chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

, radical initiators are substances that can produce radical species under mild conditions and promote

radical reaction A free-radical reaction is any chemical reaction involving free radicals. This reaction type is abundant in organic reactions. Two pioneering studies into free radical reactions have been the discovery of the triphenylmethyl radical by Moses Gomb ...

s. These substances generally possess weak bonds—bonds that have small bond dissociation energies. Radical initiators are utilized in industrial processes such as

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

synthesis. Typical examples are molecules with a nitrogen-halogen bond,

azo compound Azo compounds are organic compounds bearing the functional group diazenyl (, in which R and R′ can be either aryl or alkyl groups). IUPAC defines azo compounds as: "Derivatives of diazene (diimide), , wherein both hydrogens are substituted ...

s, and organic and inorganic peroxides.

Main types of initiation reaction

*Halogens undergo homolytic fission relatively easily.

Chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...

, for example, gives two chlorine radicals (Cl•) by irradiation with

ultraviolet light Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of th ...

. This process is used for chlorination of

alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whi ...

s. *Azo compounds (R- N=N-R') can be the precursor of two

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

-centered radicals (R• and R'•) and nitrogen gas upon heating and/or by irradiation. For example, AIBN and ABCN yield isobutyronitrile and cyclohexanecarbonitrile radicals, respectively. : AIBN-radical-2D

Organic peroxide In organic chemistry, organic peroxides are organic compounds containing the peroxide functional group (). If the R′ is hydrogen, the compounds are called hydroperoxides, which are discussed in that article. The O−O bond of peroxides easily b ...

s each have a peroxide bond (- O-O-), which is readily cleaved to give two oxygen-centered radicals. The oxyl radicals are unstable and believed to be transformed into relatively stable carbon-centered radicals. For example, di-''tert''-butyl peroxide (''t''- Bu OO''t''-Bu) gives two ''t''-butoxy radicals (''t''-BuO•) and the radicals become

methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated as ...

radicals (C H₃•) with the loss of

acetone Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...

Benzoyl peroxide Benzoyl peroxide is a chemical compound (specifically, an organic peroxide) with structural formula , often abbreviated as (BzO)2. In terms of its structure, the molecule can be described as two benzoyl (, Bz) groups connected by a peroxide ...

(( PhC)OO)₂) generates benzoyloxyl radicals (PhCOO•), each of which loses

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

to be converted into a phenyl radical (Ph•). Methyl ethyl ketone peroxide is also common, and

acetone peroxide Acetone peroxide ( also called APEX and mother of Satan) is an organic peroxide and a primary explosive. It is produced by the reaction of acetone and hydrogen peroxide to yield a mixture of linear monomer and cyclic dimer, trimer, and tet ...

is on rare occasions used as a radical initiator, too. *Inorganic peroxides function analogously to organic peroxides. Many polymers are often produced from the alkenes upon initiation with

peroxydisulfate The peroxydisulfate ion, , is an oxyanion, the anion of peroxydisulfuric acid. It is commonly referred to as persulfate, but this term also refers to the peroxomonosulfate ion, . It is also called ''peroxodisulfate''. Approximately 500,000 ton ...

salts. In solution, peroxydisulfate dissociates to give sulfate radicals: : ₃SO-OSO₃sup>2− 2 O₄sup>− The sulfate radical adds to an alkene forming radical sulfate esters, e.g. ^.CHPhCH₂OSO₃⁻, that add further alkenes via formation of C-C bonds. Many styrene and fluoroalkene polymers are produced in this way. * In atom transfer radical polymerization (ATRP), carbon-halides reversibly generate organic radicals in the presence of

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

Safety

Some radical initiators such as

s and

peroxide In chemistry, peroxides are a group of Chemical compound, compounds with the structure , where the R's represent a radical (a portion of a complete molecule; not necessarily a free radical) and O's are single oxygen atoms. Oxygen atoms are joined ...

s can detonate at elevated temperatures so they must be stored cold.

References

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