Reactions of alkoxy radicals

As alkoxy radicals are only formed at low temperatures from RO2 + RO2 reactions and the decomposition of peroxides, they are only of general importance below 650 K 

As is well known, they play a key role in atmospheric chemistry through sequences involving NO. 

RO radicals undergo only two reactions C—C homolysis reaction with O2 to give an aldehyde or ketone and an HO2 radical isomerization to give a hydroxyalkyl radical when the ring transition state involves little strain energy. These possibilities are illustrated below for 2-pentoxy radicals. 

Atkinson [54] has reviewed these reactions in the context of atmospheric chemistry, and all are important in combustion under specific conditions up to about 600 K. Arrhenius parameters for a selection of reactions are given in Table 1.19. It is clear that 1,5 isomerization is the dominant process where structurally possible. Increased ring strain in 1,4 and 1,6 isomerizations will render them much less likely. 

Methoxy radicals are important up to 800 K, where moderate yields of CH3OH can be observed in the initial products. 

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The chemistry of atkoxycarbonyloxy radicals in many ways parallels that of the aroyloxy radicals (e.g. benzoyloxy, see 3.4.2.2.1). Products attributable to the reactions of alkoxy radicals generally arc not observed. This indicates that the rate of p-scission is slow relative to the rate of addition to monomers or other... 

Cox, R.A., Patrick, K.F., and Chant, S.A. Mechanism of atmospheric photooxidation of organic compounds. Reactions of alkoxy radicals in oxidation of n-butane and simple ketones, Environ. Sci Technol, 15(5) 587-592,1981. 

The reactions of R02 with NO and with R02 generate alkoxy radicals (RO). Alkoxy radicals have several possible atmospheric fates, depending on their particular structure. These include reaction with 02, decomposition, and isomerization as we shall see, reactions with NO and N02 are unlikely to be important under most tropospheric conditions. Atkinson et al. (1995b) and Atkinson (1997b) have reviewed reactions of alkoxy radicals and /3-hydroxyalkyl radicals ... 

An alternate potential fate for the CH3SCH20 radical is the well-known abstraction reaction of alkoxy radicals with 02, forming methyl thioformate (Crutzen, 1983 Butkovskaya and Le Bras, 1994) ... 

Recently, Kabasakalian et al.138-140 have reported the nitroso dimer formation in the photolysis of primary and secondary nitrites. Both this reaction and the Barton reaction16 are explained in terms of reactions of alkoxy radicals. 

Another possible course of reaction of alkoxy radicals is addition to double bonds ... 

The mechanisms for ambient temperature oxidations become more complex as the alkyl radical R becomes more complex and as the reactions proceed. Thus, with azo-2-methylpropane (52) the pyrolysis of the tert-butoxy radical must be included, and in all but the initial stages, reactions of alkoxy radicals with products containing weak C—H bonds must be included. Numerous tertiary reactions can then occur. As with most free radical systems, useful information can be obtained only if the degree of conversion of the starting material is kept low. 

The reaction of alkoxy radicals with 02 has been postulated in many studies (29). Cerfontain and Kutschke (8), in the photolysis of (C2H5)2N2, estimated the relative importance between... 

At the end of reaction the initial hydrocarbon concentration is still important, but there are also many other hydrogen donors. Thus, in the absence of oxygen, reactions of alkoxy radicals will lead to aldehydes by pyrolysis and to alcohols by abstraction. Let us examine these two possible reactions for each alkoxy radical. 

Phillips and his co-workers have studied the reactions of alkoxy radicals between 95 and 240°C. The pyrolysis of CH3ONO at 150-240°C was examined, and CH20 was formed above 180°C.343 Thus, reaction (27) plays some role at elevated temperatures for CH30 radicals. (NO is produced in the pyrolysis.) In unpublished work (as reported in ref. 6), Phillips found that, in the pyrolysis of (CH30)2-NO mixtures, k27/k26 = 0.5 at 174°C. 

The reaction of alkoxy radicals, as the intermediates of hydrocarbon oxidation, with molecular oxygen takes place in the case of primary and secondary radicals... 

Compared with chlorination, hydrogen abstraction reactions of alkoxy radicals are relatively insensitive to solvent effects [160, 222, 223]. The results of the AIBN-initiated radical chain chlorination of 2,3-dimethylbutane with tert-butyl hypochlorite indicate a solvent effect on tert-butoxy radical reactions of much smaller magnitude, but greater selectivity in aromatic solvents [222, 223], The reduced solvent effect for this hydrogen abstraction reaction has been attributed to steric effects. Due to the bulky... 

Scheme 1.57. Chain scission (13) or elimination (14) reactions of alkoxy radicals in polypropylene resulting in terminal (13) or in-chain (14) ketone groups as well as alkyl radicals that may react further with oxygen.

Wijnen, M. H. J. Reactions of Alkoxy-Radicals. I. Photolysis of Methyl Acetate. J. Chem. Phys. 27, 710 (1957). 

Table 6-13. Left Rate Coefficient Estimates for Reactions of Alkoxy Radicals in the Atmosphere, Taken Partly from the Review of Atkinson and Lloyd (1984) Right Rates Calculated from the Rate Coefficients Assuming Number Densities n(02) = 5 x 10 8, n(N02) = 2.5 x 10 ° molecules / cmib... 

Batt, L. (1980) Reactions of alkoxy radicals relevant to atmospheric chemistry. In Eur. Symp. Physico-Chemical Behaviour Atmos. Pollutants, 1st (B. Versino and H. Ott, eds.), pp. 167-184. Commission of the European Communities, Luxembourg. 

The kinetics of the reactions of alkoxy radicals have been reviewed and evaluated by Gray et al. [369]. Results for H-abstraction reactions of methoxy radicals are shown in Table 33. It is not possible with alkoxy radicals to monitor the radical concentrations by measuring the rates of formation of the dimers since these are peroxides which are difficult to analyse. The reactions of methoxy radicals with alkanes were studied indirectly by measuring the rates of consumption of the alkanes in competitive experiments involving pairs of reactants [366]. This yielded relative Arrhenius parameters which were put on an absolute basis by... 

Alkoxy (RO) radicals are formed in the reaction of alkyl peroxy (R02) radicals with NO. Subsequent reactions of alkoxy radicals determine to a large extent the products resulting from the atmospheric oxidation of VOCs (Orlando et al. 2003). Alkoxy radicals react under tropospheric conditions via a variety of processes unimolecular decomposition, unimolecular isomerization, or reaction with 02. Alkoxy radicals with fewer than five carbon atoms are too short to undergo isomerization for these the competitive processes are unimolecular decomposition versus reaction with 02. The general alkoxy radical-02 reaction involves abstraction of a hydrogen atom by 02 to produce an H02 radical and a carbonyl species ... 

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. 

Alkoxy (RO ) radicals are formed in the reaction of alkyl peroxy (RO2 ) radicals with NO, reaction 5.59a. Subsequent reactions of alkoxy radicals determine to a large extent the... 

Williams, 1959 Gray et al., 1967 Heicklen, 1968 Howard, 1972), but no general update seems to have t peaied. However, Batt (1979) has reviewed the thermochemistry and thermal-decomposition kinetics for aikoxyl radicals and alkyl nitrites. It is the purpose of the present review to report all work through June, 1986 (and some later) on the reactions of alkoxy radicals. In addition, since aikoxyl radicals often are produced in laboratory experiments from alkyl nitrites, the photochemical and thermal decomposition of these species will also be reviewed. 

B.R. Kennedy and K.U. Ingold, Reactions of alkoxy radicals. 1. Hydrogen atom... 

The results of LACTOZ have provided an extended kinetic data base for the following classes of reactions reactions of OH with VOCs, reactions of NO3 with VOCs and peroxy radicals, reactions of O3 with alkenes, reactions of peroxy radicals (self reactions, reaction with HO2, other RO2, NO, NO2), reactions of alkoxy radicals (reactions with O2, decomposition, isomerisation), thermal decomposition of peroxynitrates. Photolysis parameters (absorption cross-section, quantum yields) have been refined or obtained for the first time for species which photolyse in the troposphere. Significantly new mechanistic information has also been obtained for the oxidation of aromatic compounds and biogenic compounds (especially isoprene). These different data allow the rates of the processes involved to be modelled, especially the ozone production from the oxidation of hydrocarbons. The data from LACTOZ are summarised in the tables given in this report and have been used in evaluations of chemical data for atmospheric chemistry conducted by international evaluation groups of NASA and lUPAC. 

Table 14 Rate constants for the reactions of alkoxy radicals with O2 at 298 K.

It is certainly difficult, at the present time, to establish relationships between structure and reactivity for reactions of alkoxy radicals. More experimental and theoretical approaches are necessary to build a general picture of alkoxy radical reactivity. 

Reactions of alkoxy radicals, including isomerisation, decomposition and reaction with O2. 

Laboratory study of reactions of alkoxy radicals in oxygen,... 

Fig. 3. The important reactions in polymer oxidation expressed as the interlocking cycles which eventually destroy the polymer through the scission reactions of alkoxy radicals.

In general, alkyl-peroxy radicals are rather stable and do not undergo side reactions. In an oxygen-saturated polymer, alkyl radicals are rapidly converted to alkyl-peroxy radicals by reaction with oxygen, so that their other reactions are not important. The most important side reaction is that of the alkoxy radical. In liquid hydrocarbons, the main reaction of alkoxy radicals is hydrogen abstraction to give... 

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