Radical (chemistry)

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Moses Gomberg (1866-1947) identified the first organic free radical, the triphenylmethyl radical, in 1900.

In chemistry, radicals (or free radicals) are atomic or molecular species with unpaired electrons in an otherwise open shell configuration. These unpaired electrons are usually highly reactive, so most radicals readily take part in chemical reactions. Being uncharged, their reactivity is different from that of ions of similar structure. The first organic free radical, the triphenylmethyl radical, was identified by Moses Gomberg in 1900.

Radicals are involved in many chemical processes, including combustion, atmospheric chemistry, polymerization, and plasma chemistry. They also play a significant role in human physiology. For example, superoxide and nitric oxide regulate many biological processes, such as controlling vascular tone.

Contents

Clarification of terms

Historically, the term "radical" has also been used for bound parts of a molecule, especially when they remain unchanged in reactions. For example, methyl alcohol was described as consisting of a methyl radical and a hydroxyl radical. Neither is a radical in the usual chemical sense, as they are permanently bound to each other, with no unpaired, reactive electrons.

The terms "radical" and "free radical" are frequently used interchangeably. However, a radical may not be "free" if it is trapped within a solvent cage or otherwise bound.

Some molecules contain multiple radical centers. A molecule that has two radical centers is called a biradical.

Formation

The formation of radicals requires covalent bonds to be broken homolytically, a process that requires significant amounts of energy. If a substance is broken down with a hail of energetic electrons, free radicals are produced and can be detected by mass spectrometry.

For example, splitting H2 into 2H has a ΔH° of +435 kJ/mol, and Cl2 into 2Cl has a ΔH° of +243 kJ/mol. This is known as the homolytic bond dissociation energy, and is usually abbreviated as the symbol DH°.

The bond energy between two covalently bonded atoms is affected by the structure of the molecule as a whole, not just the identity of the two atoms, and radicals requiring more energy to form are less stable than those requiring less energy. Homolytic bond cleavage most often happens between two atoms of similar electronegativity. In organic chemistry, this is often the O-O bond in peroxide species or O-N bonds.

However, propagation is a very exothermic reaction. Note that all free radical species are electrically neutral, although radical ions do exist.

Persistence and stability

The radical derived from α-tocopherol. The dot above one of the oxygen atoms represents an unpaired electron.

Long lived radicals can be placed into two categories:

  • Stable Radicals
Purely organic radicals can be long lived if they occur in a conjugated π system, such as the radical derived from α-tocopherol (vitamin E). Albeit, there exist hundreds of known examples of heterocyclic thiazyl radicals which show remarkable kinetic and thermodynamic stability, with only a very limited extent of π resonance stabilization.[1]
  • Persistent Radicals
Persistent radical compounds are those whose longevity is due to steric crowding around the radical center and makes it physically difficult for the radical to react with another molecule. Examples of these include Gomberg's radical (triphenylmethyl), Fremy's salt (Potassium nitrosodisulfonate, (KSO3)2NO), nitroxides, (general formula R2NO·) such as TEMPO, verdazyls, nitronyl nitroxides, and azephenylenyls. The longest-lived free radical is melanin, which may persist for millions of years.

Reactivity

Radical alkyl intermediates are stabilized by similar criteria as carbocations: the more substituted the radical center is, the more stable it is. This will direct their reactions: formation of a tertiary radical (R3C·) is favored over secondary (R2HC·) or primary (RH2C·). However, radicals next to functional groups, such as carbonyl, nitrile, and ether are even more stable than tertiary alkyl radicals.

Radicals attack double bonds, but unlike similar ions, they are slightly less directed by electrostatic interactions. For example, the reactivity of nucleophilic ions with α,β-unsaturated compounds (C=C-C=O) is directed by the electron-withdrawing effect of the oxygen, resulting in a partial positive charge on the carbonyl carbon. There are two reactions that are observed in the ionic case: the carbonyl is attacked in a direct addition to carbonyl, or the vinyl is attacked in conjugate addition, and in either case, the charge on the nucleophile is taken by the oxygen. Radicals add rapidly to the double bond, and the resulting α-radical carbonyl is relatively stable. Nonetheless, the electrophilic/neutrophilic character of radicals has been shown in a variety of instances (for example, in the alternating tendency of the copolymerization of malieic anhydride and styrene).

In intramolecular reactions, precise control can be achieved despite the extreme reactivity of radicals. Radicals will attack the closest reactive site the most readily. Therefore, when there is a choice, a preference for five-membered rings is observed: Four-membered rings are too strained, and collisions with carbons five or more atoms away in the chain are infrequent.

Combustion

The most familiar free-radical reaction is probably combustion. The oxygen molecule is a stable diradical, best represented by ·O-O·, which is stable because the spins of the electrons are parallel. The ground state of oxygen is an unreactive spin-paired (triplet) radical, but an extremely reactive spin-unpaired (singlet) radical is available. In order for combustion to occur, the energy barrier between these must be overcome. This barrier can be overcome by heat, requiring high temperatures, or can be lowered by enzymes to initiate reactions at the temperatures inside living things.

Combustion consists of various radical chain reactions that the singlet radical can initiate. The flammability of a given material is strongly dependent on the concentration of free radicals that must be obtained before initiation and propagation reactions dominate leading to combustion of the material. Once the combustible material has been consumed, termination reactions again dominate and the flame dies out. Propagation or termination reactions can be promoted to alter flammability. Tetraethyl lead was once commonly added to gasoline, because it very easily breaks up into radicals, which consume other free radicals in the gasoline-air mixture. This prevents the combustion from initiating prematurely.

Polymerization

Besides combustion, many polymerization reactions involve free radicals. As a result, many plastics, enamels, and other polymers are formed through radical polymerization.

Recent advances in radical polymerization methods, known as Living Radical Polymerization, include:

  • Reversible Addition-Fragmentation chain Transfer (RAFT)
  • Atom Transfer Radical Polymerization (ATRP)
  • Nitroxide Mediated Polymerization (NMP)

These methods produce polymers with a much narrower distribution of molecular weights.

Depicting radicals in chemical reactions

In written chemical equations, free radicals are frequently denoted by a dot placed immediately to the right of the atomic symbol or molecular formula as follows:

Cl2 + → 2 Cl·

Radical reaction mechanisms use single-headed arrows to depict the movement of single electrons:

Radical.jpg

The homolytic cleavage of the breaking bond is drawn with a "fish-hook" arrow to distinguish from the usual movement of two electrons depicted by a standard curly arrow. It should be noted that the second electron of the breaking bond also moves to pair up with the attacking radical electron; this is not explicitly indicated in this case.

In chemistry, free radicals take part in radical addition and radical substitution as reactive intermediates. Reactions involving free radicals can usually be divided into three distinct processes: initiation, propagation, and termination.

  • Initiation reactions are those which result in a net increase in the number of free radicals. They may involve the formation of free radicals from stable species as in Reaction 1 above or they may involve reactions of free radicals with stable species to form more free radicals.
  • Propagation reactions are those reactions involving free radicals in which the total number of free radicals remains the same.
  • Termination reactions are those reactions resulting in a net decrease in the number of free radicals. Typically two free radicals combine to form a more stable species, for example: 2Cl·→ Cl2

Free radicals in the atmosphere

In the upper atmosphere, free radicals are produced through dissociation of the source molecules, particularly the normally unreactive chlorofluorocarbons, by solar ultraviolet radiation or by reactions with other stratospheric constituents. These free radicals then react with ozone in a catalytic chain reaction that destroys the ozone, but regenerates the free radical, allowing it to participate in additional reactions. Such reactions are believed to be the primary cause of depletion of the ozone layer and this is why the use of chlorofluorocarbons as refrigerants has been restricted.

Free radicals in biology

Free radicals play an important role in a number of biological processes, some of which are necessary for life, such as the intracellular killing of bacteria by neutrophil granulocytes. Free radicals have also been implicated in certain cell signalling processes. The two most important oxygen-centered free radicals are superoxide and hydroxyl radical. They are derived from molecular oxygen under reducing conditions. However, because of their reactivity, these same free radicals can participate in unwanted side reactions resulting in cell damage. Many forms of cancer are thought to be the result of reactions between free radicals and DNA, 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 free-radical induced oxidation of many of the chemicals making up the body. In addition free radicals contribute to alcohol-induced liver damage, perhaps more than alcohol itself. Radicals in cigarette smoke have been implicated in inactivation of alpha 1-antitrypsin in the lung. This process promotes the development of emphysema.

Free radicals may also be involved in Parkinson's disease, senile and drug-induced deafness, schizophrenia, 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. The free radical theory of aging proposes that free radicals underlie the aging process itself.

Because free radicals are necessary for life, the body has a number of mechanisms to minimize free radical induced damage and to repair damage which does occur, such as the enzymes superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase. In addition, antioxidants play a key role in these defense mechanisms. These are often the three vitamins, vitamin A, vitamin C and vitamin E and polyphenol antioxidants. Further, there is good evidence bilirubin and uric acid can act as antioxidants to help neutralize certain free 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.[2]

Reactive oxygen species

Reactive oxygen species or ROS are species such as superoxide, hydrogen peroxide, and hydroxyl radical and are associated with cell damage.

Free radicals are also produced inside organelles of living cells, and released toward the cytosol. For example, the organelles known as mitochondria convert energy for the cell into a usable form, adenosine triphosphate (ATP). The process by which ATP is produced (called oxidative phosphorylation) inovolves the transport of protons (hydrogen ions) across the inner mitochondrial membrane by means of the electron transport chain. In this chain, electrons are passed through a series of proteins via oxidation-reduction reactions, with each acceptor protein along the chain having a greater reduction potential than the last. The last destination for an electron along this chain is an oxygen molecule. Normally the oxygen is reduced to produce water; but in about 1-2 percent of all cases, the oxygen is reduced to give the superoxide radical, ·O2-.

Superoxide needs an additional electron to make it more stable, so it steals an electron from the nearest source—such as mitochondrial DNA, the mitochondrial membrane, protein, reductants such as vitamin C or E, or antioxidants such as glutathione or thioredoxin. If too much damage is caused to the mitochondrion, the cell undergoes apoptosis, or programmed cell death.

According to the Free Radical Theory of Aging, aging occurs (via a loss of energy-producing cells) either when mitochondria begin to die out because of free radical damage, or when less functional mitochondria remain within these cells. The focus of the project is to neutralize the effect of these free radicals with antioxidants. Antioxidants neutralize free radicals by donating one of their own electrons. The antioxidant nutrients themselves do not become free radicals in this process, because they are stable in either form.

Superoxide dismutase (SOD) is present in two places naturally in the cell. SOD that is present in the mitochondria contains manganese (MnSod). This SOD is transcribed in the nucleus and has a mitochondrial targeting sequence, thereby localizing it to the miotchondrial matrix. SOD that is present in the cytoplasm of the cell contains copper and zinc (CuZnSod). The genes that control the formation of SOD are located on chromosomes 21, 6, and 4. When superoxide dismutase comes in contact with superoxide, it reacts with it and forms hydrogen peroxide. The stoichiometry of this reaction is that for each 2 superoxide radicals encountered by SOD, 1 H2O2 is formed. This hydrogen peroxide is dangerous in the cell because it can easily transform into a hydroxyl radical (via reaction with Fe2+:Fenton chemistry), one of the most destructive free radicals. Catalase, which is concentrated in peroxisomes located next to mitochondria but formed in the rough endoplasmic reticulum and located everywhere in the cell, reacts with the hydrogen peroxide and forms water and oxygen. Glutathione peroxidase reduces hydrogen peroxide by transferring the energy of the reactive peroxides to a very small sulfur containing protein called glutathione. The selenium contained in these enzymes acts as the reactive center, carrying reactive electrons from the peroxide to the glutathione. Peroxiredoxins also degrade H2O2, both within the mitochondria, cytosol and nucleus.

Diagnostics

Free Radical diagnostic techniques include:

  • Electron Spin Resonance
A widely-used technique for studying free radicals, and other paramagnetic species, is electron spin resonance spectroscopy (ESR). This is alternately referred to as "electron paramagnetic resonance" (EPR) spectroscopy. It is conceptually related to nuclear magnetic resonance, though electrons resonate with higher-frequency fields at a given fixed magnetic field than do most nuclei.
  • Nuclear magnetic resonance using a phenomenon called CIDNP
  • Chemical Labeling
Chemical labeling by quenching with free radicals, e.g. with NO or DPPH, followed by spectroscopic methods like X-ray photoelectron spectroscopy (XPS) or absorption spectroscopy, respectively.
  • Use of free radical markers
Stable, specific or non-specific derivatives of physiological substances can be measured. Examples include lipid peroxidation products (isoprostanes, TBARS), amino acid oxidation products (such as meta-tyrosine, ortho-tyrosine, hydroxy-Leu, dityrosine), peptide oxidation products (oxidized glutathione—GSSG)
  • Indirect method
Measurement of the decrease in the amount of antioxidants (such as TAS, reduced glutathione—GSH)

Notes

  1. R.T. Oakley, Prog. Inorg. Chem. 36(1998): 299.
  2. C.J. Rhodes, Toxicology of the Human Environment: The Critical Role of Free Radicals (London: Taylor and Francis, 2000).

References

  • Fossey, Jacques, Daniel Lefort, and Janine Sorba. 1995. Free Radicals in Organic Chemistry. New York: Wiley. ISBN 0471954969
  • Halliwell, Barry, and John M.C. Gutteridge. 2007. Free Radicals in Biology and Medicine. Oxford, UK: Oxford University Press. ISBN 978-0198568698
  • Parsons, Andrew F. 2000. An Introduction to Free-Radical Chemistry. Oxford: Blackwell Science. ISBN 0632052929
  • Rhodes, Christopher J., ed. 2000. Toxicology of the Human Environment: The Critical Role of Free Radicals. London: Taylor and Francis. ISBN 0748409165

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