Alkoxy radicals, peroxidation products

A number of reports on the thermal decomposition of peroxides have been published. The thermal decompositions of f-butyl peroxyacetate and f-butyl peroxypivalate, of HCOH and a kinetic study of the acid-induced decomposition of di-f-butyl peroxide in n-heptane at high temperatures and pressures have been reported. Thermolysis of substituted f-butyl (2-phenylprop-2-yl) peroxides gave acetophenone as the major product, formed via fragmentation of intermediate alkoxy radicals RCH2C(Ph)(Me)0. A study of the thermolysis mechanism of di-f-butyl and di-f-amyl peroxide by ESR and spin-trapping techniques has been reported. The di-f-amyloxy radical has been trapped for the first time. jS-Scission reaction is much faster in di-f-amyloxyl radicals than in r-butoxyl radicals. The radicals derived from di-f-butyl peroxide are more reactive towards hydrogen abstraction from toluene than those derived from di-f-amyl peroxide. 

A related procedure, which may be of value from the preparative standpoint, involves the preparation of /rans-nitrosomethane dimer by adding a solution of diacetyl peroxide in sec-butyl nitrite to warm sec-butyl nitrite [50]. From the product of the reaction it has been assumed that this preparation involves the generation of free methyl radicals which react with the nitrite to give nitrosomethane and alkoxy radicals. The latter disproportionate to ketones and alcohols, while the nitroso compound dimerizes. 

Another important kinetic feature of autoxidation is autocatalysis. One of the oxidation products is the hydroperoxide, while peroxides can also be produced in termination. Peroxides and hydroperoxides are stable within a limited temperature range, but at higher temperatures they decompose homolytically giving alkoxy radicals ... 

In 1933, M. S. Kharasch and F. W. Mayo found that some additions of HBr (but not HC1 or HI) to alkenes gave products that were opposite to those expected from Markovnikov s rule. These anti-Markovnikov reactions were most likely when the reagents or solvents came from old supplies that had accumulated peroxides from exposure to the air. Peroxides give rise to free radicals that initiate the addition, causing it to occur by a radical mechanism. The oxygen-oxygen bond in peroxides is rather weak, so it can break to give two alkoxy radicals. 

The antioxidant radical produced because of donation of a hydrogen atom has a very low reactivity toward the unsaturated lipids or oxygen therefore, the rate of propagation is very slow. The antioxidant radicals are relatively stable so that they do not initiate a chain or free radical propagating autoxidation reaction unless present in very large quantities. These free radical interceptors react with peroxy radicals (ROO ) to stop chain propagation thus, they inhibit the formation of peroxides (Equation 13). Also, the reaction with alkoxy radicals (RO ) decreases the decomposition of hydroperoxides to harmful degradation products (Equation 14). 

In thermal oxidation, initiation (1) results Irom the thermal dissociation of chemical bonds that may arise Irom intrinsically weak links formed as by-products of the polymerization reaction (e.g. head-to-head links) or impurities formed in the polymerization reactor such as hydroperoxides, POOH, or in-chain peroxides as occur in polystyrene from oxygen scavenging. Reaction (1 ) shows that POOH may produce peroxy and alkoxy radicals that may subsequently form alkyl radicals via reaction (3). 

The oxo-oxidation products from cyclohexane with tBuOOH arc cyclohcxyl peroxide and its decomposition products, cyclohexanol and cyclohexanone. The relative ratio of the hydroperoxide decomposition products depends on its decomposition mechanism (see scheme). After a homolytic 0-0 bond cleavage in the peroxide, the formed alkoxy radical can undergo disproportionation, yielding equal amounts of ol/onc. A high ketone yield results from the peroxide dehydration with a Lewis acid, such as l e(OII) formed by H2O2 decomposition on free Fe. 

Photolysis can be used to achieve homolytic fission. For example, azo compounds produce radicals via the unstable cis isomer by the absorption of light energy (Scheme 2) [3], Peroxides produce alkoxy radicals and acyloxy radicals as cleavage products on absorption of light energy. 

The reaction of alkoxy radicals with triethyl phosphite apparently is sufficiently rapid to swamp out their known rapid conversion to alcohols or ketones, since such products were not detected. It would be interesting to know whether unsymmetrical or less sterically hindered dialkyl peroxides could be induced to interact via a polar mechanism. 

In the case of aryl analogs, products may be derived either from the carboxyl radical or the radical formed by decarboxylation. Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. t-Butyl hydroperoxide is easily available, and has often been used as a radical source. Detailed studies have been reported on the mechanism of the decomposition, which is somewhat more complicated than simple unimolecular decomposition." Dialkyl peroxides give two alkoxy radicals ... 

A more general method for producing phosphoranyl radicals is by reacting a trivalent phosphorus compound with oxygen-containing radicals. The reaction of phosphites with peroxides (6.333) is believed to proceed by decomposition of the peroxide to alkoxy radicals followed by attack on the phosphite and decomposition of the product by P scission (13.182)-(13.184). 

Mansuy et al. employed alkyl peroxides and iodosylbenzene as oxygen donors and examined the oxidation products of cyclohexane in the presence of a series of M(TPP) complexes (M=Fe, Mn, Co, Rh, and Cr) [284]. As listed in Table 10, cyclohexanol and cyclohexanone are the major products however, the ratio of the alcohol and ketone was dependent on the oxidant employed. For the oxidation by peroxides, involvement of alkoxy radical (RO ) due to the homolytic 0-0 bond cleavage of R-OO-M was proposed. In order to trap possibly formed radical intermediates. He and Bruice examined the oxidation of cw-stilbene and (Z)-l,2-bis(fran5-2,tran5-3-diphenylcyclopropyl)ethane by iron porphyrin/f-BuOOH system [285]. In separate experiments, AIBN was used as a radical chain initiator for the oxidation of the alkenes by t-BuOOH. According to the product distribution, they concluded the products to be derived from initial combination of t-BuOO, rather than 0=Fe (por)", with olefin. 

Ethoxyethylperoxyls were prepared by a variety of methods and react to give ethyl acetate, ethanol, acetaldehyde and ethyl formate. (Table V). There was no evidence for the formation of di-(1-ethoxyethyl) peroxide, a result that is not entirely incompatible with Diaper s work in view of the large difference in the size the peroxy radicals studied by the two groups of workers. The absolute yields of the products were difficult to ascertain with any degree of certainty because of secondary reactions. The high yield of ethyl formate is, however, diagnostic for the intermediacy of alkoxy radicals and indicates that at least 20 of the self-reactions of 1-ethoxyethylperoxyls occur via the radical mechanism. 

The alkoxy radical is usually described as a typical product of the thermal decomposition of hydroperoxides. Nevertheless, in the post-irradiation oxidation process at room temperature, it cannot originate from this reaction because all the formed products follow a kinetic similar to that of ketone formation [21]. The reaction between the alkyl macroradical and the peroxy macroradical forms peroxides (Scheme 9, Reaction 20), but we can also hypothesize Reaction 21, Scheme 9. Literature studies demonstrate that the alkoxy radical can give beta-scission (Reaction 28) forming a primary alkyl radical and CO, a product that is found during the irradiation of PE (Scheme 10, Reaction 29) [24]. The activation energy of this reaction is around 50kJ/mole. 

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