Alkyl peroxides photolysis

Because di-/ fZ-alkyl peroxides are less susceptible to radical-induced decompositions, they are safer and more efficient radical generators than primary or secondary dialkyl peroxides. They are the preferred dialkyl peroxides for generating free radicals for commercial appHcations. Without reactive substrates present, di-/ fZ-alkyl peroxides decompose to generate alcohols, ketones, hydrocarbons, and minor amounts of ethers, epoxides, and carbon monoxide. Photolysis of di-/ fZ-butyl peroxide generates / fZ-butoxy radicals at low temperatures (75), whereas thermolysis at high temperatures generates methyl radicals by P-scission (44). 

The present discussion is by no means exhaustive. It is designed to provide a summary of the most significant and reliable kinetic data, at least those that appear so to the author. There is a variety of methods for producing alkyl radicals, and, naturally, there will be certain restrictions on experimental conditions depending on the method chosen. Some of the common methods for generation of methyl radicals, for example, include photolysis of acetone, pyrolysis of di-ferf-butyl peroxide, photolysis of biacetyl, photolysis of azomethane and decarbonylation of acetaldehyde. In the majority of cases discussed here, the reactions were followed by product determinations, employing gas chromatography. 

The photodecomposition of the various oxidation products of the alkanes, alkenes, and the aromatic hydrocarbons play important roles in the chemistry of the urban, mral, and remote atmospheres. These processes provide radical and other reachve products that help drive the chemistry that leads to ozone generation and other important chemistty in the troposphere. In this chapter, we have reviewed the evidence for the nature of the primary processes that occur in the aldehydes, ketones, alkyl nitrites, nittoalkanes, alkyl nitrates, peroxyacyl nitrates, alkyl peroxides, and some representative, ttopospheric, sunlight-absorbing aromatic compounds. Where sufficient data exist, estimates have been made of the rate of the photolytic processes that occur in these molecules by calculation of the photolysis frequencies ory-values. These rate coefficients allow estimation of the photochemical lifetimes of the various compounds in the atmosphere as well as the rates at which various reactive products are formed through photolysis. 

A teehnique that is a convenient source of radieals for study by EPR involves photolysis of a mixture of di-t-butyl peroxide, triethylsilane, and the alkyl bromide corresponding to the radieal to be studied. Photolysis of the peroxide gives t-butoxy radieals, whieh selectively abstract hydrogen from the silane. This reactive silicon radieal in turn abstracts bromine, generating the alkyl radieal at a steady-state eoncentration suitable for EPR study. 

Photolysis of symmetrical diacyl peroxides [109] was used for generation in inert matrices of a number of alkyl radicals (see Pacansky et al., 1991 Pacansky and Waltman, 1989, and references cited therein). Thus, ethyl. 

The usual sources used for the homolytic aromatic arylation have been utilized also in the heterocyclic series. They are essentially azo- and diazocompounds, aroyl peroxides, and sometimes pyrolysis and photolysis of a variety of aryl derivatives. Most of these radical sources have been described in the previous review concerning this subject, and in other reviews concerning the general aspects of homolytic aromatic arylation. A new source of aryl radicals is the silver-catalyzed decarboxylation of carboxylic acids by peroxydisulfate, which allows to work in aqueous solution of protonated heteroaromatic bases, as for the alkyl radicals. 

Methylation is taken as illustrative of alkylation for comparative purposes in Table 25 however, a wide range of other alkylations have been studied (76MI20503). Photolysis of di-r-butyl peroxide in a mixture of cyclohexane and pyridine gives cyclohexylation (equation 170) (7lCR(C)(272)854>. The relative rates for homolytic substitution of pyridines by cyclic alkyl radicals have been obtained (74JCS(P2)1699). A striking contrast can be seen (Table 26)... 

Recent laser flash photolysis (LFP) studies have provided absolute rates of addition of perfluoro-n-alkyl radicals to a variety of alkenes in solution [ 114,115]. In these studies, C2Fj, C3F7, and n-C7F,5 were generated instantaneously by photolysis of the respective diacyl peroxides. The initially-formed perfluoroacyloxyl radicals decarboxylated rapidly to yield the perfluoroalkyl radicals, after which the additions of these radicals to styrene, a-methylstyrene, etc. were monitored directly via observation of the growth of UV absorption due to the transient benzylic radicals. 

One of the reasons for that is the high stability of the O-H bond in the newly formed silanol group (125-130 kcal/mol), and H-r can be not only a hydrocarbon molecule, but also H-OH, H-NH2, etc. The diamagnetic dioxasily-rane groups are also the generators of alkyl radicals (see subsection 4.2). Many intermediates can be obtained as the products of thermal or thermo oxidative transformations or photo transformations of other initial structures. For example, vinoxyl radicals were obtained by the photolysis of peroxide radicals of the vinyl type [119] ... 

Feldhues, M. and Schafer, H.J. (1985) Selective mixed coupling of carboxylic adds (I). Electrolysis, thermolysis and photolysis of unsymmetrical diacyl peroxides with acyclic and cydic alkyl groups. Tetrahedron, 41, 4195M212. 

Photolysis of acyl peroxides apparently leads to formation of alkyl radicals and carbon dioxide as observed by ESR (196) ... 

Compounds having absorption bands in the visible or near-ultraviolet spectrum may be electronically excited to such a degree that weak covalent bonds undergo homolysis (Scheme 2.29). Photolysis of the peroxides gives alkoxy radicals. Many azo compounds are important source of alkyl radicals. Acetone in vapour phase is decomposed by light having a wavelength... 

For the selective preparation of unsymmetrical coupling products on a small scale, the photolysis of peroxides is a favorable alternative to the mixed Kolbe electrolysis. By photolysis of unsymmetrical diacyl peroxides at -60 to -70 C in the solid state two different alkyl groups can be joined selectively. "... 

Free radical reagents usually react with tetraalkyltin compounds at a hydrogen centre in one of the alkyl ligands to give a stannylalkyl radical. P-Stannylalkyl radicals prepared in this way can then undergo P-scission to break the Sn-C bond. Thus the photolysis of di-t-butyl peroxide in the presence of trimethylisobutyltin and ethylene or an alkyl bromide, gives rise to the ESR spectrum of the P-stannylethyl radical or of the alkyl radical respectively, which are formed by the sequence of reactions shown in equations 5-38.50... 

Cleavage of an alkyl-tin bond by an Sh2 reaction (see Section 5.3.6) occurs more readily with the alkyltin halides (and carboxylates) than with the tctraalkylslannancs.87 88 These reactions were first identified by ESR spectroscopy in the photolysis of di-t-butyl peroxide in the presence of organotin halides, which provides a very convenient technique for ESR studies of alkyl radicals, for example equations 11-28 and 11-29.89... 

Benzocyclopropene reacts with a variety of radical reagents (for example A -bromosuccinimide carbon tetrachloride bromotrichloromethane bromoform/benzoyl peroxide alkyl sulfide and ethane-1,2-dithiol with photolysis) to afford products derived from cleavage of the cyclopropane ring. The preferential mode of reaction consists of a chain reaction initiated by radical addition at Cl a followed by opening of the cyclopropyl radical to afford a benzyl radical. Yields are generally low except for the addition of the alkylsulfanyl radical, e.g. formation of 1, and no products derived from addition to the central tt-bond are formed. Cyclopropa[A]naphthalene reacts similarly with radicals and gives 2-methylnapthalene derivatives, while no addition to the central 7i-bond is observed. ... 

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