Peroxy radical interactions

Pohlman, A. A. and T. Mill. 1983. Peroxy radical interaction with soil constituents. Soil Sci. Soc. Amer. J. 47 922-927. 

The reaction with NO leads to the formation of CO2 and a methyl radical that is oxidized to formaldehyde by reactions (16)-(20). In addition, the oxidation of CH3 regenerates HO c so that the oxidation cycle continues. Association with NO2 produces peroxyacetyl nitrate (PAN). Its lifetime is longer than that of alkylperoxy nitrates, but strongly temperature dependent, ranging from 1 hr at 298 K to 140 d at 250 K. Thus, PAN can be transported over a great distance before undergoing thermal decomposition. Under conditions of lowNOj concentrations acetyl peroxy radicals interact also with HO2 radicals... 

Peroxy radicals interact rapidly with antioxidants which may be present to give monohydroperoxides (cf. 3.7.3.1). Thus, it is not only the chain reaction which is inhibited by antioxidants, but also P-fragmentation and peroxy radical cyclization. Fragmentation occurs when a hydroperoxidee-pidioxide is heated, resulting in formation of aldehydes and aldehydic acids. For example. 

First the interaction of selected tetramethylpiperidine (TMP) derivatives with radicals arising from Norrish-type I cleavage of diisopropyl ketone under oxygen was studied. These species are most probably the isopropyl peroxy and isobutyryl peroxy radicals immediately formed after a-splitting of diisopropyl ketone and subsequent addition of O2 to the initially generated radicals. Product analysis and kinetic studies showed that the investigated TMP derivatives exercise a marked controlling influence over the nature of the products formed in the photooxidative process. The results obtained point to an interaction between TMP derivatives and especially the isobutyryl peroxy radical. 

From the foregoing degradation scheme for DiPK in the presence of nitroxide, as well as from what we know about DBK photooxidation, it would be expected that the two radicals a and b primarily formed would be captured by oxygen and then give rise to the isobutyryl and isopropyl peroxy radicals c and d. Theoretically, these could interact to form isobutyric acid and acetone (reaction (8)) ... 

Kinetic analysis of the results of ketone oxidation in the presence of amine II reveals that the velocity constant of the oxidation of amines by acyl per-oxy radicals must be greater (by a factor of 2 - 3) than that of the interaction of these radicals with the nitroxide-i. In this reaction, acyl peroxy radicals are captured and destroyed by amines. 

Since the reactions occur under oxygen saturation, the principal stabilizing steps are the interactions of the HALS derivatives with the alkoxy and peroxy radicals and with the hydroperoxides. 

ROS and other radical intermediates dictate the oxidative decomposition of fuels. We have noted that peroxy radical intermediates provide an enormous amount of flexibility in the combustion of a given compound, specifically in the unimolecular steps available to that compound. In an instructive display of the interaction of experimental and theoretical techniques, rearrangement pathways of the peroxy radicals have been modeled computationally and provide justification for several unexpected products. 

A wealth of evidence (6, 7,13,19, 23, 46, 48) now supports the idea originally introduced by Blanchard (12) that tertiary peroxy radicals undergo both terminating and nonterminating interactions. 

There is excellent agreement between the decay constants obtained by ceric ion oxidation of secondary hydroperoxides and the rate constants for chain termination in hydrocarbon autoxidation determined by the rotating sector. The agreement suggests that secondary peroxy radicals do not undergo many nonterminating interactions, so that most self-reactions of secondary peroxy radicals must be chain terminating. 

Oxygen has two possible interactions during the polymerization process [94], and these reactions are illustrated in Fig. 2. The first of these is a quenching of the excited triplet state of the initiator. When this quenching occurs the initiator will absorb the light and move to its excited state, but it will not form the radical or radicals that initiate the polymerization. A reduction in the quantum yield of the photoinitiator will be observed. The second interaction is the reaction with carbon based polymerizing radicals to form less reactive peroxy radicals. The rate constant for the formation of peroxy radicals has been found to be of the order of 109 1/mol-s [94], Peroxy radicals are known to have rate constants for reaction with methyl methacrylate of 0.241/mol-s [100], while polymer radicals react with monomeric methyl methacrylate with a rate constant of 5151/mol-s [100], This difference implies that peroxy radicals are nearly 2000 time less reactive. Obviously, this indicates that even a small concentration of oxygen in the system can severely reduce the polymerization rate. 

From my estimates on the thermodynamic properties of peroxy and polyoxide molecules and radicals, we can estimate that the bond dissociation energy of the tetroxide is about 5 kcal. Thus, at room temperature, or even at dry ice temperature, the tetroxide is extremely unstable and should redissociate into the more stable (from a thermodynamic point of view) peroxy radicals. The competing step would be a concerted decomposition into an RO and an R03 (Step 14) radical, which would be uphill by 20 kcal., or else a concerted decomposition into 2 RO radicals and 02 (Step 14 ). The latter is almost thermoneutral. If we take the current data at face value, it provides, from the reported activation energy at least, strong evidence that the propagating interaction of two alkylperoxy radicals proceeds in a concerted fashion. 

C. Walling In the discussion at this session an interesting contradiction has arisen which needs to be resolved. Extensive data on liquid phase autoxidations near room temperature indicate that bimo-lecular interactions of peroxy radicals (particularly tertiary peroxy radicals ) are slow processes with appreciable activation energies. In contrast, analysis of high temperature gas-phase processes indicate fast interactions occurring at almost every collision. 

Peroxy radicals are formed in the troposphere through the interaction of sunlight with certain molecules or as products of other radical reactions. 

The discussion that follows is divided into two sections. The Spectroscopic Methods section includes those measurement techniques that involve the interaction of a photon with a peroxy radical. The Chemical Conversion Methods section describes the measurement of another molecule or radical to which a peroxy radical has been converted. 

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... 

Nangia PS, Benson SW (1980) The kinetics of the interaction of peroxy radicals. II. Primary and secondary alkyl peroxy. Int J Chem Kinet 12 43-53... 

Russell GA (1957) Deuterium-isotope effects in the autoxidation of aralkyl hydrocarbons. Mechanism of the interaction of peroxy radicals. J Am Chem Soc 79 3871-3877... 

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