Inhibited oxidation rate, hydroperoxide effect

The catalyst is preliminarily oxidized to the state of the highest valence (vanadium to V5+ molybdenum to Mo6+). Only the complex of hydroperoxide with the metal in its highest valence state is catalytically active. Alcohol formed upon epoxidation is complexed with the catalyst. As a result, competitive inhibition appears, and the effective reaction rate constant, i.e., v/[olefin][ROOH], decreases in the course of the process due to the accumulation of alcohol. Water, which acts by the same mechanism, is still more efficient inhibitor. Several hypothetical variants were proposed for the detailed mechanism of epoxidation. 

We have seen earlier that in the presence of an antioxidant the rate of hydroperoxide formation is related to the ratio of lipid concentration to that of the antioxidant concentration, which is dependent on the rate of inhibition reaction (4). The effectiveness of an antioxidant is generally based on the balance between the inhibition rate (k ) of reaction (4) and the transfer reactions ( ), (9) and (13). Therefore, the effect of antioxidants on hydroperoxide decomposition reactions (10) and (11) is an important property that needs to be evaluated. However, most studies of antioxidant actions measure initial events of lipid oxidation based on oxygen absorption, hydroperoxide formation, and peroxide values (Chapters 5 and 7). Very few studies have measured the effect of antioxidants on decomposition products of hydroperoxides, such as aldehydes and carbonyl compounds. Yet these volatile decomposition aldehydes are most relevant to the development of rancidity and to the ultimate quality and stability of food lipids. 

Work in this laboratory has shown also that the Ru(poip)(0)2 complexes (porp = TMP, TDCPP, and TDCPP-Clg) are practically inactive for thermal 02-oxygenation of saturated hydrocarbons . Some activity data for 0.2 mM Ru solutions in benzene under air at 25°C for optimum substrates such as adamantane and triphenylmethane at 6 mM did show selective formation of 1-adamantol and trityl alcohol, respectively, but with turnover numbers of only -0.2 per day the maximum turnover realized was -15 after 40 days for the TDCPP system Nevertheless, this was a non-radical catalytic processes there was < 10% decomposition of the Ru(TDCPP)(0)2, and a genuine O-atom transfer process was envisaged . Quite remarkably (and as mentioned briefly in Section 3.3), at the much lower concentration of 0.05 mM, Ru(TDCPP-Clg)(0)2 in neat cyclooctene gave effective oxidation. For example, at 90°C under 1 atm O2, an essentially linear oxidation rate over 55 h gave about -70% conversion of the olefin with - 80% selectivity to the epoxide however, the system was completely bleached after - 20 h and, as the activity was completely inhibited by addition of the radical inhibitor BHT, the catalysis is operating by a radical process, but in any case the conversion corresponds to a turnover of 110,000 As in related Fe(porp) systems (Section 3.3, ref. 121), the Ru(porp) species are considered to be very effective catalysts for the decomposition of hydroperoxides (eqs. 

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. 

For initiated oxidation, the inhibitory criterion could be defined as the ratio v0/v or (v0/ v — v/v0), where v0 and v are the rates of initiated oxidation in the absence and presence of the fixed concentration of an inhibitor, respectively. Another criterion could be defined as the ratio of the inhibition coefficient of the combined action of a few antioxidants / to the sum of the inhibition coefficients of individual antioxidants when the conditions of oxidation are fixed (fx = IfiXi where f, and x, are the inhibition coefficient and molar fraction of z th antioxidant terminating the chain). It should, however, be noted that synergism during initiated oxidation seldom takes place and is typical of autoxidation, where the main source of radicals is formed hydroperoxide. It is virtually impossible to measure the initial rate in the presence of inhibitors in such experiments. Hence, inhibitory effects of individual inhibitors and their mixtures are usually evaluated from the duration of retardation (induction period), which equals the span of time elapsed from the onset of experiment to the moment of consumption of a certain amount of oxygen or attainment of a certain, well-measurable rate of oxidation. Then three aforementioned cases of autoxidation response to inhibitors can be described by the following inequalities (r is the induction period of a mixture of antioxidants). 

In the development of effective catalytic oxidation systems, there is a qualitative correlation between the desirability of the net or terminal oxidant, (OX in equation 1 and DO in equation 2) and the complexity of its chemistry and the difficulty of its use. The desirability of an oxidant is inversely proportional to its cost and directly proportional to the selectivity, rate, and stability of the associated oxidation reaction. The weight % of active oxygen, ease of deployment, and environmental friendliness of the oxidant are also key issues. Pertinent data for representative oxidants are summarized in Table I (4). The most desirable oxidant, in principle, but the one with the most complex chemistry, is O2. The radical chain or autoxidation chemistry inherent in 02-based organic oxidations, whether it is mediated by redox active transition metal ions, nonmetal species, metal oxide surfaces, or other species, is fascinatingly complex and represents nearly a field unto itself (7,75). Although initiation, termination, hydroperoxide breakdown, concentration dependent inhibition... 

To give a specific example, the advantages of styrene as a substrate for peroxyl radical trapping antioxidants are well known" (i) Its rate constant, kp, for chain propagation is comparatively large (41 M s at 30 °C) so that oxidation occurs at a measurable, suppressed rate during the inhibition period and the inhibition relationship (equation 14) is applicable (ii) styrene contains no easily abstractable H-atom so it forms a polyper-oxyl radical instead of a hydroperoxide, so that the reverse reaction (equation 21), which complicates kinetic studies with many substrates, is avoided and (iii) the chain transfer reaction (pro-oxidant effect, equation 20) is not important with styrene since the mechanism is one involving radical addition of peroxyls to styrene. 

Antioxidants behave differently when oxidation is initiated with metals as compared to azo radical initiators. Thus, BHT was a more effective antioxidant in a phosphatidylcholine liposome oxidized with copper (II), in the presence of t-butyl-hydroperoxide, than in the same liposome oxidized with AAPH or with AMVN (Table 10.9). The inhibition by BHT was better with the water-soluble initiator AAPH than the oil-soluble initiator AMVN. This result suggests that radicals produced in the water phase by copper (II) were trapped by BHT in the same way as the radicals produced by AAPH. The rates of oxygen absorption in this liposome system were increased three fold with 100 M copper (II) compared to four fold with 2 mM AAPH and about five fold wi 2 mM AMVN. Such results must be interpreted with caution, because at sufficiently high concentrations metals react stoichiometrically and promote chain termination, a condition which is not observed at low and trace concentrations in... 

The macroradical formed to reaction (16) interacts with oxygen, leading to a normal chain reaction and the formation of hydroperoxides. Accordii to the data of [85], the quantum yield of carbonyl groups in the photolysis of polyethylene is no greater than 0.1. This means that the kinetic chains in photooxidation are not very long. In view of this, termination of the chains by means of antioxidants is hindered in practice, since in the case of short chains and a high rate of initiation, effective inhibition is impossible. The formation of unsaturated compoimds during photolysis [reactions (15) and (16)] facilitates further oxidation of the polyolefin. 

---