Peroxides, photolytic decomposition

Chemical Properties. Diacyl peroxides (20) decompose when heated or photoly2ed (<300 mm). Although photolytic decompositions generally produce free radicals (198), thermal decompositions can produce nonradical and radical iatermediates, depending on diacyl peroxide stmcture. Symmetrical aUphatic diacyl peroxides of certain stmctures, ie, diacyl peroxides (20, = alkyl) without a-branches or with a mono-cx-methyl... 

The radical is generated by photolytic decomposition of di-/-butyl peroxide in methylcy-clopropane, a process that leads to selective abstraction of a methyl hydrogen from methylcyclopropane ... 

The reaction of EtsSiH with [l.l.l]propellane under photolytical decomposition of di-tert-butyl peroxide afforded products 17 and 18 in 1 3 ratio (Reaction 5.15) [36]. A rate constant of 6.0 x 10 M s at 19 °C for the addition EtsSi radical to [l.l.l]propellane was determined by laser flash photolysis [37]. Thus, it would appear that [l.l.l]propellane is slightly more reactive toward attack by EtsSi radicals than is styrene, and significantly more reactive than 1-hexene (cf. Table 5.1). 

Liao C-H, Gurol M D (1995) Chemical Oxidation by Photolytic Decomposition of Hydrogen Peroxide, Enviromental Science Technology 29 3007-3014. 

Liao, C.H. and Gurol, M.D., Chemical oxidation by photolytic decomposition of hydrogen peroxide, Environ. Sci. Technol., 29(12), 3007-3014, 1995. 

Phthaloyl peroxide (2.93) on photolytic decomposition generates benzyne via lactone intermediate. 

A number of reactions including only first- and second-row atoms have also been studied. Some illustrative examples are the intramolecular transformation of methylene peroxide to dioxirane the photolytic decomposition of oxathiirane the reaction of singlet molecular oxygen with ethene and the dissociation of diimide . 

Photolytic decomposition of peroxides is not v y efBcient in crosslinking. An enhancement effect on the extent of photocrosslinking of polyolefins in the presence of peroxides is displayed by aromatic hydrocarbons such as naphthalene. These transfer the exdtation energy absorbed to a peroxide. This procedure, however, does not represent an important improvement when compared with that refored to earlier, namely the photoreduction of pdyethylene with aromatic ketones and quinones [84. From aromatic ketones and quinones, particulariy bena>phenone [32], chlorinated benzophenones, benzoyl-l-( dohexanol [82], a, -dimethot - hen acdr henone, 2,4,6-trimethyl benzoyl phenyl phosphinic ethyl ester [85], anthrone [86], anthraqui-none [87], naphthoquinone, benzoquinone, and their d vatives have all been examined. 

The rest of the photo-oxidation on the glycol portion is continued in Scheme 18.3. The cleavage of the 0-0 bond should be very facile. Both photolytic and thermal decomposition of these peroxides are possible. 

Almost all organic peroxides are thermally and photolytically sensitive owing to the facile cleavage of the weak oxygen-oxygen bond. This cleavage is a unimolecular (first-order) reaction. The thermal decomposition rates are affected by the structure of the organic peroxide and the decomposition conditions. 

Chemical Properties. Acyclic di-ferf-alkyl peroxides efficiently generate alkoxy free radicals by thermal or photolytic homolysis. Primary and secondary dialkyl peroxides undergo thermal decompositions more rapidly than expected owing to radical-induced decompositions. Such radical-induced peroxide decompositions result in inefficient generation of free radicals. 

Kolbe electrolysis also allows some comparisons with analogous homogeneous reactions with regard to dimerization, substitution, or addition reactions of the generated radicals. Photolytic or thermal decarboxylation of diacylperoxides is a source of alkyl radicals similar to those afforded by the Kolbe electrolysis. The anodic oxidation of propionate has been compared with the thermal decomposition of dipropionyl peroxide [28]. Examination of the yields shows that reaction between radicals is favored in the electrochemical process, whereas in peroxide decomposition hydrogen atom abstraction from the solvent or the substrate occurs to a higher extent. This illustrates the effect of the higher radical concentration at the electrode. 

Aryl radicals, that is, CeHs, which have been generated thermally by the decomposition of, for example, aryl diazonium salts (ArN2 ) in the presence of copper(I) salts (Equation 6.113) and the decomposition of diacyl peroxides (Equation 6.114), or photolytically from aryl iodides (e.g., iodobenzene, CeHsl) (Equation 6.115) and arylthallium trifluoroacetates as shown in Equation 6.116, react with benzene (CeHe) to give biphenyl (CeHs-CeHs) (Scheme 6.97). 

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