Alkyl hydroperoxides reaction temperature

As the temperature is increased through the NTC zone, the contribution of alkylperoxy radicals falls. Littie alkyl hydroperoxide is made and hydrogen peroxide decomposition makes a greater contribution to radical generation. Eventually the rate goes through a minimum. At this point, reaction 2 is highly displaced to the left and alkyl radicals are the dominant radical species. 

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]. 

At elevated temperature alkyl hydroperoxides undergo thermal decomposition to alcohols [Eqs. (9.9)—(9.11)]. This decomposition serves as a major source of free radicals in autoxidation. Because of side reactions, such as p scission of alkylperoxy radicals, this process is difficult to control. Further transformation of the alkoxy... 

Figure 6 shows the variation of peroxide concentration in methyl ethyl ketone slow combustion, and similar results, but with no peracid formed, have been found for acetone and diethyl ketone. The concentrations of the organic peroxy compounds run parallel to the rate of reaction, but the hydrogen peroxide concentration increases to a steady value. There thus seems little doubt that the degenerate branching intermediates at low temperatures are the alkyl hydroperoxides, and with methyl ethyl ketone, peracetic acid also. The tvfo types of cool flames given by methyl ethyl ketone may arise from the twin branching intermediates (1) observed in its combustion. 

The major product of this chain is the alkyl hydroperoxide. The secondary products which are observed are the results of the reactions of alkyl radicals in the system with the hydroperoxide or of the secondary spontaneous breakdown of the hydroperoxide if the temperature is sufficiently high. This chain mechanism predominates in the temperature regime from about 30° to about 250°C., for the gas phase or in relatively inert solvents. 

This region of negative temperature coefficient can be quantitatively ascribed to the failure of the system to produce alkyl hydroperoxide as a product at the higher temperatures. Instead, with increasing temperature because of the reversibility of Reaction 1, the equilibrium concentration of alkyl peroxy radicals decreases in favor of alkyl radicals, and the high temperature mechanism supersedes the low temperature mechanism (Reactions 4 and 5). 

The result of this change in mechanism is that the major products at high temperatures are olefins and hydrogen peroxide and their secondary decomposition products, which of course include water. The relatively unstable alkyl hydroperoxide produced by the low temperature chain is replaced by the much more stable hydrogen peroxide. The result is that the secondary initiation, responsible for the cool flames, is replaced by a much slower initiation—the second-order decomposition of hydrogen peroxide (Reaction 6). 

When the alkyl hydroperoxide has been fully formed, only one half of the oxidizing power of the oxygen has been utilized. Alkyl hydroperoxides are therefore unstable, the stability being dependent upon the structure. Tertiary hydroperoxides are the most, and primary hydroperoxides the least, stable. The degradation reaction, which is essentially a second stage in the oxidation, may be either inter- or intramolecular the degradation may be either bi- or monomolecular. The rate of degradation is a function of the temperature and is easily subject to catalysis. 

Alkyl hydroperoxides, including ethyl hydroperoxide, cuminyl hydroperoxide, and tert-butyl hydroperoxide, are not used by V-BrPO to catalyze bromination reactions [29], These alkyl hydroperoxides have the thermodynamic driving force to oxidize bromide however, they are kinetically slow. Several examples of vanadium(V) alkyl peroxide complexes have been well characterized [63], including [V(v)0(OOR)(oxo-2-oxidophenyl) salicylidenaminato] (R = i-Bu, CMe2Ph), which has been used in the selective oxidation of olefins to epoxides. The synthesis of these compounds seems to require elevated temperatures, and their oxidation under catalytic conditions has not been reported. We have found that alkyl hydroperoxides do not coordinate to vanadate in aqueous solution at neutral pH, conditions under which dihydrogen peroxide readily coordinates to vanadate and vanadium( V) complexes (de la Rosa and Butler, unpublished observations). Thus, the lack of bromoperoxidase reactivity with the alkyl hydroperoxides may arise from slow binding of the alkyl hydroperoxides to V-BrPO. 

Characterization of Photooxidation Products Formed in Isooctane. Among the lodometncally titratable species formed in the photooxidation of isooctane, a substance, developed in appreciable amounts, was observed which was easily distinguishable from ordinary alkyl-hydroperoxides. This substance is, at room temperature, readily destroyed by addition of olefins to the reaction medium. This is a reaction which is typical and specific for peracids (10) and can be employed for quantitative assessment of peracids in the presence of alkylhydroperoxides. In contrast to alkylhydroperoxides, peracids react with olefins forming the corresponding epoxide, according to the equation... 

Careful investigation [25,26] in which the catalyst was removed by filtration at the reaction temperature and the filtrate was left to react further revealed, however, that the observed catalysis was by soluble chromium(VI) leached from the framework. In separate experiments it was shown that a few ppm of soluble chromium VI) could account for the observed catalysis. It was further shown that leaching was a result of reaction of framework chromium atoms with the alkyl hydroperoxide. 

A few C-H activations by gold are known. Thus, an initial example observed in nature is the existence of Au atoms in the core of a bacterial protein that oxidizes methane to methanol. In a reaction that tries to mimic this behavior, the formation of alkyl hydroperoxides was achieved with NaAuCL or [AuCUPPhs)] in acetonitrile solution with H2O2 at relatively low temperatures (75 °C). The formed hydroperoxides decomposed partially in the reaction mixture to give the corresponding alcohols and ketones, but were subsequently reduced to the alcohols by different methods. The oxidation of methane by gold - oxygenated complexes has also been studied theoretically. "... 

Although alkyl peroxides are usually intermediates and products autoxidation of alkanes, the reactions between alkanes and alkyl hydroperoxides need relatively high temperature or a metal complex as a catalyst. Sometimes other compounds can induce such reactions. Eor example, -pentane and n-hcxane are oxidized by tcrf-butyl hydroperoxide in benzene solution if (t-Bu)jAl is present in the system [44]. 

It has been proposed that the crucial step of the oxidation by the reagent O2 - H2O2 - VOj"- pyrazine-2-carboxylic acid is the very efficient generation of HO radicals [15]. These radicals abstract a hydrogen atom from the alkane, RH, to generate the alkyl radical, R. The latter reacts rapidly with an O2 molecule affording the peroxo radical, ROO. This radical is then transformed simultaneously into three products alkyl hydroperoxide, ketone, and alcohol. The relative content of the last two products is increased if the reaction temperature is higher. 

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