Alkyl peroxide radical

Maeda H, Katsuki T, Akaike T and Yasutake R. 1992. High correlation between lipid peroxide radical and tumor-promoter effect—suppression of tumor promotion in the Epstein-Barr-virus lymphocyte-B system and scavenging of alkyl peroxide radicals by various vegetable extracts. Jpn J Cancer Res 83(9) 923-928. 

The 0x0 species abstracts a hydrogen from cyclohexane to form Fe" -OH and c clohexyl radical which rebounds at rates as high as lO /sec to form cyclohexanol and Fe porphyrin is regenerated". Even though free radicals are involved, the oxidation is not a chain reaction and it does not involve alkyl peroxide radicals as in the commercial processes described earlier. 

Thus a comprehensive base is available for assessing the rates at which alkanes will react with hydroxyl radicals. It is also useful to be able to define the product(s) of these reactions. Hydrogen abstraction produces an alkyl radical that in air reacts with oxygen to produce an alkyl peroxide radical ... 

The process then continues with formation of alkyl peroxide radicals, isomerization, breakdown, and formation of oxygen-containing compounds like aldehydes, and ketones. 

The hindered phenol antioxidants Formula 4.4 and aromatic amines (e.g., p-phenylene diamines, Formula 4.1) both operate by kinetic chain-breaking processes. They donate a hydrogen to an alkyl peroxide radical and break the free radical chain with reactions such as below. 

Chemical Properties. Acychc di-Z f/-alkyl peroxides efftciendy generate alkoxy free radicals by thermal or photolytic homolysis. 

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). 

Primary and secondary dialkyl peroxides react much mote readily than di-/ fZ-alkyl peroxides (66,76). Products derived from the free radical are also produced in these reactions. 

The salts of alkyl xanthates, A/,A/ -di-substituted dithio-carbamates and dialkyidithiophosphates [26] are effective peroxide decomposers. Since no active hydrogen is present in these compounds, an electron-transfer mechanism was suggested. The peroxide radical is capable of abstracting an electron from the electron-rich sulfur atom and is converted into a peroxy anion as illustrated below for zinc dialkyl dithiocarbamate [27] ... 

Anti-oxidants can be divided into two classes depending on which part of the radical chain they quench. Primary anti-oxidants are radical scavengers and will react with alkyl chain radicals (R ) or hydroperoxides (ROOH). Secondary antioxidants work in combination with primary anti-oxidants and principally act by converting peroxide radicals (ROO ) into non-radical stable products. Synergism often works when both classes are used together. 

The experimental data on the reactions of ketyl radicals with hydrogen and benzoyl peroxides were analyzed within the framework of IPM [68]. The elementary step was treated as a reaction with the dissociation of the O—H bond of the ketyl radical and formation of the same bond in acid (from acyl peroxide), alcohol (from alkyl peroxide), and water (from hydrogen peroxide). The hydroperoxyl radical also possesses the reducing activity and reacts with hydrogen peroxide by the reaction... 

Ditellurides, 24 422 Diterpene glycosides, 24 239 Diterpenoid acids, 24 552 Diterpenoids, 24 550-555 labdane family of, 24 573 Di -ferf-alkyl peroxides, 23 439-441 as free-radical initiators, 14 288... 

The differences in the rates of decomposition of the various initiators are related to differences in the structures of the initiators and of the radicals produced. The effects of structure on initiator reactivity have been discussed elsewhere [Bamford, 1988 Eastmond, 1976a,b,c Sheppard, 1985, 1988]. For example, k,i is larger for acyl peroxides than for alkyl peroxides since the RCOO- radical is more stable than the RO radical and for R—N=N—R, kd increases in the order R = allyl, benzyl > tertiary > secondary > primary [Koenig, 1973]. 

The first step is a very slow dissociation step, with an activation energy >80 kcal/mole. However, once the radical R is made, it will rapidly react with O2 to form the aUcylperoxy radical, which is also very reactive and can abstract an H atom from any organic molecule in the solution to form the relatively stable (until someone shakes the bottle) alkyl peroxide. 

We call this type of reaction autooxidation because it is a an autocatalytic process (the reaction generates radical intermediates that propagate chain reactions) and it is an oxidation that converts alkanes into alkyl peroxides. 

TABLE 9. Rate constants for the formation of di-tertiary alkyl peroxides from tertiary alkylperoxy radicals ... 

Amidation is quite a general reaction, and substitution usually occurs at C-2 (Table 29) (76MI20503). The radicals are generated from the amide and hydrogen peroxide or an alkyl peroxide (Scheme 212). 

It is possible to obtain anti-Markovnikov products when HBr is added to alkenes in the presence of free radical initiators, e.g. hydrogen peroxide (HOOH) or alkyl peroxide (ROOR). The free radical initiators change the mechanism of addition from an electrophilic addition to a free radical addition. This change of mechanism gives rise to the anh-Markovnikov regiochemistry. For example, 2-methyl propene reacts with HBr in the presence of peroxide (ROOR) to form 1-bromo-2-methyl propane, which is an anh-Markovnikov product. Radical additions do not proceed with HCl or HI. 

The other important property affecting lipid oxidation is the chelating effect of chlorogenic acids. It is important to keep in mind that the influence of biometals (Fe, Cu etc.) on lipid free radical oxidation is essential. It is well known that iron can react with hydrogen peroxide by the Fenton reaction (Equation 3). The hydroxyl radical formed in the Fenton reaction is capable of reacting with lipid and PUFA as the initiation stage. Iron can also participate in alkyl peroxide or lipid peroxide decomposition. Therefore, the nature of iron chelation in a biological system is an important aspect in disease prevention. 

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. 

Because di-tert-alkyl peroxides are less susceptible to radical-induced decompositions, they tire safer and more efficient radical generators Ilian primary or secondary dialkyl peroxides. They are the preferred clialkyl peroxides for generating free radicals for commercial applications. 

The formation of the alkoxide ligand occurred with 50% retention (R ) and 50% racemization (R). This finding was suggested to occur by a two-step reaction, the first step involving formation of a reactive alkyl peroxide intermediate by a radical process involving racemization, followed by a bimolecular reaction, proceeding with retention (equations 36 and 37). 

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