chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

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

radical reaction In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...

s. These substances generally possess weak bonds—bonds that have small

bond dissociation energies The bond-dissociation energy (BDE, ''D''0, or ''DH°'') is one measure of the strength of a chemical bond . It can be defined as the standard enthalpy change when is cleaved by homolysis to give fragments A and B, which are usually radical s ...

. Radical initiators are utilized in industrial processes such as

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

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 with the 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 betwee ...

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

ultraviolet light Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiation i ...

. This process is used for

chlorination Chlorination may refer to: * Chlorination reaction In chemistry, halogenation is a chemical reaction that entails the introduction of one or more halogens into a compound. Halide-containing compounds are pervasive, making this type of transform ...

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 which ...

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

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...

-centered radicals (R• and R'•) and nitrogen gas upon heating and/or by irradiation. For example, AIBN and

ABCN 1,1′-Azobis(cyclohexanecarbonitrile) or ACHN is a radical initiator. The molecular formula is NCC6H10N=NC6H10CN. It is a white solid that is soluble in aromatic solvents. ACHN has a 10-hour half-life in toluene Toluene (), also known as tol ...

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 . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in many ...

radicals (C H₃•) with the loss of

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

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 (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

to be converted into a phenyl radical (Ph•).

Methyl ethyl ketone peroxide Methyl ethyl ketone peroxide (MEKP) is an organic peroxide with the formula CH3)(C2H5)C(O2H)sub>2O2. MEKP is a colorless oily liquid. It is widely used in vulcanization (crosslinking) of polymers. It is derived from the reaction of methyl ethy ...

is also common, and acetone peroxide 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 tons o ...

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 Atom transfer radical polymerization (ATRP) is an example of a reversible-deactivation radical polymerization. Like its counterpart, ATRA, or atom transfer radical addition, ATRP is a means of forming a carbon-carbon bond with a transition metal cat ...

(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. They are the elements that can ...

catalyst Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...

Safety

Some radical initiators such as

s and

peroxide In chemistry, peroxides are a group of compounds with the structure , where R = any element. The group in a peroxide is called the peroxide group or peroxo group. The nomenclature is somewhat variable. The most common peroxide is hydrogen p ...

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

References

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