Phenols oxidation rate constants

Oxidation of the same set of substituted phenols listed in Table 17.3 was also undertaken employing the organic oxidant, cumylperoxyl radical (Cum ). Unlike the metal based oxidation. Fig. 17.11 shows that the phenol oxidation rate constants do not correlate well with the substrate army better than with substrate BDE. This behavior implicates a more synchronous HAT mechanism for the organic radical oxidant. 

That is, hox(AtXH)= - / (ArX ). For this type of reaction, E (ArX ) is positive. Hence, the more positive this value, the more difficult it is to oxidize the compound. For many phenols and anilines, polarographic half-wave potentials, Zs1/2(ArX"), determined at pH values where the compound is present in its neutral form, are available. These values should reasonably parallel the oxidation potentials of the compounds, and therefore can also be used to relate oxidation rate constants ... 

Based on their chemical structure, the organic chemicals were divided into a number of categories alkanes, alkenes, amines, aromatic hydrocarbons, benzenes, carboxylic acids, halides, phenols, and sulfonic acid. Linear regression analysis has been applied using the method of least-squares fit. Each correlation required at least three datapoints, and the parameters chosen were important to ensure comparable experimental conditions. Most vital parameters in normalizing oxidation rate constants for QSAR analysis are the overall liquid volume used in the treatment system, the source of UV light, reactor type, specific data on substrate concentration, temperature, and pH of the solution during the experiment. 

Correlation of oxidation rate constants of halogenated phenols with log P. TABLE 7.7... 

Oxidation rate constant k, for gas-phase second order rate constants, Icqjj for reaction with OH radical, k os with NO3 radical and ko3 with O3 or as indicated, data at other temperatures see reference k < 4 X 10 M h for singlet oxygen, 1.1 x 10 M fh for peroxy radical at 25°C (Mabey et al. 1982) photooxidation = 77-3840 h in water, based on reported reaction rate constants for ROj radicals with the phenol class (Mill Mabey 1985 selected, Howard et al. 1991) photooxidation ty, = 8.0 h in air, based on reaction with photochemically produced hydroxyl radical in air (GEMS 1986 selected, Howard 1989) koH = 71.5 X 10 cm molecule- s- at 296 2 K (Atkinson 1989)... 

Temperature of the process (T) time of contact (t) ethanol concentration in condensate (C jqij) phenol concentration in the condensate (CpjjQjj) degree of oxidation of organic substances and DPS oxidation rate constant (k). 

For the case of CF , the rate constants have been measured at pH 1 for a series of -substituted phenols, the value for phenol being 2.5 x 10 M s . The rate constants increase with increasing electron-donating power of the substituent. A plot of the rate constants vi the Hammett a values yields p = — 1.5, indicating an electron transfer mechanism for the formation of the phenoxyl radicals . The weaker oxidant Br2 reacts with phenol more slowly, k = 6x 10 M s . However, upon increasing the reducing power by going from phenol to phenolate, the rate constant increases to ca 4 X 10 s . (SCN)2 and 12 are even weaker oxidants than Br2 and... 

Phenolic degradation, thermal and thermo-oxidative, 418-425 Phenolic-epoxy networks, 413 Phenolic monomers, second-order reaction rate constants of formaldehyde with, 403... 

Assuming that ks = k9 = 3x 108Lmol 1s 1 (the key value), the diversity of the rate constants of the reactions of phenols and phenoxyl radicals (7, —7, 10, 11, and 12) can be reduced to only two parameters, k7 and T. This allows one to get the universal formulae for the oxidation rate v, into which these parameters enter as functions of k2, k7, T, and ambient conditions (Table 14.7). When considering this table, it should be taken into account that mechanism VII is possible only for 2,4,6-tris-alkylphenols, while mechanism IX holds only for o- and p-alkoxyphenols. 

Mahoney and DaRooge [57] studied the kinetics of oxidation of 9,10-dihydroanthracene at 333 K in the presence of some phenols and estimated the ratio of rate constants ks/k2. From these ratios, the rate constants ks were calculated for several para-substituent phenols 4-YC6H4OH using 2 = 850L mol-1 s 1. 

It is seen that the rate constant ks is lower for compounds with electron-accepting substituents than with electron-donating substituents, which implies a dependence of the rate of Ar20 and R02 recombination on the electron density at the para- and ort/zo-positions of the benzene ring of the phenoxyl radical. The activation energies of this reaction vary from -33 to 10 kJ mol-1 however, the concurrent variation in the pre-exponential factor from 103 to 1010 L mol-1 s-1 causes a strong compensatory effect. It can also be seen that phenoxyl radicals readily react with peroxyl radicals k= 10s—109 L mol-1 s-1), whereas the disproportionation of peroxyl radicals is sufficiently slower (see Chapter 2). Hence, during the oxidation of hydrocarbons in the presence of phenols when k7[ArOH] > /c2[RH], the recombination reaction of ArO with R02 is always faster than the reaction of disproportionation of peroxyl radicals. 

Reactions of phenoxyl and aminyl radicals with RH and ROOH are chain propagation steps in oxidation inhibited by phenols and amines (see Chapter 14). Both reactions become important when their rates are close to the initiation rate (see Chapter 14). Mahoney and DaRooge [57] studied the oxidation of 9,10-dihydroanthracene inhibited by different phenols. He went on to estimate the values of rate constants ratio of the reaction of ArO with RH and the reaction In + In (reactions (9) and (10), see Chapter 14) by the kinetic study. The values of kw for the reaction... 

Of these reactions, the reaction of the peroxyl radical with phosphite is the slowest. The rate constant of this reaction ranges from 102 to 103 L mol 1 s 1 which is two to three orders of magnitude lower than the rate constant of similar reactions with phenols and aromatic amines. Namely, this reaction limits chain propagation in the oxidation of phosphites. Therefore, the chain oxidation of trialkyl phosphites involves chain propagation reactions with the participation of both peroxyl and phosphoranylperoxyl radicals ... 

The values of calculated activation energies and rate constants of the >NO reactions with chosen phenols and amines are given in Table 18.6. The hydroxylamine formed by the reaction of the nitroxyl radical with InH reacts with peroxyl radicals very rapidly (see Table 18.7). So, two reactions of chain termination occur in oxidized RH in the presence of >NO and InH and chain termination includes the following cycles of reactions. 

FIGURE 19.2 The correlation of rate constants of various free radical reactions with molecular mobility of nitroxyl radical in the polymer matrix of different polymers with addition of plastificator I in IPP, II in preliminary oxidized IPP, III in PE, and IV in PS. Line 1 for the reaction of 2,6-bis(l,l-dimethy-lethyl)phenoxyl radical with hydroperoxide groups at T — 295 K line 2 for the reaction of 2,2,6, 6-tetramethyl-4-bcnzoyloxypiperidinc-/V-oxyl with 1-naphthol at T = 333 K line 3 for the reaction of 2,2,6,6-tetramethyl-4-benzoyloxypiperidine-iV-oxyl with 2,6-bis(l,l-dimethylethyl)phenol at T = 333 K line 4 for the same reaction at 7 — 303 K line 5 for the same reaction at T = 313 K and line 6 for the same reaction at T — 323 K [18]. 

The mechanism of antioxidant action on the oxidation of carbon-chain polymers is practically the same as that of hydrocarbon oxidation (see Chapters 14 and 15 and monographs [29 10]). The peculiarities lie in the specificity of diffusion and the cage effect in polymers. As described earlier, the reaction of peroxyl radicals with phenol occurs more slowly in the polymer matrix than in the liquid phase. This is due to the influence of the polymeric rigid cage on a bimolecular reaction (see earlier). The values of rate constants of macromolecular peroxyl radicals with phenols are collected in Table 19.7. 

The following correlation was established for IPP oxidation at T= 473 K inhibited by phenols between the induction period t and the rate constant of the reaction of the same phenols with cumylperoxyl radicals k7 in cumene at T 333 K [48] ... 

Now, we will consider the major reactions of peroxynitrite with biomolecules. It was found that peroxynitrite reacts with many biomolecules belonging to various chemical classes, with the bimolecular rate constants from 10-3 to 10s 1 mol 1 s 1 (Table 21.2). Reactions of peroxynitrite with phenols were studied most thoroughly due to the important role of peroxynitrite in the in vivo nitration and oxidation of free tyrosine and tyrosine residues in proteins. In 1992, Beckman et al. [112] have showed that peroxynitrite efficiently nitrates 4-hydroxyphenylacetate at pH 7.5. van der Vliet et al. [113] found that the reactions of peroxynitrite with tyrosine and phenylalanine resulted in the formation of both hydroxylated and nitrated products. In authors opinion the formation of these products was mediated by N02 and HO radicals. Studying peroxynitrite reactions with phenol, tyrosine, and salicylate, Ramezanian et al. [114] showed that these reactions are of first-order in peroxynitrite and zero-order in phenolic compounds. These authors supposed that there should be two different intermediates responsible for the nitration and hydroxylation of phenols but rejected the most probable proposal that these intermediates should be NO2 and HO. ... 

Several compounds can be oxidized by peroxidases by a free radical mechanism. Among various substrates of peroxidases, L-tyrosine attracts a great interest as an important phenolic compound containing at 100 200 pmol 1 1 in plasma and cells, which can be involved in lipid and protein oxidation. In 1980, Ralston and Dunford [187] have shown that HRP Compound II oxidizes L-tyrosine and 3,5-diiodo-L-tyrosine with pH-dependent reaction rates. Ohtaki et al. [188] measured the rate constants for the reactions of hog thyroid peroxidase Compounds I and II with L-tyrosine (Table 22.1) and showed that Compound I was reduced directly to ferric enzyme. Thus, in this case the reaction of Compound I with L-tyrosine proceeds by two-electron mechanism. In subsequent work these authors have shown [189] that at physiological pH TPO catalyzed the two-electron oxidation not only L-tyrosine but also D-tyrosine, A -acetyltyrosinamide, and monoiodotyrosine, whereas diiodotyrosine was oxidized by a one-electron mechanism. 

Compound 10 has also been used to quantify double Lewis acid activation by two cobalt (HI) ions [37]. In 12, the RNA analogue 2-hydroxypropyl-phenyl phosphate (HPPP) is coordinated to the dinu-clear cobalt site. It is well known that in this substrate the hydroxypropyl group is an efficient intramolecular nucleophile. Release of phenol by intramolecular cyclization is much faster than the reaction by nucleophilic attack of bridging oxide, as observed in 11. At pH >8, transesterification rate is linearly dependent on hydroxide concentration since OH" acts as an intermolecular base for the deprotonation of the hydroxypropyl group. The second order rate constant for the hydroxide-dependent cleavage is 4 x 105 times larger than the second-order rate constant for the hydroxide-dependent spontaneous transesterification of hy-droxypropyl-phenyl phosphate. 

An alternative electrochemical method has recently been used to obtain the standard potentials of a series of 31 PhO /PhO- redox couples (13). This method uses conventional cyclic voltammetry, and it is based on the CV s obtained on alkaline solutions of the phenols. The observed CV s are completely irreversible and simply show a wave corresponding to the one-electron oxidation of PhO-. The irreversibility is due to the rapid homogeneous decay of the PhO radicals produced, such that no reverse wave can be detected. It is well known that PhO radicals decay with second-order kinetics and rate constants close to the diffusion-controlled limit. If the mechanism of the electrochemical oxidation of PhO- consists of diffusion-limited transfer of the electron from PhO- to the electrode and the second-order decay of the PhO radicals, the following equation describes the scan-rate dependence of the peak potential ... 

Tratnyek, P.G. and Hoigne, J. Oxidation of substituted phenols in the environment a QSAR analysis of rate constants for reaction with singlet oxygen. Environ. Sci. Tecbnol., 25(9) 626-631, 1991. 

Mahoney studied this kinetics by the oxidation of 9,10-dihydroanthracene inhibited by several substituted phenols [23,31,32,37,38,49]. 9,10-Dihydroanthracene possesses weak C—H bonds that are easily attacked not only by peroxyl radicals but also by phenoxyl radicals as well (for the rate constants of reaction (10), see Chapter 15). 

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