, for example manganese-mediated coupling reactions. Elimination reactionsAnslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry. p. 596,
With regards to rearrangements of radicals, prominent examples are the "radical clock reactions, e.g. the ring-opening of cyclopropylcarbinyl radical.Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry. p. 683, Fragmentations and rearrangements involving radicals are relevant to mass spectrum analysis.
Combustion
Combustion consists of radical chain reactions. Because combustion occurs at high temperatures, a range of radicals are generated that are not observed at room temperature.
Lead deactivates radicals. When gasoline-air mixtures are combusted in the presence of tetraethyl lead, the lead atoms suppress uncontrolled combustion of unburnt residues (engine knocking) or premature ignition (preignition).
In biology and medicine
Radicals play important roles in biology. Some nitrogen oxides are involved in cell signalling processes, known as redox signaling.
A majority of natural products are generated by radical-generating enzymes. In living organisms, the radicals superoxide and nitric oxide and their reaction products regulate many processes, such as the control of vascular tone (and thus blood pressure). They also play a key role in the intermediary metabolism of various biological compounds. A radical may be trapped within a solvent cage or be otherwise bound.
Fats and fatty acids Unsaturated fatty acids and especially polyunsaturated fatty acids and their derived triglycerides have a rich radical chemistry. For example, radical attack of linoleic acid produces a series of 13-hydroxyoctadecadienoic acids and 9-hydroxyoctadecadienoic acids, which appear to regulate inflammatory and/or healing responses, pain perception, and the proliferation of malignant cells. Radical attacks on arachidonic acid and docosahexaenoic acid produce a similar but broader array of signaling products.
Disease Radicals may also be involved in Parkinson's disease, senile and drug-induced deafness, and Alzheimer's. The classic free-radical syndrome, the iron-storage disease hemochromatosis, is typically associated with a constellation of free-radical-related symptoms including movement disorder, psychosis, skin pigmentary melanin abnormalities, deafness, arthritis, and diabetes mellitus. The free-radical theory of aging proposes that radicals underlie the aging process. Similarly, the process of mitohormesis suggests that repeated exposure to radicals may extend life span.
Because radicals are necessary for life, the body has a number of mechanisms to minimize radical-induced damage and to repair damage, such as the enzymes superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase. Antioxidants such as vitamins A, C, and E also play a key role in these defense mechanisms. There is good evidence indicating that bilirubin and uric acid can act as antioxidants to help neutralize certain radicals. Bilirubin comes from the breakdown of red blood cells' contents, while uric acid is a breakdown product of purines. Too much bilirubin, though, can lead to jaundice, which could eventually damage the central nervous system, while too much uric acid causes gout.An overview of the role of radicals in biology and of the use of electron spin resonance in their detection may be found in Additionally, polyphenols are theorized to work as antioxidants.
Reactive oxygen species Reactive oxygen species (ROS) are species such as superoxide, hydrogen peroxide, and hydroxyl radical, commonly associated with cell damage. ROS form as a natural by-product of the normal metabolism of oxygen and have important roles in cell signaling. Two important oxygen-centered radicals are superoxide and hydroxyl radical. They derive from molecular oxygen under reducing conditions.
Because of their reactivity, these radicals can participate in unwanted side reactions resulting in cell damage. Excessive amounts of these radicals can lead to cell injury and death, which may contribute to many diseases such as cancer, stroke, myocardial infarction, diabetes and major disorders. Many forms of cancer are thought to be the result of reactions between radicals and DNA, potentially resulting in mutations that can adversely affect the cell cycle and potentially lead to malignancy. Some of the symptoms of aging such as atherosclerosis are also attributed to radical induced oxidation of cholesterol to 7-ketocholesterol.
Oxybenzone has been found to form radicals in sunlight, and therefore may be associated with cell damage as well. This only occurred when it was combined with other ingredients commonly found in sunscreens, like titanium oxide and octyl methoxycinnamate.
Reactive oxygen species are also used in controlled reactions involving singlet dioxygen known as type II photooxygenation reactions after Dexter energy transfer (triplet-triplet annihilation) from natural triplet dioxygen and triplet excited state of a photosensitizer. Typical chemical transformations with this singlet dioxygen species involve, among others, conversion of cellulosic biowaste into new poylmethine dyes.
Inorganic, main group, and related non-carbon radicals
Radicals consisting of main group elements are more common than for organic radicals. Radicals is especially common for the compounds rich in most electronegative atoms, N, O, and the halogens. The N- and O-based radicals are described below as well as under the topics of atmospheric chemistry.
Some main-group radicals exist in notional equilibrium with closed-shell dimers. For example, nitrogen dioxide equilibrates with dinitrogen tetroxide, and tributyltin radicals equilibrate with .
Hydrogen radical