Decomposition hydroperoxide

The early work of Kennerly and Patterson [16] on catalytic decomposition of hydroperoxides by sulphur-containing compounds formed the basis of the preventive (P) mechanism that complements the chain breaking (CB) process. Preventive antioxidants (sometimes referred to as secondary antioxidants), however, interrupt the second oxidative cycle by preventing or inhibiting the generation of free radicals [17]. The most important preventive mechanism is the nonradical hydroperoxide decomposition, PD. Phosphite esters and sulphur-containing compounds, e.g., AO 13-18, Table la are the most important classes of peroxide decomposers. 

With some systems, the hydroperoxide is reduced to hydroperoxy radical by the metal ion in its higher oxidation state (Scheme 3.39). Thus, it is possible to set up a catalytic cycle for hydroperoxide decomposition. 

Increase the oxidation rate of polymers, e.g. metal ions which increase the hydroperoxide decomposition rate. Photodegradation and thermal degradation are enhanced by transition metal ion containing pro-oxidants, such as iron dithiocarbamate (as opposed to nickel dithiocarba-mate, which acts as a photo-antioxidant). 

The mechanoradical produced will react with the small amount of oxygen to form hydroperoxides these are subsequently utilised as radical generators in the second stage. The resulting hydroxyl radical (from hydroperoxide decomposition) abstracts a hydrogen from the substrate to form macroradical which, in turn, will react with more of the thiyl radical to form more bound antioxidant. The polymer bound antioxidant made in this way is very much more resistant to solvent leaching and volatilisation when compared to commercial additives (13). see Figure 2. 

Consequently conventional antioxidant mechanisms must be expected to protect against photo-oxidation. Thus hydroperoxide decomposition to inert molecular products will reduce the rate of photoinitiation and scavenging of any of the free radical species will be beneficial, although the effectiveness of conventional antioxidants in photo-oxidations is limited by their own stability and the photo-sensitizing propensity of their products (3,). 

Unsubstituted hydroxylamines ( NOH) which may be formed in reactions 6 and 8 are powerful antioxidants, both by peroxyl radical scavenging and by hydroperoxide decompositions. 

Secondly, the interaction of hindered amines with hydroperoxides was examined. At room temperature, using different monofunctional model hydroperoxides, a direct hydroperoxide decomposition by TMP derivatives was not seen. On the other hand, a marked inhibitory effect of certain hindered amines on the formation of hydroperoxides in the induced photooxidation of hydrocarbons was observed. Additional spectroscopic and analytical evidence is given for complex formation between TMP derivatives and tert.-butyl hydroperoxide. From these results, a possible mechanism for the reaction between hindered amines and the oxidizing species was proposed. 

Decomposition of Peroxides by Various Stabilizers. The efficiency of tert-butyl hydroperoxide decomposition in tert-butyl alcohol by various additives was determined (Table 9). Under the conditions of these experiments, the phenolic antioxidants and dilauryl thiodipropionate had little or, often, no effect on the hydroperoxide decomposition. The three zinc salts effectively inhibited peroxide decomposition. This effect might briefly inhibit the onset of substrate oxidation under weathering-test conditions, but the peroxide would decompose whenever its concentration reached a sufficient level to permit significant light... 

This sequence correlates quite well with the reactivity of these ions in the Haber-Weiss cycle of hydroperoxide decomposition. 

The set of the rate constants k determined for experimental runs of Figure 15 and their comparison with the rate constants of hydroperoxide decomposition determined by other methods may be seen in Table 3. When we take into account that PPs of different origin were examined, the agreement seems quite satisfactory. This agreement is valid for faster decomposing peroxides, which are the species determining the resulting rate of oxidation [49]. 

The effect of oxygen concentration is thus included in a constant m, which modifies both the resulting maximum of the chemiluminescence intensity and the apparent rate constant k of hydroperoxide decomposition. 

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

This test is used for both in vitro and in vivo determinations. It involves reacting thiobarbituric acid (TBA) with malondialdehyde (MDA), produced by lipid hydroperoxide decomposition, to form a red chromophore with peak absorbance at 532 nm (Fig. 10.1). The TBARS reaction is not specific. Many other substances, including other alkanals, proteins, sucrose, or urea, may react with TBA to form colored species that can interfere with this assay. 

Bimolecular reaction of hydroperoxide decomposition to free radicals was discovered ROOH + ROOH —> R0 + H20 + R00 L. Bateman, H. Hughes, and A. Moris [67]... 

A very serious problem was to clear up the formation of hydroperoxides as the primary product of the oxidation of a linear aliphatic hydrocarbon. Paraffins can be oxidized by dioxygen at an elevated temperature (more than 400 K). In addition, the formed secondary hydroperoxides are easily decomposed. As a result, the products of hydroperoxide decomposition are formed at low conversion of hydrocarbon. The question of the role of hydroperoxide among the products of hydrocarbon oxidation has been specially studied on the basis of decane oxidation [82]. The kinetics of the formation of hydroperoxide and other products of oxidation in oxidized decane at 413 K was studied. In addition, the kinetics of hydroperoxide decomposition in the oxidized decane was also studied. The comparison of the rates of hydroperoxide decomposition and formation other products (alcohol, ketones, and acids) proved that practically all these products were formed due to hydroperoxide decomposition. Small amounts of alcohols and ketones were found to be formed in parallel with ROOH. Their formation was explained on the basis of the disproportionation of peroxide radicals in parallel with the reaction R02 + RH. 

PRODUCTS OF HYDROPEROXIDE DECOMPOSITION 1.4.1 Hydroperoxides as the Intermediates of Hydrocarbon Oxidation... 

The bimolecular reaction of hydroperoxide decomposition to free radicals... 

Different chain mechanisms of hydroperoxide decomposition are known with the participation of alkyl, alkoxyl, and peroxyl radicals [9]. 

The chain decomposition of hydroperoxides was proved and studied for hydroperoxides produced by the oxidation of polyesters such as dicaprilate of diethylene glycol and tetra-valerate of erythritol [140], The retarding action of phenolic antioxidant on the decay of hydroperoxides was observed. The initial rate of hydroperoxide decomposition was found to depend on the hydroperoxide concentration in accordance with the kinetic equation typical for the induced chain decomposition. 

The kinetics of hydroperoxide decomposition obeys the following equation ... 

If radicals are produced in the reactions of unimolecular hydroperoxide decomposition and the reaction of ROOH with hydrocarbon whose concentration at the initial stages of oxidation is virtually constant, the production of radicals from ROOH can be regarded as a pseudo-monomolecular process occurring at the rate V = [ROOH] = + iRH[RH]). The... 

In addition to hydroperoxide decomposition by the reaction of the first-order bimolecular decomposition was observed in cyclohexanol at [H202] > 1 M [60], The bimolecular radical generation occurs with the rate constant k 6.8 x 108 exp(—121.7/R7) L mol-1 s-1. The following mechanism was suggested as the most probable. 

Ketones play an important role in the decomposition of peroxides to form radicals in alcohols undergoing oxidation. The formed hydroxyhydroperoxide decomposes to form radicals more rapidly than hydrogen peroxide. With an increase in the ketone concentration, there is an increase in the proportion of peroxide in the form of hydroxyhydroperoxide, with the corresponding increase in the rate of formation of radicals. This was proved by the acceptor radical method in the cyclohexanol-cyclohexanone-hydrogen peroxide system [59], The equilibrium constant was found to be K — 0.10 L mol 1 (373 K), 0.11 L mol 1 (383 K), and 0.12 L mol 1 (393 K). The rate constant of free radical generation results in the formation of cyclohexylhydroxy hydroperoxide decomposition and was found to be ki = 2.2 x 104 exp(—67.8/7 7) s 1 [59]. 

Effective Rate Constants of Hydroperoxide Decomposition in Solution of Subsequent Either [68]... 

Apparently, hydrogen bonding is a preliminary stage of free radical generation through hydroperoxide decomposition. This is in agreement with the kinetic equation for hydroperoxides decomposition to free radicals [33] ... 

Quite another equation was established for the initiation rate for hydroperoxide decomposition in dimethylacetamide [34] ... 

The accumulation of hydroperoxide accelerates the ester oxidation. As in hydrocarbon oxidation, this acceleration is the result of hydroperoxide decomposition into free radicals. The most probable is the bimolecular reaction of hydroperoxide with the weakest C—H bond of saturated ester (see Chapter 4). 

Rate Constants of Ester Hydroperoxides Decomposition into Free Radicals... 

In real systems (hydrocarbon-02-catalyst), various oxidation products, such as alcohols, aldehydes, ketones, bifunctional compounds, are formed in the course of oxidation. Many of them readily react with ion-oxidants in oxidative reactions. Therefore, radicals are generated via several routes in the developed oxidative process, and the ratio of rates of these processes changes with the development of the process [5], The products of hydrocarbon oxidation interact with the catalyst and change the ligand sphere around the transition metal ion. This phenomenon was studied for the decomposition of sec-decyl hydroperoxide to free radicals catalyzed by cupric stearate in the presence of alcohol, ketone, and carbon acid [70-74], The addition of all these compounds was found to lower the effective rate constant of catalytic hydroperoxide decomposition. The experimental data are in agreement with the following scheme of the parallel equilibrium reactions with the formation of Cu-hydroperoxide complexes with a lower activity. 

The Yield of Free Radical Generation (e) in Reactions of Hydroperoxide Decomposition Catalyzed by Transition Metal Complexes... 

The increase in the amount of catalyst introduced in oxidized cumene (353 K) increases the oxidation rate, decreases the amount of the formed hydroperoxide, and increases the yield of the products of hydroperoxide decomposition methylphenyl ethanol and acetophenone. Similar mechanism was proposed for catalysis by copper phthalocyanine in cumene oxidation [254],... 

Depending on the electron affinity of the catalyst, one of these two routes predominates. The dependence of the hydroperoxide decomposition rate on [ROOH] is in agreement with the conception of preliminary equilibrium sorption of hydroperoxide on the catalyst surface (Me2PhCOOH, AgO, 16m2 L 343 K) [263]). The equilibrium constant was estimated to be K 1 mol L and effective rate constant of described ROOH decomposition is /cis = 70s I[263]. 

The [S]cr value depends on the nature of the catalyst, its surface area per unit of weight, the ratio of the rates of hydroperoxide decomposition into free radicals and molecular products, hydrocarbon and dioxygen concentrations, the method of catalyst preparation, and the chemical treatment of the surface. 

The effect of jumping of the maximal hydroperoxide concentration after the introduction of hydrogen peroxide is caused by the following processes. The cumyl hydroperoxide formed during the cumene oxidation is hydrolyzed slowly to produce phenol. The concentration of phenol increases in time and phenol retards the oxidation. The concentration of hydroperoxide achieves its maximum when the rate of cumene oxidation inhibited by phenol becomes equal to the rate of hydroperoxide decomposition. The lower the rate of oxidation the higher the phenol concentration. Hydrogen peroxide efficiently oxidizes phenol, which was shown in special experiments [8]. Therefore, the introduction of hydrogen peroxide accelerates cumene oxidation and increases the yield of hydroperoxide. 

The kinetic study of cumyl hydroperoxide decomposition in emulsion showed that (a) hydroperoxide decomposes in emulsion by 2.5 times more rapidly than in cumene (368 K, [RH] [H20] = 2 3 (v/v), 0.1 N Na2C03) and (b) the yield of radicals from the cage in emulsion is higher and close to unity [19]. The activation energy of ROOH decomposition in cumene is Ed = 105 kJ mol-1 and in emulsion it is lower and equals Ed 74 kJ mol 1 [17]. 

The reverse micelles stabilized by SDS retard the autoxidation of ethylbenzene [27]. It was proved that the SDS micelles catalyze hydroperoxide decomposition without the formation of free radicals. The introduction of cyclohexanol and cyclohexanone in the system decreases the rate of hydroperoxide decay (ethylbenzene, 363 K, [SDS] = 10 3mol L [cyclohexanol] =0.03 mol L-1, and [cyclohexanone] = 0.01 mol L 1 [27]). Such an effect proves that the decay of MePhCHOOH proceeds in the layer of polar molecules surrounding the micelle. The addition of alcohol or ketone lowers the hydroperoxide concentration in such a layer and, therefore, retards hydroperoxide decomposition. The surfactant AOT apparently creates such a layer around water moleculesthat is very thick and creates difficulties for the penetration of hydroperoxide molecules close to polar water. The phenomenology of micellar catalysis is close to that of heterogeneous catalysis and inhibition (see Chapters 10 and 20). 

The formation of free radicals and alcohol (in addition to the products of hydroperoxide heterolysis) implies that the catalytic decomposition of hydroperoxide occurs both hetero-lytically and homolytically. The mechanism of homolytic hydroperoxide decomposition was proposed by Van Tilborg and Smael [48]. 

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