Unimolecular decomposition alkoxy radical

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

Above 250°C. we approach, in the gas phase, what is known as the cool flame regime. This is characterized by induction periods and by the appearance of pressure peaks and luminescent phenomena in the oxygen-hydrocarbon system. The consensus of present data seems to support the contention that these cool flames arise from the secondary decomposition of the hydroperoxides produced by the low temperature chain. The unimolecular decomposition of the hydroperoxide yields active alkoxy and hydroxyl radicals ... 

Tertiary peroxy radicals have no alpha hydrogen that can be abstracted by 02 in the second step. However, a unimolecular decomposition of the alkoxy radical results in the formation of a peroxy radical, which can be measured. 

The resulting 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 do not undergo isomerization for these, the competitive processes are unimolecular decomposition versus reaction with 02. The general alkoxy radical—02 reaction is... 

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 ... 

In the presence of O2 there appear to exist also other routes of OH radical formation in ozonolysis. Here unimolecular decomposition of secondary alkoxy-peroxy radicals seem to be involved. For isoprene a sequence as given by (4)-(6) may be considered ... 

Furthermore, as for alkoxy radicals with a carbon number > C4, it is known that the following type of unimolecular decomposition to rupture C-C bond between the carbon attached to carboxyl group and the adjacent carbon atom occurs. 

Since the unimolecular decomposition rate constants for the secondary alkoxy radicals with > 4 carbons, are 10" s (Atkinson and Arey 2003), the reactions of the types (7.23), (7.24) and (7.25) can occur in parallel to give hydroxyl aldehyde, aldehyde with the same carbon number as the reactant alkanes, and aldehydes with one carbon less than the original alkane, respectively. The rate constants of for the isomerization reaction, reaction with O2, and the unimolecular... 

Reaction Scheme 7.2 summarizes the reaction mechanism for 1-butene (l-C4Hg) as an example of alkenes. The hydroxyalkyl radicals formed by the pathways (a) and (b) is a kind of alkyl radicals mentioned in the previous Sect. (7.2.2), and exclusively forms hydroxyperoxy radicals by the reaction with O2 in the atmosphere. From the hydroxyperoxy radicals, oxyradicals (hydroxybutoxy radicals) and NO2 (pathways (d), (k)), and partially hydroxybutyl nitrate (pathways (e), (1)) are produced by the reaction with NO as in the case of alkylperoxy radicals described in the previous paragraph. The yields of hydroxylalkyl nitrates are 2-6 % for C4-C6 alkenes (O Brien et al. 1998), which is about half of those for alkyl nitrates from the alkoxy radicals. Hydroxy alkoxy radicals formed in pathways (d) and (k) are known to follow the three reaction pathways, unimolecular decomposition ((g), (n)), H-atom abstraction by O2 ((h), (o)), and dihydroxyl radical formation by isomerization (p), corresponding to reactions (7.25), (7.24) and... 

However, less is know about the quantitative details of these processes than is the case for the alkanes. In general, unimolecular decomposition of the alkoxy radicals [e.g., CH3CH20CH(0 )CH3 in the above reaction sequence] is more rapid than for an alkane-derived radical of similar structure. The major end product of ether oxidation is quite often an ester, the analog to the carbonyl compounds (aldehydes and ketones) generated from alkane oxidation. 

The efficient decomposition of hydroperoxides by a non-radical pathway can greatly increase the stabilizing efficiency of a chain-breaking antioxidant. This generally occurs by an ionic reaction mechanism. Typical additives are sulfur compounds and phosphite esters. These are able to compete with the decomposition reactions (either unimolecular or bimolecular) that produce the reactive alkoxy, hydroxy and peroxy radicals and reduce the peroxide to the alcohol. This is shown in the first reaction in Scheme 1.69 for the behaviour of a triaryl phosphite, P(OAr)3 in reducing ROOH to ROH while itself being oxidized to the phosphate. 

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