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In chemistry free radicals are uncharged atomic or molecular species that contain an unpaired electron. These unpaired electrons are generally highly reactive, so free radicals can take part in take part in chemical reactions. The term free radical is generally shortened to radical, though it should be noted that most, but not all radicals are free radicals. Importantly, because radicals are neutral species radical reactions are usually not affected by solvent effects unlike ionic reactions. Free radicals play an important role in combustion, atmospheric chemistry, polymerization, plasma chemistry, biochemistry, and many other chemical processes.It is possible for a molecule to be charged and have an unpaired electron, a radical species that is positively charged is called a radical cation, while a radical species that also bears a negative charge is called a radical anion.
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:
- H2 + hν → 2 H·
This represents the unpaired electron and is derived from Lewis dot notation.
- Initiation reactions are those which result in a net increase in the number of radicals. There are 5 different general methods for the generation of radical:
- Thermolysis
- Photolysis
- Radiation
- Electrolytic
- Redox
- Propagation reactions are those reactions involving no overall change in the number of radical species.
- Fragmentation
- Addition
- Atom transfer
- Rearrangement
- Termination reactions are those reactions resulting in a net decrease in the number of radicals.
- Combination, for example: 2H· → H2
- Redox
- Disproportionation
Initiation A common method of generating radicals is by homolysis (from the Greek homo meaning the same and lysis meaning to break) of a covalent bond. Homolysis is simply the breaking of a bond so that the electrons in the bond are divided evenly (ie. one to each of the fragments), as opposed to heterolysis (the formation of ions) which is where both electrons move towards the same atom.
Image:Homolysis.PNG
Homolysis of Cl2
The formation of radicals requires covalent bonds to be broken homolytically, a process that requires significant amounts of energy. For example, splitting Cl2 into 2Cl· has a ΔH° of +243 kJ/mol.
This is known as the homolytic bond dissociation energy (or more correctly bond dissociation enthalpy BDE) and is usually abbreviated as the symbol ΔH°. 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. If a covalent bond is strong then more energy is required to break it than a weaker bond, this also means that the radical species formed will have more energy, and therefore be more reactive (less stable. In the gas phase, homolytic cleavage of a covalent bond requires less energy than heterolytic cleavage, however, it should be noted that this is not true in polar solvents where solvation stabilizes the charged products.
To homolytically cleave a covalent bond such as a C-H bond requires large amounts, therefore radical reactions are usually started using an initiator compound. An initiator is a molecule which contains one or more weak bond(s), meaning that relatively little energy is required to break the bond and generate a radical, these initial radicals are then used to propagate the desired radical reaction. Examples of types of molecules that can be used as initiators are; peroxides, azo-compounds.
Insert diagram of initiators
The energy to decompose an initiator, may come from heating the reaction (thermolysis), irradiation with light of a particular wavelength (photolysis) or from a radioactive source (radiolysis).
Another method for generating radicals is by a single electron transfer (SET) process, this is simply the addition or removal of one electron from a molecule or ion. Addition of an electron – Reduction When an electron is given to a neutral molecule it results in the formation of a radical anion. This species may fragment to give a neutral radical and an anion, or may go on to react further.
When an electron is given to a cation a neutral radical is formed.
Removal of an electron – Oxidation When an electron is removed from a neutral molecule it results in the formation of a radical cation. This species may fragment to give a neutral radical and an cation, or may go on to react further.
Image:Oxidation.PNG
When an electron is removed from an anion a neutral radical is formed.
In electrolysis, electron transfer occurs at the electrodes, reduction processes occur at the negatively charged cathode, while oxidation processes occur at the positively charged anode.
One example of an electrolytic radical reaction in the Kolbe electrolysis reaction.
Redox reactions
Metals especially transition metals, can change oxidation state, by loss or gain of an electron (this is driven by the attainment of a more stable electronic configuration, in the case of
Na → Na+ + e-, the loss of an electron gives an ion with a noble gas like electronic configuration.)
Ne 1s2 2s2 2p6 Na+ 1s2 2s2 2p6
As previously mentioned, many of the reactions that take place in combustion of a fuel involve radical species. Until the mid 1980's petrol (gas) contained metal additives (primarily tetraethyl lead and related compounds), their role was to generate radicals inside the combustion chamber of the engines pistons, this improved the efficiency of the engine by making the resulting explosion smoother.
Propagation Propagation may though of as the transformation of one radical species into another different radical. A radical reaction may involve several propagation steps. If the propagation steps are occuring one after another, in a stepwise fashion this is a tandem or cascade process, theoretically there are no limits to the number of propagation steps that can occur in one reaction, though practically, because each step is less than 100% efficient cascades involving 10 or more propagation steps are usually low yielding. Polymerization reaction may be though of as an exception as the radical may undergo thousands of propagation steps to form a polymer chain. If the a radical can undergo more than one propagation step for example addition to an alkene or hydrogen atom abstraction, the two processes are competing reaction pathways, and the relative rates of the two reactions will determine the outcome of the reaction. There are three general cases that can occur;
- Rate of addition to an alkene >> Rate of hydrogen atom abstraction --> Addition reaction dominates
- Rate of addition to an alkene << Rate of hydrogen atom abstraction --> Hydrogen atom abstraction reaction dominates
- Rate of addition to an alkene ≈ Rate of hydrogen atom abstraction --> Mixture of both products formed
Need diagrams here
Fragmentation A radical or radical ion may fragment to generate a new radical and a stable molecule, or in the second case a new neutral radical and a charged species. Such a reaction will always occur to produce a net reduction in the energy of the species, though this does not mean the all the species formed after a fragmentation are of lower energy (more stable).
Example radical decarboxylation
Fragmentation processes are most commonly observed in mass spectrometry, and can provide useful information on the sorts of functional groups in an unknown molecule.
Addition The addition of a radical to an unsaturated (double or triple) bond is generally an energetically favourable process. Eg. The addition of a carbon-centred radical to an alkene, results in the breaking of a π-bond (~230 kJ mol-1) and the formation of a new sigma-bond (~370 kJ mol-1), this is an overall exothermic process.
Atom transfer As the name implies atom transfer reactions involve the transfer of an atom from a functional group to a radical, generating a new radical species and a new non-radical species.
The driving force for such reaction is again thermodynamic stability, a reactive (high energy) radical is exchanged for a more stable (lower energy) product radical. The most commonly observed atom transfer processes are hydrogen atom transfer and halogen atom transfer. The atom may be transferred from a different molecule (intermolecular) or from a different part if the radical species (intramolecular) – see rearrangement.
As well as atom transfer, it is possible in special cases to transfer certain functional groups such as an allyl substituent, though such reactions are more properly described as addition-fragmentation reactions.
Rearrangement Radical rearrangements are essentially any intramolecular radical process, such as intramolecular atom transfer, intramolecular addition (ie cyclisation) and intramolecular addition-fragmentation reactions (functional group transfers).
Termination A termination reaction involves the transformation of a radical species into a non-radical species, in this sense atom transfer propagation steps are like a special subset of termination process as involve the trapping of a radical but do not result in a net decrease in the number of radicals.
Combination Radical combination involves two radical species meeting and both donate their unpaired electron to form a new covalent bond this is the reverse of homolytic bond cleavage. If the radicals are identical the process is often called radical dimerisation. Because this reaction requires two radicals to meet (collide) it is usually not a significant reaction, unless there is a high concentration of radicals. Combination, for example: 2H· → H2
Redox Just as radicals can be formed by single electron transfer (SET) processes, they can also be transformed into non-radical species by a SET process. Hence, reduction of a radical (donation of an electron) leads to formation of an anion. Likewise, oxidation of a radical (removal of an electron) gives rise to a cation.
Disproportionation Disproportionation is a special case of redox reaction, rather than electron transfer from/to a reagent or electrode, an electron is transferred between two radicals resulting in a positively charged species and a negatively charged species. Essentially, one radical is being oxidised and the other reduced. Once again this reaction involves two radical species meeting and so is unlikely to occur in dilute conditions.
Many polymerization reactions involve free radicals, many million tons of plastics and other polymers are formed each industrially by radical processes. In the upper atmosphere free radicals are produced through dissociation of 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 which 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 refridgerants has been restricted.
Relatively stable, persistent free radical compounds include Fremys salt (Potassium nitrosodisulfonate, (KSO3)2NO·)and nitroxides, (general formula R2NO·).
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.
[edit] 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.
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. 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.
