Alkylperoxyl radicals

Phenols are important antioxidants, with vitamin E being the most important endogenous phenolic membrane-bound antioxidant. Membrane levels of vitamin E are maintained through recycling of the vitamin E radical with ascorbate and thiol reductants. Vitamin E is a mixture of four lipid-soluble tocopherols, a-tocopherol being the most efiective radical quencher. The reaction of a-tocopherol with alkyl and alkylperoxyl radicals of methyl linoleate was recently reported. These are facile reactions that result in mixed dimer adducts (Yamauchi etal., 1993). 

Adhikari, S., Kapoor, S., Chattopadhyay, S., and Mukheijee, T. 2000. Pulse radiolytic oxidation of P-carotene with halogenated alkylperoxyl radicals in a quaternary microemulsion Formation of retinal. Biophys. Chem. 88 111-117. 

Under conditions of liquid-phase oxidation, alkylperoxyl radicals are stable and react rapidly with RH. They do not decompose into olefin and hydroperoxyl radical. However, some peroxyl radicals have a weak C—OO bond and decompose back to R and dioxygen ... 

The situation with polyarylmethanes is very similar. Due to the stabilization of free valence in arylmethyl radicals, the bond dissociation energy (BDE) of the bond C—02 for example, in triphenylmethyl radical is sufficiently lower than in alkylperoxyl radicals. This radical is decomposed under oxidation conditions (room temperature), and the reaction of Ph3C with dioxygen is reversible ... 

Alkylperoxyl radicals produced by addition reactions can be destructively isomerized with the formation of epoxides [13,139,154,155] ... 

The chain termination is a result of tertiary alkylperoxyl radical recombination in the solvent cage. The values of the rate constants for chain termination through the disproportionation of tertiary peroxyl radicals are collected in Table 2.15. They vary in the range 103 to 105 L mol 1 s 1 at room temperature. The probability of a pair of alkoxyl radicals to escape cage recombination is sufficiently higher than that of cage recombination. The values of rate constants of the reaction 2 R02 > 2 RO + 02 measured by the EPR technique are presented in Table 2.16. 

The chain termination by reactions of tertiary alkylperoxyl radicals is complicated by their decomposition with production of ketone and the alkyl radical, for example ... 

Chain propagation in oxidized alcohols proceeds with the participation of a mixture of H02 and R(0H)02 radicals. Using the data of Table 7.4 and Table 7.7 we can compare the activity of the H027H0R02 mixture and alkylperoxyl radicals toward the same alcohol (R OO is the peroxyl radical of cyclohexene, T = 333 K). 

Energies of a-C—H Bonds Dissociation in Ethers and Enthalpies of Secondary Alkylperoxyl Radical Reactions with Ethers [2]... 

Along with tertiary hydroperoxide of ether, the BDE of the O—H bonds of alkoxy hydroperoxides are higher than that of similar hydrocarbons. Very valuable data were obtained in experiments on ether oxidation (RiH) in the presence of hydroperoxide (RiOOH). Peroxyl radicals of oxidized ether exchange very rapidly to peroxyl radicals of added hydroperoxide ROOH and only R02 reacts with ether (see Chapter 5). The rate constants of alkylperoxyl radicals with several ethers are presented in Table 7.18. The reactivity of ethers in reactions with peroxyl radicals will be analyzed in next section. 

The Values of a-C—H bond Dissociation Energy in Acids (Dc H) and Enthalpies (AH) of Secondary Alkylperoxyl Radical (R1R2CHOO ) Reaction with Acids [2]... 

The Values of AH, E, AEH, and Geometric Parameters of Secondary Alkylperoxyl Radicals Reactions with Phenols, Calculated by IPM Method (Equations (15.3)-(15.6))... 

The question why the aminyl radicals ensure cyclic chain termination in those systems in which the hydroperoxyl and hydroxyalkylperoxyl radicals are formed, but not in the oxidation of hydrocarbons where alkylperoxyl radicals are the chain-propagating species deserves special attention [22 24]. Indeed, the disproportionation of the aminyl and peroxyl radicals... 

FIGURE 16.1 The dependence of activation energy E on reaction enthalpy A He for reaction of hydrogen atom abstraction by aminyl radical from the C—H bond of alkylperoxyl radical and O—H bond of hydroperoxyl radical calculated by IPM method (see Chapter 6). The points fix the reactions with minimum and maximum enthalpy among known aromatic aminyl radicals. 

At the same time, quinones do not practically retard oxidation of hydrocarbons, since alkyl radicals react very rapidly with dioxygen (see Chapter 4) to give alkylperoxyl radicals, which scarcely react with quinones. Quinones exhibit their inhibiting properties as alkyl radical acceptors only in the oxidation of polymers (see Chapter 19). However, quinones were found to decelerate the oxidation of alcohols very efficiently and for long periods by ensuring cyclic chain termination via the following reactions [31-34] ... 

The question arises why quinones terminate chains via the reaction with hydroperoxyl and hydroxyalkylperoxyl radicals and do not via the reaction with the (3-C—H bond of the alkylperoxyl radical. 

We see, at first, that the reaction enthalpy for quinone abstraction reactions with the C—H bond of alkylperoxyl radicals is higher than with the O—H bond of the hydroperoxyl radical. The second important factor is different triplet repulsions in these two types of abstraction reactions. Indeed, the reaction with R02 proceeds via TS of the C H O type. Such reaction is characterized by the high thermally neutral activation energies Eeo = 62.9 kJ mol-1. The value of Ee0 for the reaction involving the O H O TS reaction center is much lower (27.3 kJ mol-1). With the rate constants have a very low value, the reaction Q + R02 cannot influence the oxidative chain termination in comparison with the interaction of two R02 radicals. Indeed, the rate constant for the latter is 105—107 L mol-1 s-1 and, in these cases, the inequality (2k6v )1/2 2k[Q] always holds. The reason for such high Ee0 values and, hence,... 

Nitroxyl radicals (AmO ) are known to react rapidly with alkyl radicals and efficiently retard the radical polymerization of hydrocarbons [7]. At the same time, only aromatic nitroxyls are capable of reacting with alkylperoxyl radicals [10,39] and in this case the chain termination in the oxidation of saturated hydrocarbons occurs stoichiometrically. However, in the processes of oxidation of alcohols, alkenes, and primary and secondary aliphatic amines in which the chain reaction involves the HOT, >C(0H)02 , and >C(NHR)02 radicals, possessing the... 

Principally, nitroxyl radicals can disproportionate with alkylperoxyl radicals. 

The reaction of AmO with H02 occurs with AH < A//c min and, subsequently, with a low activation energy (E=0.5RT) and a high rate constant. The latter is higher than 2kt for peroxyl radicals (see Chapter 6), which is important for cyclic chain termination. The inverse situation takes place in reactions of nitroxyl radical disproportionation with alkylperoxyl radicals. For these reactions we observe inequality AH > A//c min and, subsequently, relatively a high activation energy (E> 0.5RT) and a low rate constant. The latter are lower than 2kt for... 

It is clearly seen that for the known nitroxyl radicals E 0 for reactions of nitroxyls with the hydroxyl group containing peroxyl radicals and E>0 for all nitroxyl reactions with alkylperoxyl radicals. 

Alkylperoxyl radicals have also been trapped at low temperatures using nitrosobutane (Howard and Tait, 1978), although the adduct is detected only below 0°C (see also Pfab, 1978). 

A number of NO-derived reactive species can initiate lipid peroxidation, including nitrogen dioxide and, most notably, ONOO , which displays unique properties as a mediator of lipid oxidation. On a molecular basis, ONOO is a more potent lipid oxidant than hydrogen peroxide and, unlike H2O2, it does not require metal catalysis. The one-electron oxidants such as metals, as well as heme proteins and peroxynitrite, are assumed to play an important role in many diseases associated with oxidative stress. Heme proteins such as horseradish peroxidase (HRP) can produce alkylperoxyl radicals through two sequential... 

The rate constants for hydrogen abstraction from Rh H, O H, and C-H bonds by chromyl ions and Craq002+ are summarized in Table VI. Also listed in the table are selected relative rate constants for hydrogen abstraction by tert-butoxyl and tert-butylperoxyl radicals, expressed as .buo/AbuOO- The difference between the two sets of data is striking in that alkoxyl radicals are 105-107 times more reactive than alkylperoxyl radicals, but in the chromium series the ratio kcrolkcrOO is only about 102 for all the reactions studied. This ratio is preserved over about 103-fold change in absolute rate constants within each series. 

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