Catalytic decomposition of ROOH

According to the Haber-Weiss scheme [11], in the framework of which we considered the catalytic decomposition of ROOH, all ROOH molecules decompose under the action of ions... 

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. 

Since the catalytic decomposition of ROOH with Co(II) is much faster than the thermal decomposition of ROOH, the latter reaction becomes almost irrelevant in the overall kinetics of oxidation. The steady-state concentrations of RO and ROOH are reached much faster, significantly reducing the observed induction period compared with uncatalyzed oxidation. The theoretical maximum rate in this case is described by Eq. (4-II) [11,13]. A comparison between Eqs. (4-1) and (4-II) shows that the theoretical maximum rate of uncatalyzed oxidation is four times higher than that of cobalt-catalyzed oxidation. Practically, the theoretical maximum rate of oxidation is rarely reached due to very long induction periods. 

According to the Haber—Weiss scheme, in the framework of which we considered the catalytic decomposition of ROOH, all ROOH molecules decompose under the action of ions only to free radicals, i.e., one-electron redox decomposition occurs. Both rates of catalytic decomposition of ROOH (from the consumption of ROOH) and the rate of generation of free radicals (from the consumption of the acceptor of free radicals or initiation rate of the chain process of RH oxidation or CH2=CHX polymerization) were measured for a series of systems (ROOH-catalyst-solvent). The comparison of these two processes showed that there are many systems, indeed, where the rate of ROOH decomposition and radical generation virtually coincide (v, = Vj). This is observed, for example, in the systems... 

As in the case with catalytic decomposition of hydrogen peroxide, radical generation by the reaction of metal ions with hydroperoxide consists of several steps. In an aqueous solution, first ROOH is substituted in the internal coordination sphere of the ion followed by the transfer of an electron from the ion to ROOH accompanied by the subsequent cleavage of hydroperoxide to RO and OH, for example,... 

In the case of cobalt ions, the inverse reaction of Co111 reduction with hydroperoxide occurs also rather rapidly (see Table 10.3). The efficiency of redox catalysis is especially pronounced if we compare the rates of thermal homolysis of hydroperoxide with the rates of its decomposition in the presence of ions, for example, cobalt decomposes 1,1-dimethylethyl hydroperoxide in a chlorobenzene solution with the rate constant kd = 3.6 x 1012exp(—138.0/ RT) = 9.0 x 10—13 s—1 (293 K). The catalytic decay of hydroperoxide with the concentration [Co2+] = 10 4M occurs with the effective rate constant Vff=VA[Co2+] = 2.2 x 10 6 s— thus, the specific decomposition rates differ by six orders of magnitude, and this difference can be increased by increasing the catalyst concentration. The kinetic difference between the homolysis of the O—O bond and redox decomposition of ROOH is reasoned by the... 

It was shown in the previous section that hydrocarbon oxidation catalyzed by cobalt salts occurs under the quasistationary conditions with the rate proportional to the square of the hydrocarbon concentration and independent of the catalyst (Equation [10.9]). This limit with respect to the rate is caused by the fact that at the fast catalytic decomposition of the formed hydroperoxide, the process is limited by the reaction of R02 with RH. The introduction of the bromide ions into the system makes it possible to surmount this limit because these ions create a new additional route of hydrocarbon oxidation. In the reactions with ROOH and R02 the Co2+ ions are oxidized into Co3+, which in the reaction with ROOH are reduced to Co2+ and do not participate in initiation. 

The reverse emulsion stabilized by sodium dodecylsulfate (SDS, R0S03 Na+) retards the autoxidation of dodecane [24] and ethylbenzene [21,26,27]. The basis for this influence lies in the catalytic decomposition of hydroperoxides via the heterolytic mechanism. The decay of hydroperoxides under the action of SDS reverse micelles produces olefins with a yield of 24% (T=413 K, 0.02mol L 1 SDS, dodecane, [ROOH]0 = 0.08 mol L 1) [27], The thermal decay gives olefins in negligible amounts. The decay of hydroperoxides apparently occurs in the ionic layer of a micelle. Probably, it proceeds via the reaction of nucleophilic substitution in the polar layer of a micelle. 

Once aryl phosphite has oxidized into phosphate followed by the catalytic decomposition of hydroperoxide [6,10,19,20], The catalyst is obviously stable, since additional hydroperoxide introduced into the system immediately breaks down [10]. Repeatedly, ROOH decomposes via the acid-catalyzed reaction with the products of phosphate hydrolysis acting as catalysts [6,10],... 

The oxidation of sulfides is a complex process involving a number of conversions [32,46], Disulfides are oxidized by hydroperoxide via the intermediate thiosulfinate RSSOR, which is very reactive to ROOH [32,52-54], The interaction of ROOH with phenolsulfoxides also gives rise to intermediate catalytic compounds, as a result of which the reaction proceeds as an autocatalytic process [46,55], The rate of the catalytic decomposition of R OOH is described by one of the following equations ... 

These complexes (205) were found to be good models for the reactive intermediates involved in the catalytic decomposition of alkyl hydroperoxides (Haber Weiss mechanism), and in the catalytic hydroxylation of hydrocarbons by ROOH. 

While the detailed mechanism of H2O2 decomposition by 1 is not known yet, we are interested if related organic hydroperoxides can be decomposed. The surprising results, shown in Figure 4, is that 1, which contains the Mn(II)Mn(III)3 core, does not decompose significantly either cumene hydroperoxide (CHP) or t-butyl hydroperoxide (TBHP). In contrast, Mn(II) and Mn(III) acetates decompose CHP. The lack of decomposition of ROOH by 1 combined with its catalytic activity reconmiend Mn4 complexes as candidates for oxidation catalysts. 

We suggest the original method for evaluation of catalytic activity of complexes formed in situ at the beginning of reaction and in developed process, at elementary stages of oxidation process [33, 90-93] by simplified scheme assuming quadratic termination of chain and equality to zero of rate of homolytic decomposition of ROOH. In the If amework of radical-chain mechanism the chain termination rate in this case will be Eq. (1) ... 

The selective oxidation of hydrocarbons into hydroperoxides, primary products of oxidation is the most difficult problem because of the high catalytic activity of the majority of applied catalysts in ROOH decomposition. At the same time, the problem of selective oxidation of alkylarens (ethylbenzene and cumene) with molecular 02 in ROOH, is of current importance from the practical point of view in connection with ROOH use in large-tonnage productions such as production of propylene oxide and styrene (a-phenylethylhydroperoxide, PEH), or phenol and acetone (cumyl hydroperoxide) [1],... 

These experiments were repeated in the presence of CaC03 to see if the base would affect the activity of the sulfur compounds as peroxide decomposers. The initial reaction of sulfenic acid with the hydroperoxide was slowed, as shown in Figure 2, but ultimately consumed two moles of ROOH per mole of RSOH. The subsequent catalytic decomposition was almost completely stopped with excess of base consistent with neutralization of an acid catalyst, presumed to be the sulfonic acid formed by oxidation of the sulfenic acid ... 

The product study272) and the kinetic study261) form the experimental basis for the discussion of complex transformations of thiobisphenolic antioxidants. Their retardation effect in polypropylene after the induction period and the mechanism of formation of intermediates or products catalytically active in ROOH decomposition indicate the reactivity of both different centres in molecule and provide partial data for the discussion of intramolecular synergism of thiobisphenols5 57,258,26 ). 

The Et4>rBr addition into the reaction medium in concentration which is one order less (6.00 10 M) than peroxides concentration ([ROOH]o=5.0010- M, [ROOR]o = 5.2410 M) leads to significant increasing of reaction rate as compared with peroxides thermal decomposition (343 K). The salt concentration during the reaetion proceeding and in the end of reaction remains constant. This fact shows the catalytic character of tetraethylammonium bromide aetion upon a-oxycyelohexylperoxides decomposition. 

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