Hydroperoxide homolytic decomposition

Other Hydroperoxides. Several hydrotrioxides including alkyl hydrotrioxides, R—OOOH, have been reported (63,64). There is strong spectroscopic evidence that a-cumyl hydrotrioxide [82951-48-2] is produced in the low temperature ozonization of cumene. Homolytic decomposition of a-cumyl hydrotrioxide in cumene/acetone-hindered phenol resulted in cumyl alcohol as the only organic product (65). Based on the... 

Propagation. Propagation reactions (eqs. 5 and 6) can be repeated many times before termination by conversion of an alkyl or peroxy radical to a nonradical species (7). Homolytic decomposition of hydroperoxides produced by propagation reactions increases the rate of initiation by the production of radicals. 

Autoca.ta.Iysis. The oxidation rate at the start of aging is usually low and increases with time. Radicals, produced by the homolytic decomposition of hydroperoxides and peroxides (eqs. 2—4) accumulated during the propagation and termination steps, initiate new oxidative chain reactions, thereby increasing the oxidation rate. 

Thermally induced homolytic decomposition of peroxides and hydroperoxides to free radicals (eqs. 2—4) increases the rate of oxidation. Decomposition to nonradical species removes hydroperoxides as potential sources of oxidation initiators. Most peroxide decomposers are derived from divalent sulfur and trivalent phosphoms. 

Due to the unimolecular and bimolecular homolytic decomposition of hydroperoxide, such as... 

The rate constants of homolytic decomposition of the two hydroperoxides in butylacetamide media are given in Table 9.8. 

Along with homolytic decomposition, hydroperoxides are decomposed in an acetamide solution by the chain mechanism under the action of formed free radicals [33,35]. 

Another probable reaction of homolytic decomposition of ester hydroperoxide is the intramolecular interaction of the hydroperoxide group with the carbonyl group of ester with the formation of labile hydroxyperoxide succeeded the splitting of the weak O—O bond (see decomposition of hydroperoxides in oxidized ketones in Chapter 8). 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

The homolytic decomposition of hydroperoxides was proved to be catalyzed by Bronsted as well as Lewis acids (for example, BF3, A1C13, SbCls) [230]. Experimental data on acid catalysis of the homolytic decomposition of hydroperoxides are collected in Table 10.9. 

Acid Catalysis of the Homolytic Decomposition of Hydroperoxides (Experimental Data)... 

The formation of the H0S 02 radical was demonstrated by EPR spectroscopy [51]. The homolytic decomposition of hydroperoxide catalyzed by S02 underlies an oscillating pattern of the M-hcxylbenzcne oxidation inhibited by thiophene or BaS04 [48]. Radicals can also be... 

Autoxidation of alkanes may be carried out by metal catalysis.2,14 17 Although metal ions participate in all oxidation steps, their main role in autoxidation is not in their ability to generate free radicals directly by one-electron oxidation [Eq. (9.14)] but rather their activity to catalyze the homolytic decomposition of the intermediate hydroperoxide according to Eqs. (9.15) and (9.16). As a result of this decomposition, metal ions generate chain-initiating radicals. The overall reaction is given in Eq. (9.17) ... 

Another polymer which is easily peroxidized is polyacetaldehyde it has a polyacetalic structure and is characterized by the presence of some side hydroperoxide groups (1 to 4°/00) due to traces of peracetic acid when the polymer is prepared. The homolytic decomposition of these peroxide groups yields macroradicals to which methyl methacrylate could be grafted [73, 74). 

It is possible to divide the metals that react with alkyl hydroperoxides into four groups (i) metals that effect reaction (53), (ii) metals that participate in reaction (54), (iii) metals that are involved in both, and (iv) metals that effect heterolytic reactions of the hydroperoxide (see Section III.B). Other routes that do not involve changes in the oxidation state of the catalyst are also possible for the homolytic decomposition of alkyl hydroperoxides. The following scheme, for example, is presented speculatively ... 

The selectivity to epoxide is determined by the realtive rates of reaction of the catalyst-hydroperoxide complex with the olefin [Eq. (311)] in competition with its homolytic decomposition [Eq. (312)]. 

In the early stages of oxidation, the concentration of RO2H may be very low. Initiating events other than the homolytic decomposition of hydroperoxides are critical at this early stage. Once decomposition of RO2H has occurred, the rate of hydrogen abstraction from... 

In such processes the metal ion acts (in combination with ROOH) as an initiator rather than a catalyst. It is important to note that homolytic decomposition of alkyl hydroperoxides via one-electron transfer processes is generally a corn-... 

These reactions must be distinguished from homolytic decomposition by heat and LfV light that break the 0—0 bond by energy deposition, yielding alkoxyl and hydroxyl radicals ( OH). The 0—0 in organic hydroperoxides (BDE = 25-38 kCal/mole) begins decomposing at about 50°C and is completely decomposed at 160°C (297). 

In both reactions with CrAPO-5 and CrS-1 a substantial amoimt of 2-cyclohexen-l-ol was formed. The formation of 2-cyclohexen-l-ol can be explained by a competing homolytic decomposition of the peroxide catalyzed by a CrVi species. The 2-cyclohexen-l-one, on the other hand, can be formed from the intramolecular heterolytic decomposition of the cyclohexenyl hydroperoxide over CrVi or by an intermolecular oxidation of 2-cyclohexen-l-ol by CrVi followed by a reoxidation of the formed Criv by cyclohexenyl hydroperoxide (see Figure 1). 

Metals with low oxidation potentials and high Lewis acidity in their highest oxidation states are superior catalysts and show the following order of reactivity Mo > W > V > Ti. Metals which readily promote homolytic decomposition of alkyl hydroperoxides via one-electron pathways, e.g., Co, Mn, Fe, and Cu, are not effective. Certain main group elements, e.g., B and Sn, exhibit activity, albeit significantly lower than molybdenum. 

Radicalar polymerizations are very commonly used. In this case, the active site is an atom bearing a unpaired electron (free bond). The initiator is an organic material which can spontaneously split by homolytic breaking and in this way can produce radicals able to attack the monomer and thus to initiate the process. Actually, as initiators, one uses peroxides (R -O-O-R"), hydroperoxides (R -O-OH) (for instance hydrogen peroxide H202, or cumene hydroperoxide) and aliphatic azoics (R -N=R"). For each initiator, there is a domain of temperature often narrow (about 10 degrees) in which the homolytic decomposition occurs with an acceptable velocity. 

Thus, it has been proposed that the homolytic decomposition of hydroperoxides can be induced by sulfenic acid (12,13). There is evidence that various carboxylic acids can promote radical formation from hydroperoxides at elevated temperatures (II, 14). The intermediate thiosul-furous acid (Reaction 7) itself may function as the source of radicals, since sulfinic acid is known to initiate the radical polymerization of vinyl monomers at 20°C (15). Based on the AIBN-initiated oxidation of cumene, Koelewijn and Berger (16) proposed that pro-oxidant effects arise from catalysis of the radical decomposition of hydroperoxides by intermediate compound formation between the hydroperoxide and sulfoxide. However, under our conditions hydroperoxide was stable in the presence of sulfoxide alone. 

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