Oxygen reaction with alkyl radicals

It was established that the sulphur-containing antioxidants completely lose their catalytic activity in the decomposition of CHP reacting with peroxide radicals. However, their activity is not significantly decreased in the reaction with alkyl radicals and oxygen (Fig. 9.8). 

The reaction rate of molecular oxygen with alkyl radicals to form peroxy radicals (eq. 5) is much higher than the reaction rate of peroxy radicals with a hydrogen atom of the substrate (eq. 6). The rate of the latter depends on the dissociation energies (Table 1) and the steric accessibiUty of the various carbon—hydrogen bonds it is an important factor in determining oxidative stabiUty. 

Oxidizing agents, e.g., quinones, which were shown to be able to retard oxidation [13] can function as antioxidants (via a chain breaking acceptor process, CB—A) if they can compete with oxygen for the alkyl radicals (Scheme 4). In the case of polymers, reaction 4a can... 

It can be seen that the steric effect is profound in radical reactions of Ar2OH with peroxyl and methyl radicals. It will be shown later that the steric effect exists in other free radical reactions of Ar2OH. The AES values of the reactions of alkyl radicals with Ar2OH are considerably higher than those for phenols reacting with oxygen-centered radicals. The steric effect can also manifest itself in the inverse reactions of sterically hindered phenoxyl radicals Ar20 with various molecules (see later). 

Results of a chemical activation induced by ultrasound have been reported by Nakamura et al. in the initiation of radical chain reactions with tin radicals [59]. When an aerated solution of R3SnH and an olefin is sonicated at low temperatures (0 to 10 °C), hydroxystannation of the double bond occurs and not the conventional hydrostannation achieved under silent conditions (Scheme 3.10). This point evidences the differences between radical sonochemistry and the classical free radical chemistry. The result was interpreted on the basis of the generation of tin and peroxy radicals in the region of hot cavities, which then undergo synthetic reactions in the bulk liquid phase. These findings also enable the sonochemical synthesis of alkyl hydroperoxides by aerobic reductive oxygenation of alkyl halides [60], and the aerobic catalytic conversion of alkyl halides into alcohols by trialkyltin halides [61]. 

Triethylborane in the presence of very small amounts of oxygen is an excellent initiator for radical chain reactions. For a long time it has been known that trialkylboranes R3B react spontaneously with molecular oxygen to give alkyl radicals (Reaction 4.7), but only recently has this approach successfully been applied as the initiation [22]. The reactions can be run at temperatures as low as — 78 °C, which allow for a better control of stereoselectivity (see below). 

It is the purpose of the present article to consider the evidence that the rate parameters offer concerning the nature of the transition states involved in the various radical reactions and how these in turn are affected by the chemical nature of the species involved. Although our principal concern shall be with alkyl radical reactions, we shall also consider some molecular reactions which are closely related and finally the behavior of some systems containing oxygen and halogen atoms as well. 

Carlier fundamental studies of autoxidations of hydrocarbons have concentrated on liquid-phase oxidations below 100 °C., gas-phase oxidations above 200°C., and reactions of alkyl radicals with oxygen in the gas phase at 25°C. To investigate the transitions between these three regions, we have studied the oxidation of isobutane (2-methylpropane) between 50° and 155°C., emphasizing the kinetics and products. Isobutane was chosen because its oxidation has been studied in both the gas and liquid phases (9, 34, 36), and both the products and intermediate radicals are simple and known. Its physical properties make both gas- and liquid -phase studies feasible at 100°C. where primary oxidation products are stable and initiation and oxidation rates are convenient. 

Small radicals such as tert-butylperoxy and ethylperoxy can, however, react via 1,4 H-transfer only the strain energy involved in O-heterocycle formation is 28 kcal. per mole. In this case, k.4(x — 106 sec."1 whereas krta = 10r> 4 sec. 1 and when [02] = 200 mm. of Hg, ko[02] = 105,3 sec. 1, so that k.4ct < < (tkr,a + k [02]). The result is that in the oxidation of small alkyl radicals, the route via alkylperoxy radicals will be blocked because reverse Reaction —4 competes successfully with Reaction 5. Reaction 2 will thus be a more effective mode of reaction of alkyl radicals with oxygen and the conjugate alkene will be a major product. 

It is possible that H02 reacts with alkyl radicals, but it cannot be the sole reaction responsible for the formation of oxygenated products during the pic darret since the H02 radical concentration becomes very high at 450°-500°C., while the pic darret disappears (Figure 4). 

The reaction of alkyl radicals with molecular oxygen should formally be considered as an addition reaction ... 

The formation of oxyl radicals in reaction of alkyl radicals with oxygen is important for the possible occurrence of branching of oxidation reaction due to oxygen atoms. This aspect is not, however, obvious from the experiments performed. It seems that oxyl radicals in HDPE are not formed in propagation reaction of alkyl radicals and oxygen but as the product of termination of two peroxyl radicals. 

Rate coefficients for reactions of alkyl radicals with oxygen (From ref. 158.)... 

The initiation reaction is the hemolytic abstraction of hydrogen to form a carbon-centered alkyl radical in the presence of an initiator. Under normal oxygen pressure, the alkyl radical reacts rapidly with oxygen to form the peroxy radical, which in turn reacts with more unsaturated lipids to form hydroperoxides. The lipid-free radical thus formed can further react with oxygen to form a peroxy radical. Hence, the autoxidation is a free radical chain reaction. Because the rate of reaction between the alkyl radical and oxygen is fast, most of the free radicals are in the form of the peroxy radical. Consequently, the major termination takes place via the interaction between two peroxy radicals. 

Since the reaction of alkyl radicals with oxygen (pi) is much faster than reaction p2, most of the radicals present in the system at a particular time will be peroxyl radicals, ROO. so the main route for destruction of the radicals will be the bimolecular reaction between the peroxyl radicals, i.e. ku... 

The most important daytime loss process for the biogenics is reaction with OH radicals. Rate constants for the OH reaction with isoprene, a-and /3-pinene are 1.01 X 10-10 cm3 molecule-1 s-1, 5.37 X 10-11 cm3 molecule-1 s-1, and 7.89 X 10 11 cm3 molecule-1 s-1, respectively. Hydroxyl radical reactions with dialkenes and monoterpenes proceed primarily via OH addition across the double bond. Subsequent addition of oxygen to the radical produces a peroxy radical. The reactions of the resulting peroxy radical proceed in a manner similar to those of alkyl peroxy radicals. 

In the propagation sequence (Reactions 12.3 and 12.4), given an adequate supply of oxygen, the reaction between alkyl radicals and molecular oxygen is very fast and peroxyl radicals are formed (ROO ). These react with another fatty acid molecule producing hydroperoxides (ROOH) and new free radicals that contribute to the chain by reacting with another oxygen molecule. Hydroperoxide molecules can decompose in the presence of metals to produce alkoxyl radicals (RO ), which cleave into a complex mixture of aldehydes and other products, i.e., secondary oxidation products."... 

Photoreactions of aromatic ketones and quinones take place both in vacuum and in air. The subsequent reactions of alkyl radicals with oxygen significantly affect cross-linking since parent ketones are regenerated from ketyl radicals and simultaneously, hydrogen peroxyl radicals are formed. 

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