And decomposition of ROOH

Rate Constants of the Decomposition of ROOH (k-,) and R1C(OH)(OOR)R2 (k2) and Equilibrium Constants K of Hydroxyalkyl Peroxide Formation from ROOH and Ketone... 

The reaction of Co2+ with ROOH limits initiation. In the quasistationary regime, the rate constants of formation and decomposition of hydroperoxide given by the following equations ... 

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... 

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 resulting products, such as sulfenic acid or sulfur dioxide, are reactive and induce an acid-catalyzed breakdown of hydroperoxides. The important role of intermediate molecular sulfur has been reported [68-72]. Zinc (or other metal) forms a precipitate composed of ZnO and ZnS04. The decomposition of ROOH by dialkyl thiophosphates is an autocata-lytic process. The interaction of ROOH with zinc dialkyl thiophosphate gives rise to free radicals, due to which this reaction accelerates oxidation of hydrocarbons, excites CL during oxidation of ethylbenzene, and intensifies the consumption of acceptors, e.g., stable nitroxyl radicals [68], The induction period is often absent because of the rapid formation of intermediates, and the kinetics of decomposition is described by a simple bimolecular kinetic equation... 

Radical trapping. To allow for stabilizaton by this mechanism, another reaction (number 49) was included to allow easy abstraction of a hydrogen atom from an additive (QH) by a peroxy radical to form a hydroperoxide and a harmless adduct. With the same value of the rate constant as for energy transfer and for concentrations as low as 10 M, the photooxidation process was efficiently slowed. Figure 9 shows the linear dependence of the time to failure (5% oxidation) as the concentration of QH is altered. Note that the trap is consumed in the process and the apparent induction time is associated with its removal. The stabilization is less effective for higher intensity (and probably higher temperature) because the faster photo (or thermal) decomposition of ROOH continues the degradation process. 

Fe(II)(acac)2)x(R4NBr)y at the micro stages of radical-chain ethylbenzene oxidation by simplified scheme assuming quadratic termination of chain and equality to zero of rate of homolytic decomposition of ROOH [4,7,11,13,22],... 

If R is alkyl in sulphinic acid CC (obtained by transformation of CXCVIIb), S02 is readily eliminated and further oxidized to S03. This compound is according to2 9) the real catalyst of the decomposition of ROOH (Scheme 24). 

Transition metal ions (Co, in particular) catalyze decomposition of ROOH with formation of alkoxy and peroxy radicals, as shown in reactions (4.10) and (4.11) [11, 12]. We would like to note here that the decomposition of hydroperoxides and peracids on cobalt(II) and (III) is more complex than what are shown in Eqs. (4.10) and (4.11) and involves several steps and two equivalents of Co(II) [16, 17, 18a, 19]. 

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. 

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. 

This fact may be expected from the analysis of the scheme of catalyzed oxidation including participation of catalyst in chain initiation under catalyst interaction with ROOH, in chain propagation (Cat + RO ) and assuming the chain decomposition of ROOH. In this case the rate of reaction should be decreased, and [ROOH] should be increased with decrease of [Cat]p [8]. But the growth in is not accompanied the growth in... 

Autoacceleration takes place in the presence of air due to the decomposition of ROOH. ROOH decomposition is classically accelerated by heat, light and metal ions such as cobalt, iron, manganese and copper which can provide electrons to the process ... 

Make mathematic model for a process and calculate kinetic curves for aU given substances for [ROOH o = 0.1 M. Show that for given values of rate constants application of the quasistationery principle is justified. Determine the values of the constant concentrations of the radicals and the value of the constant rate of the decomposition of ROOH. 

In the presence of the initiator I or under the initiating action of light or radiation, chains are generated with the rate v,-, and the oxidation of RH proceeds as a chain process including reactions (1)—(6) (see Section 13.11). Usually v, is so high that the decomposition of ROOH to radicals insignificantly contribute to initiation, so that during the experiment v,- = const, and oxidation develops as a chain non-branched reaction. 

The scheme of oxidation in the presence of the inhibitor InH, which reacts with peroxyl radicals, and the inhibitor Q, which is an acceptor of alkyl radicals, includes several reactions involving radicals In- formed from the inhibitor, namely, reactions of radical decay (In- + In-, In- - RO-2) and reactions of chain propagation of the type In- -I- RH, In- + ROOH, and decomposition of In- to the molecule and radical capable of propagating the chain. This can be exemplified by the reaction... 

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