Peroxides as radical sources

Interaction of ethylene and carbon tetrachloride at elevated temperatures and pressures, initiated with benzoyl peroxide as radical source, caused violent explosions on several occasions. Recommended precautions include use of minimum pressure and quantity of initiator, maximum agitation, and presence of water as an inert moderator of high specific heat. 

Azo compounds (diazenes) as sources of free radicals 19 Diacyl peroxides as radical sources 20 Peroxy esters 20... 

Vinyl acetate is normally inhibited with hydroquinone to prevent polymerisation. A combination of too low a level of inhibitor and warm, moist storage conditions may lead to spontaneous polymerisation. This process involves autoxidation of acetaldehyde (a normal impurity produced by hy droly sis of the monomer) to a peroxide which initiates exothermic polymerisation as it decomposes. In bulk, this may accelerate to a dangerous extent. Other peroxides or radical sources will initiate the exothermic polymerisation. 

The peroxide and azo thermal initiators also are photochemically unstable and have been used as radical sources at well below their normal thermal decomposition temperatures. However, their industrial use as photoinitiators has been limited because their light-absorption characteristics frequently are unsuitable and because of the obvious potential complication owing to their slow thermal decomposition, which leads to poor shelf-life and nonreproducible photoactivity in given formulations (88). Further information on photoinitiators can be found in the literature (92). 

Free radical alkylation procedures have proved a useful route to alkylpurines which are not readily available by other methods. Thus 6-substituted purines including adenine and hypoxanthine may be converted into 8-methyl derivatives with t-butyl hydroperoxide in the presence of iron(II) ions and acid (74T2677), although small amounts of 2-methyl and 2,8-dimethyl derivatives were formed simultaneously. Adenosine and guanosine similarly furnished the corresponding 8-methyl derivatives with diacetyl peroxide (as a source of methyl radicals) and iron(II) ions (76T337). 

Diacyl peroxides, which are known as radical sources, can decompose by an ionic mechanism in the presence of strong acids. Thus, benzoyl peroxide 282 can be converted into phenyl benzoate in a process whose first step involves a Lewis acid catalyzed carboxy inversion reaction to the mixed carbonate 283 (equation 134). ... 

Recently, Li s group reported an unexpected efficient Pd-catalyzed methylation of aryl C—H bonds using a peroxide as a source of methyl groups (Equation 11.35) [74], This study provided a new avenue for the direct alkylation of aryl C—H bonds using alkyl radicals rather than organometallic reagents. Moreover, this work presents an excellent example of the control of radical decomposition. 

This ladical-geneiating reaction has been used in synthetic apphcations, eg, aioyloxylation of olefins and aromatics, oxidation of alcohols to aldehydes, etc (52,187). Only alkyl radicals, R-, are produced from aliphatic diacyl peroxides, since decarboxylation occurs during or very shortiy after oxygen—oxygen bond scission in the transition state (187,188,199). For example, diacetyl peroxide is well known as a source of methyl radicals (206). 

Organic peroxides are used in the polymer industry as thermal sources of free radicals. They are used primarily to initiate the polymerisation and copolymerisation of vinyl and diene monomers, eg, ethylene, vinyl chloride, styrene, acryUc acid and esters, methacrylic acid and esters, vinyl acetate, acrylonitrile, and butadiene (see Initiators). They ate also used to cute or cross-link resins, eg, unsaturated polyester—styrene blends, thermoplastics such as polyethylene, elastomers such as ethylene—propylene copolymers and terpolymers and ethylene—vinyl acetate copolymer, and mbbets such as siUcone mbbet and styrene-butadiene mbbet. 

Thermally induced homolytic decomposition of peroxides and hydroperoxides to free radicals (eqs. 2—4) increases the rate of oxidation. Decomposition to nonradical species removes hydroperoxides as potential sources of oxidation initiators. Most peroxide decomposers are derived from divalent sulfur and trivalent phosphoms. 

The quantitative phenylation of pyridine has been studied by two groups of workers. Dannley and Gregg showed that 2-, 3-, and 4-phenylpyridine are formed in relative amounts 58 28 14 in the phenylation of pyridine with dibenzoyl peroxide, as estimated by infrared spectrophotometry. Hey and his co-workers obtained the ratios shown in Table I for the phenylation of pyridine using four different sources of phenyl radicals. ... 

They are initiated or accelerated by typical free-radical sources, such as the peroxides referred to, or by light. In the latter case the concept of quantum yield applies (p. 1316). Quantum yields can be quite high, for example, 1000, if each quantum generates a long chain, or low, in the case of nonchain processes. 

Another vulcanizing agent for diene rubbers is m-phenylenebismaleimide. A catalytic free-radical source such as dicumyl peroxide or benzothiazyldisulfide (MBTS) is commonly used to initiate the reaction [61]. Phenolic curatives, benzoquinonedioxime, and m-phenylenebismaleimide are particularly useful where thermal stability is required. 

All dangerous reactions mentioned in this paragraph are radical. Therefore, there will be greater dangers in the presence of light, excessive heat, radical sources (in the last case, organic peroxides are frequently used as catalysts). 

There is a discussion of some of the sources of radicals for mechanistic studies in Section 11.1.4 of Part A. Some of the reactions discussed there, particularly the use of azo compounds and peroxides as initiators, are also important in synthetic chemistry. One of the most useful sources of free radicals in preparative chemistry is the reaction of halides with stannyl radicals. Stannanes undergo hydrogen abstraction reactions and the stannyl radical can then abstract halogen from the alkyl group. For example, net addition of an alkyl group to a reactive double bond can follow halogen abstraction by a stannyl radical. 

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