Radical acetyl peroxide

The thermal decomposition of diacyl peroxides has been the most frequently employed process for the generation of alkyl radicals. The rate and products of the unimolecular decomposition of acetyl peroxide have been the subject of many studies. Acetyl peroxide decomposes at a convenient rate at 70-80°C both in the solution and in the gas... 

The competitive method employed for determining relative rates of substitution in homolytic phenylation cannot be applied for methylation because of the high reactivity of the primary reaction products toward free methyl radicals. Szwarc and his co-workers, however, developed a technique for measuring the relative rates of addition of methyl radicals to aromatic and heteroaromatic systems. - In the decomposition of acetyl peroxide in isooctane the most important reaction is the formation of methane by the abstraction of hydrogen atoms from the solvent by methyl radicals. When an aromatic compound is added to this system it competes with the solvent for methyl radicals, Eqs, (28) and (29). Reaction (28) results in a decrease in the amount... 

The kinetic form of the decomposition in various solvents indicates competing unimolecular homolysis of the peroxide link (a) and radical induced decomposition (b). Other diacyl peroxides behave similarly, except that, in the case of acetyl peroxide, induced dceomposition is much less important. More highly branched aliphatic or a-phenyl-substituted diacyl peroxides decompose more readily, partly because induced decomposition is more important again and partly because of the occurrence of decomposition involving cleavage of more than one bond (for a mechanistic discussion of these cases, see Walling et al., 1970). 

For benzoyl and acetyl peroxides, loss of carbon dioxide occurs in a stepwise process. Estimates of the rate constants for step c in Scheme 1 are 7 x 10 sec (benzene, 60°). The corresponding process for acetyl peroxide has k = 2x 10 sec (n-hexane, 60°), so that the lifetime of radical pairs containing acetoxy radicals is comparable to the time necessary for nuclear polarization to take place (Kaptein, 1971b Kaptein and den Hollander, 1972 Kaptein et al., 1972). Propionoxy radicals are claimed to decarboxylate 15-20 times faster than acetoxy radicals (Dombchik, 1969). 

The polarization of biphenyl, deserves special comment. If, as indicated in Scheme 2, its immediate precursor is a radical pair consisting of two phenyl radicals, then it should be formed without detectable net polarization since if Ag = 0. Analogous results have been reported in the decomposition of other peroxides for example, ethane formed from acetyl peroxide shows net emission. To account for this, it has been suggested (Kaptein, 1971b, 1972b Kaptein et al., 1972) that nuclear spm selection which occurs in the primary radical pair—in... 

The back recombination of the pair of acetoxyl radicals with the formation of parent diacetyl peroxide was observed in special experiments on the decomposition of acetyl peroxide labelled by the lsO isotope on the carbonyl group [78,79]. The reaction of acetyl peroxide with NaOCH3 produces methyl acetate and all lsO isotopes are contained in the carbonyl... 

Acetoxy radicals from acetyl peroxide undergo partial decarboxylation and radical combination within the solvent cage leads to stable products incapable of producing radicals ... 

Homolytic substitution has been little studied, and work has been confined to the reaction of dibenzofuran with carboxymethyl radicals produced from acetyl peroxide or di-tcrt-butyl peroxide in boiling acetic acid or by pyrolysis of chloroacetylpolyglycolic acids. The method of analysis of the resultant mixture of 1- (55%), 4- (30%), and 3-dibenzofuranacetic acid (15%) was crude, but the results were in accord with simple HMO calculations. The amount of the 1-substituted product is perhaps surprising in view of the steric hindrance at this position. 

Few examples have been reported (5, 8, 9, 10, 12, 24) of cage recombination of simple alkoxy or acyloxy radicals to form O—O bonds in isolable molecules. This paper explores further the implications of the observed (17, 22, 23) scrambling of label seen in acetyl peroxide carbonyl-18O recovered after partial decomposition. 

It is clear (23) that three products result from reactions of acetoxy radical pairs within the solvent cage acetyl peroxide (estimated at 38% ), methyl acetate (12.4% ), and ethane (2.9% ). The implied near equality of rates for decarboxylation of the acetoxy radicals and diffusion from the cage has been given quantitative expression in work of Braun, Rajben-bach, and Eirich (2). These workers studied the variation in the amounts of ethane and methyl acetate formed from acetyl peroxide as a function of solvent viscosity, and they derived a rate constant for the decarboxylation of acetoxy radical at 60°C. of 1.6 X 109 sec."1. 

They further noted (2) that the over-all rate constant for disappearance of acetyl peroxide decreases monotonically with increasing solvent viscosity. The attractive hypothesis (19, 22, 23) that the observed rate decrease with increased viscosity in this homologous series of hydrocarbon solvents reflects the increased importance of cage recombination of acetoxy radicals in the more viscous solvents is subject to further test from the data of this paper. 

Figure 4. The prohahility that a radical generated at time zero from acetyl peroxide will be an acetoxy radical (A), a methyl radical (M) or will have diffused from the solvent cage, a quantity proportional to (f). These curves were used to duplicate the observed product yields...

Besides the polymer, the reaction products contained ketene, acetic acid, and formaldehyde. No acetyl peroxide was found, so that the CH3CO radical was probably not formed. Attempts to detect biacetyl in the reaction of 0 atoms with CH3CHO at high pressure60 also were a failure. 

Goldschmidt and Beer.372 The 2-alkylated product is the main one formed. This orientation is not unexpected since, compared with the phenyl radical, an alkyl radical should have some nucleophilic character. The j8-/y- ratio is also lower than in the phenylation, as expected for a more nucleophilic radical.371 Pyridine has a methyl affinity of 3 compared with benzene.373 This, however, does not represent the relative amount of picolines and toluene formed with acetyl peroxide.371... 

Estimates of the probability of escape of radical pairs in conventional solvents have been made by product analysis of the decomposition of diacyl peroxides. For example, Braun et al. [22] estimated that 60 to 80% of the methyl radicals produced in the thermolysis of acetyl peroxide escape geminate cage recombination. However, Guillet and Gilmer [25] showed that for longer chain and Cjj radicals the probability was much lower, ranging from 5% at 760C to 16% at 262<>C (Table VI). 

A point of view which brings together some of the essential features of both the foregoing theories, d at the same time accounts for many of the facts which these theories fail to explain, is similar to that previously described in connection with the anodic oxidation of thiosulfate. It is suggested that hydroxyl radicals, or hydrogen peroxide formed by their combination in pairs, react with the acetate ions to form acetate radicals, which then combine, possibly with the intermediate formation of acetyl peroxide, to yield ethane and carbon dioxide thus,... 

In non-aqueous solutions the Kolbe electrosynthesis takes place with high eflSciency at platinized platinum and gold, as well as at smooth platinum, anodes increase of temperature and the presence of catalysts for hydrogen peroxide decomposition, both of which have a harmful effect in aqueous solution, have relatively little influence. The mechanism of the reaction is apparently quite different in non-aqueous solutions and aqueous solutions in the former no hydroxyl ions are present, and so neither hydroxyl radicals nor hydrogen peroxide can be formed. It is probable, therefore, that direct discharge of acetate ions occurs at a potential which is almost independent of the nature of the electrode material in a given solvent. The resulting radicals probably combine in pairs, as in aqueous solution, to form acetyl peroxide, which subsequently decomposes as already described. ... 

It is, however, appropriate to point out that, whereas McDowell and Sifniades claim to have demonstrated by gas chromatography the formation of traces of acetyl peroxide, Calvert and Hanstdid not detect this peroxide when using infrared spectrophotometry. By use of infrared analysis, Osborne, Pitts, and Fowler were equally unable to detect acetyl peroxide among the photooxidation products obtained from acetone. The exact chemical nature of the product resulting from the interaction of two CHsCOa- radicals does not therefore seem to have been clarified. 

The radical initiator, diacetyl peroxide, homolyzes at the O—O bond to form carboxy radicals, which then readily lose COj to give methyl radicals. In fact, between 60°C and 100°C, acetyl peroxide can be a convenient source of methyl radicals. 

Martin et that attempts to trap the acetoxy radical with iodine, galvinoxyl or diphenylhydrazyl were either unsuccessful or ambiguous. However, isotopic labeling experiments show that cage recombination does occur and the mechanism of acetyl peroxide decomposition may be formulated as... 

Secondary isotope studies are consistent with the suggested mechanism for acetyl peroxide decomposition. It was concluded from the data given in Table 90 that little or no methyl radical character was developed in the rate-determining step . This view has been contested with data from a study of and 0-iso-tope effects in the decomposition of acetyl peroxide. The carbon isotope effect in... 

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