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Radical chemistry - The chemistry of radicals

Radical chemistry - The chemistry of radicals: Encyclopedia II - Radical chemistry - The chemistry of radicals

Radical chemistry - Terminology. In chemistry free radicals take part in radical addition and radical substitution as reactive intermediates. Reactions involving free radicals are usually divided into three categories: 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 o ...

Radical chemistry: Encyclopedia II - Radical chemistry - The chemistry of radicals

Radical chemistry - The chemistry of radicals

Radical chemistry - Terminology

In chemistry free radicals take part in radical addition and radical substitution as reactive intermediates. Reactions involving free radicals are usually divided into three categories: 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

Radical chemistry - Formation

The formation of radicals requires covalent bonds to be broken homolytically, a process that requires significant amounts of energy. 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 species are electrically neutral although radical ions do exist.

Radical chemistry - Persistence and stability

Long lived radicals can be placed into two categories

Stable Radicals

Radicals can be long lived if they occur in a conjugated π system, such as the radical derived from α-tocopherol (vitamin E)

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 (trimethylphenyl), Fremy's salt (Potassium nitrosodisulfonate, (KSO₃)₂NO·)and nitroxides, (general formula R₂NO·) such as TEMPO. The longest-lived free radical is melanin, which may persist for millions of years.

Radical chemistry - Combustion

Probably the most familiar free-radical reaction for most people is combustion. In order for combustion to occur the relatively strong O=O double bond must be broken to form oxygen free radicals. It is noteworthy that oxygen is actually a diradical with two unpaired electrons in the outer orbitals. Reactivity is limited because these electrons have parallel spins. However, this barrier is overcome by enzymes in the body (respiration) and by energy (heat). 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.

Radical chemistry - Polymerization

In addition to 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 such as:

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.

Radical chemistry - Atmospheric radicals

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 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 refrigerants has been restricted.

Other related archives

-yl, Alzheimer's, Biochemistry, CIDNP, Chemical bonding, DNA, Environmental chemistry, Free-radical theory, Fremy's salt, Living Radical Polymerization, Moses Gomberg, NO, Nuclear magnetic resonance, Parkinson's disease, TEMPO, X-ray photoelectron spectroscopy, absorption spectroscopy, aging, alcohol, alpha 1-antitrypsin, antioxidants, atherosclerosis, atmospheric chemistry, atomic, bilirubin, biochemistry, bond dissociation energy, cancer, catalase, cell cycle, cell signalling, chain reaction, chemistry, chlorofluorocarbons, cigarette, combustion, defense mechanisms, double bond, electron paramagnetic resonance, electron spin resonance, electrons, emphysema, enzymes, exothermic reaction, glutathione peroxidase, glutathione reductase, gout, hemochromatosis, homolytically, hydroxyl radical, jaundice, liver, lung, magnetic field, melanin, molecular, mutations, neutrophil granulocytes, nitroxides, nuclear magnetic resonance, open shell, ozone, ozone layer, peroxide, polymerization, purines, radical addition, radical ions, radical polymerization, radical substitution, reactive, reactive intermediates, red blood cells, schizophrenia, smoke, solvent cage, spectroscopy, superoxide, superoxide dismutase, ultraviolet radiation, uric acid, vitamin A, vitamin C, vitamin E

Adapted from the Wikipedia article "The chemistry of radicals", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki

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