Hydroperoxide rate-limiting step

Pinene hydroperoxide (PHP) when compared with r-butyl hydroperoxide has been proposed as an excellent mechanistic probe in metal-catalysed oxidations. " If inter-molecular oxygen transfer from a peroxometal species to the substrate is rate limiting, the bulky PHP is unreactive, but for reaction of an oxometal species as the rate-limiting step, little or no difference is observed and only small differences in reactivity are observed when re-oxidation of the catalyst by ROOH to an active oxometal species is the rate-limiting step. 

Cyclohexane is obtained either by the hydrogenation of benzene, or from the naphtha fraction in small amounts. Its oxidation to the KA Oil dates back to 1893 and was first industrialized by DuPont in the early 1940s. Oxidation is catalyzed by Co or Mn organic salts (e.g., naphthenate), at between 150 and 180 °C and 10-20 atm. Indeed, this reaction is a two-step process (an oxidation and a deperoxidation step), and two variants are currently in use [2,3]. The oxidation step can be performed with or without a catalyst. The deperoxidation step always uses a catalyst (Co(II) or NaOH). The overall performance of both variants is almost identical, although the selectivity in the individual steps may be different. For example, in a first reactor, cyclohexane is oxidized to cyclohexylhydroperoxide the concentration of the latter is optimised by carrying out the oxidation in passivated reactors and in the absence of transition metal complexes, in order to avoid the decomposition of the hydroperoxide. In fact, the synthesis of the hydroperoxide is the rate-limiting step of the process, and, on the other hand, alcohol and ketone are more reactive than cyclohexane. The decomposition of the hydroperoxide is then carried out in a second reactor, in which the catalyst amount and reaction conditions are optimised, thus allowing the Ol/One ratio to be controlled. 

Many metals or ions of metals are catalysts of oxidative degradation of plastics. The conversion of peroxides (RiOO ) to hydroperoxides (RiOOH) via abstraction of hydrogen from an adjacent polymer molecule (R2H) is typically the rate-limiting step for the propagating chain reaction of oxidation... 

The last reaction is the rate-limiting step in the sequence of alkane transformations into cyclohexyl hydroperoxide. If we assume that the concentration of HO is quasi-stationary and concentrations of all iron complexes are in quasi-equilibrium and take into account conditions [Fe(PCA)] [FeCpjlo [Fe2(PCA)2] [FeCp2]o, we obtain the equation for the... 

Lipoxygenase catalyzes the oxygenation of l,4-cis,a 5-pentadiene units on long-chain fatty acids to l-hydroxy-2,4-trans,cis-pentadienes. The addition of molecular oxygen involves removal of the central bisallylic hydrogen in the rate-limiting step, with the probable involvement of one iron atom on the enzyme. A possible mechanism of action is that a hydroperoxide... 

Carbon-centered radicals generally react very rapidly with oxygen to generate peroxy radicals (eq. 2). The peroxy radicals can abstract hydrogen from a hydrocarbon molecule to yield a hydroperoxide and a new radical (eq. 3). This new radical can participate in reaction 2 and continue the chain. Reactions 2 and 3 are the propagation steps. Except under oxygen starved conditions, reaction 3 is rate limiting. 

In the initial period the oxidation of hydrocarbon RH proceeds as a chain reaction with one limiting step of chain propagation, namely reaction R02 + RH. The rate of the reaction is determined only by the activity and the concentration of peroxyl radicals. As soon as the oxidation products (hydroperoxide, alcohol, ketone, etc.) accumulate, the peroxyl radicals react with these products. As a result, the peroxyl radicals formed from RH (R02 ) are replaced by other free radicals. Thus, the oxidation of hydrocarbon in the presence of produced and oxidized intermediates is performed in co-oxidation with complex composition of free radicals propagating the chain [4], A few examples are given below. 

It is particularly remarkable that at small concentrations of the inhibitor (at less than 10 M), the reaction rate of the peroxyl radical with the phenol (step 11) decreases drastically and becomes the rate-limiting stage in the inhibition of ethylbenzene oxidation. As a consequence, the positive contribution of this step becomes substantial (see Figure 7.8). At the same time, when a sufficient amount of hydroperoxide is accumulated in the reaction system, the negative contribution of the reaction between the phenoxyl radical and the hydroperoxide (step 17) increases as if it is counteracting step (11). Such a picture is common for the reaction at 60 °C and 120 °C. 

Two principal classes of antioxidant are effective in thermal oxidation. Chainbreaking or primary antioxidants limit the rate of the chain propagation steps (Eqs. 3-2 and 3-3) by trapping carbon- or oxygen-centered free radicals. Hydroperoxide decomposing or secondary antioxidants prevent chain initiation by interfering with ROOH. Photoantioxidants protect plastics exposed to photo-oxidation. 

The first step, R6, converts the HALS initially added to the clearcoat, parent HALS, into inhibition cycle, R7 and R8, products. These reactions compete with R2 and R3 lowering the stationary radical concentration, which in turn lowers the hydroperoxide concentration and the photooxidation rate. The rate constants and radical concentrations are such that only a small fraction (—5%) of the HALS stabilizer is in the form of nitroxide. Although nitroxides are thermally stable, they are not pho-tolytically stable. Nitroxides absorb light, and excited-state nitroxides can abstract hydrogen atoms to initiate free-radical formation. These reactions have been discussed in detail. "Reactions R9 and RIO are important both for the nitroxide decay measurement of free radical formation and in limiting the ultimate effectiveness ofHALS.i° i5... 

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