chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, structure, proper ...

, radical initiators are substances that can produce radical species under mild conditions and promote radical reactions. 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 (; 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 compounds, 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 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 them. Chlorine i ...

, 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 radiatio ...

. 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 wh ...

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—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon ma ...

-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 at 88 °C. See also * A ...

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 ...

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 miscibl ...

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 t ...

to be converted into a phenyl radical (Ph•). Methyl ethyl ketone peroxide 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 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. 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 azo compounds 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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