Alkyl peroxy radical reactivity

It was again observed that rearrangement pathways comprise a substantial portion of the oxidation routes for alkylated aromatics.Since this phenomenon is mainly due to peroxy radical reactivity rather than to identity of the parent compound, it is clear that comparable rearrangements would be factors for PAHs, as well as for nitrogen-, oxygen-, and sulfur-containing heteroaromatic rings and their alkylated derivatives. 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

Mention has already been made of the relatively small reactivity of allyl peroxy radicals compared with other alkyl peroxy radicals. Jost (88, 96) has reasoned that paraffins react by a small number of long chains, whereas olefins oxidize by a large number of short chains. Olefins are thus attacked more readily than paraffins but form less reactive allyl radicals. In addition, during oxidation chain transfer occurs in which alkyl radicals are replaced by allyl radicals. Shorter chains would then be expected. Comparison of the precombustion products of iso-octane and diisobutylene (154) indicates that marked self-inhibition of reaction chains was occurring in the latter case. 

Bauer G (2000) Reactive oxygen and nitrogen species efficient, selective and interactive signals during intercellular induction of apoptosis. Anticancer Res 20 4115-4140 Beckwith AU, Davies AG, Davison IGE, Maccoll A, Mruzek MH (1989) The mechanisms of the rearrangements of allylic hydroperoxides 5a-hydroperoxy-3p-hydrocholest-6-ene and 7a-hydro-peroxy-3(1-hydroxycholest-5-ene. J Chem Soc Perkin Trans 2 815-824 Behar D, Czapski G, Rabani J, Dorfman LM, Schwarz HA (1970) The acid dissociation constant and decay kinetics of the perhydroxyl radical. J Phys Chem 74 3209-3213 Benjan EV, Font-Sanchis E, Scaiano JC (2001) Lactone-derived carbon-centered radicals formation and reactivity with oxygen. Org Lett 3 4059-4062 Bennett JE, Summers R (1974) Product studies of the mutual termination reactions of sec- alkylper-oxy radicals Evidence for non-cyclic termination. Can J Chem 52 1377-1379 Bennett JE, Brown DM, Mile B (1970) Studies by electron spin resonance of the reactions of alkyl-peroxy radicals, part 2. Equilibrium between alkylperoxy radicals and tetroxide molecules. Trans Faraday Soc 66 397-405... 

The stabilizing activity of phenols is based on scavenging of the alkyl-peroxy radicals (POO ) generated in oxidizing polymers. Participation of POO in the oxidation chain transfer is thus reduced. Reactivity of phenolics with POO results, however, in chemical transformation of the original phenolic structure (antioxidant consumption). Hence, the protection of the polymer matrix is diminished in stepwise fashion. Crossconjugated dienoide compounds, e.g. quinone methides, account... 

Although the above reactions generate a few fi radicals, most of the oxidation of nitric oxide to nitrogen dioxide is carried out by the alkyl-peroxy, RO, and hydroperoxy radicals that are formed in later reactions involving reactive hydrocarbons, aldehydes, or even carbon monoxide. One such example is shown in Figure 2-7. There is still considerable uncertainty as to the mechanism of these secondary reactions. The modeling studies should be consulted for details. ... 

Their work adequately supports their qualitative conclusion that peroxy radicals have about the same reactivity irrespective of the nature of the alkyl substituent if the term about admits a factor of 2 or 3 and if we overlook the factor of 50 for Tetralin-Decalin. Their work leaves unanswered the question of whether these factors are real or experimental. 

Figure 3 shows the correlation obtained by plotting logio (relative rate) as a function of log Q — 0.5 (e + 0.8) for the seven monomers. Over a range of oxidation rates varying by a factor of 100 the relation predicts the rate from Q,e values to less than a factor of 3. This is less precise than the correlation with excitation energies used for alkyl-subsituted ethylenes (18), but is probably all that can be expected, since the Q,e system is an empirical relation and the assumption of equal reactivities and termination rate constants for primary and secondary peroxy radicals is imprecise (9). 

Peroxidation of membrane lipids involves the production of labile fatty acid hydroperoxides from free radical intermediates, which can readily decompose in the presence of transition metals to give rise to extremely reactive free radicals such as alkyl peroxy (R00 )> alkoxyl (RO ) and hydroxyl ( 0H) radicals (2). 

Peroxy nitric adds and organic peroxy nitrates are another precursors of free radicals, which may be introduced into polymers from polluted atmosphere. They produce both peroxy radicals and reactive nitrogen oxides (NO and N02) on decomposition. With alkyl peroxynitrates, decomposition proceeds via OO—N bond fission having activation energy 87 kJ/mol, their half-life being several seconds at 0 °C [12]. 

The reactivity of 02 - with alkyl halides in aprotic solvents occurs via nucleophilic substitution. Kinetic studies confirm that the reaction order is primary > secondary > tertiary and I > Br > Cl > F for alkyl hahdes, and that the attack by 02 - results in inversion of configuration (Sn2). Superoxide ion also reacts with CCI4, Br(CH2)2Br, CeCle, and esters in aprotic media. The reactions are via nucleophilic attack by 02 on carbon, or on chlorine with a concerted reductive displacement of chloride ion or alkoxide ion. As with all oxyanions, water suppresses the nucleophilicity of 02 (hydration energy, lOOkcalmoL ) and promotes its rapid hydrolysis and disproportionation. The reaction pathways for these compounds produce peroxy radical and peroxide ion intermediates (ROO and ROO ). 

The nucleophilicity of O2 - toward primary alkyl halides (Scheme 7-2) results in an Sn2 displacement of halide ion from the carbon center. The normal reactivity order, benzyl>primary>secondary>tertiary, and leaving-group order, I>Br>OTs>Cl, are observed, as are the expected stereoselectivity and inversion at the carbon center. In dimethylformamide the final product is the dialkyl peroxide. The peroxy radical (ROO), which is produced in the primary step and has been detected by spin trapping,25 is an oxidant that is readily reduced by O2 -to form the peroxy anion (ROO ). Because the latter can oxygenate Me2SO to its sulphone, the main product in this solvent is the alcohol (ROH) rather than the dialkyl peroxide. 

The key reaction in the propagation sequences is the reaction of polymer alkyl radicals (P ) with oxygen to form polymer peroxy radicals (2). This reaction is very fast. The next propagation step, reaction (3), is the abstraction of a hydrogen atom by the polymer peroxy radical (POO-) to yield a polymer hydroperoxide (POOH) and a new polymer alkyl radical (P ). In polypropylene (Scheme 1.56), hydrogen abstraction occurs preferentially from the tertiary carbon atoms since they are the most reactive, as shown in reaction (11). 

These antioxidants include the hindered phenols and are considered to be most effective when the chain-carrying (propagation) radical is an -oxy radical such as alkyl peroxy, R02. Thus, in reactive processing, they would be expected to be of value in suppressing the oxidation reactions which can occur in the earlier zones of a reactive extruder. The chemistry of these systems has been studied in detail (Al-Malaika, 1989, Scott, 1993b), and it has been found in the case of hindered phenols that the effectiveness of these stabilizers is dependent on the chemistry of the oxidation product rather than the simple donor reaction of the phenol hydrogen atom to the propagating radical. 

This leads to a hydroperoxide and an alkyl radical, which can again react with oxygen according to Reaction (4.2). The rate of Reaction (4.3) is slow, ks = 10 -10 1 mor s at 30°C, dependent on the type of hydrocarbon, when compared with Reaction (4.2), and is therefore the rate-determining step for chain propagation. Due to their low reactivity, peroxy radicals are present in relatively high concentration in the system when compared with other radicals, determined via electron spin resonance. 

Radical polymerization inhibitors are therefore molecules that are able of reacting with the radicals present in the monomer (either alkyl radicals or peroxy radicals) to give very low reactive radicals which will stop the chain growth. It should be noted that the formation of these products (inert in terms of the polyaddition reaction) can result from several basic and consecutive reactions. This is why inhibition mechanism can sometimes be rather complex. 

In contrast, available information on the reactions of alkyl and P-hydroxy peroxy radicals with NO [6, 14, 15] indicate a progressive decrease in reactivity as the carbon number increases, suggesting that the reactions with HO2 become progressively more favoured over the reactions with NO, as the size of the organic group becomes larger. 

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