Rate constant for oxidation

Fig. 7.36 Effect of alloying on the rate constant for oxidation of nickel at 900°C ...

It has been shown that the association of [Pt(NH3)5] with parsley PCu(I) decreases with increasing protonation at the remote site [96]. Rate constants for oxidation of the adduct PCu(I), [Pt(NH3)6] with [Co(phen)3] also decrease with pH, an effect which stems from protonation at the active site. The pKg s derived are both close to 5.8. 

The second-order rate constant for oxidation of Fe(CN)g by OH" radicals, produced by low-intensity-pulse radiolysis of water, varies with pH as in the accompanying table. Determine the pA for acid dissociation of the OH" radical in aqueous solution. (This is difficult to obtain by any other method.)... 

Fig. 5. Lowering of the rate constant for oxidation of reduced GO by ferricenium ions ki2 with elongation of the alkyl radical of H(CH2) Fc + (pH 7.0 0.1 M phosphate 25 °C). From Ref. (90).

Cyclic voltammetry has been also used for estimation of the rate constants for oxidation of water-soluble ferrocenes in the presence of HRP (131). There is a perfect match between the data obtained spectrophotometrically and electrochemically (Table IV), which proves that the cyclic voltammetry reveals information on the oxidation of ferrocenes by Compound II. It is interesting to note that an enzyme similar to HRP, viz. cytochrome c peroxidase, which catalyzes the reduction of H202 to water using two equivalents of ferrocytochrome c (133-136), is ca. 100 times more reactive than HRP (131,137). The second-order rate constant equals 1.4 x 106 M-1 s 1 for HOOCFc at pH 6.5 (131). There is no such rate difference in oxidation of [Fe(CN)e]4- by cytochrome c peroxidase and HRP (8). These comparisons should not however create an impression that the enzymatic oxidation of ferrocenes is always fast. The active-R2 subunit of Escherichia coli ribonucleotide reductase, which has dinuclear nonheme iron center in the active site, oxidizes ferrocene carboxylic acid and other water-soluble ferrocenes with a rate constant of... 

Somewhere in the temperature range 450° to 600°C. pyrolysis must compete on nearly equal terms with oxidation of alkyl radicals. The work of Baldwin is therefore particularly important since the rate constants for pyrolysis of alkyl radicals are reasonably well established. There is therefore the strong possibility that we shall soon possess rate constants for oxidation reactions of alkyl radicals at high temperatures. Examination of the oxidation products of the higher alkanes by the Baldwin method should go far toward resolving the problem of the source of fragmentation products at lower temperatures. 

Buxton GV, Greenstock C L, Helman WP, Ross A B (1988) Critical Review of Rate Constants for Oxidation of Hydrated Electrons, Hydrogen Atoms and Hydroxyl Radicals (0H°/0° ) in Aqueous Solutions, Journal of Physical and Chemical Reference Data 17 513-886. 

The o scale was developed to incorporate the effect of through-resonance. Both types of correlations included ortho- as well as meta- and para-substituted phenoxides, and the only outliers are compounds that exhibit strong intramolecular hydrogen bonding because this effect is not incorporated in most molecular descriptors. The rate constants for oxidation of phenoxide anions give good Hammett correlations to cr constants, as shown in the following equation ... 

This regression line is illustrated in Figure 5.32. The second-order rate constants for oxidation of the undissociated forms of substituted phenols are about six orders of magnitude smaller than the corresponding values for phenoxide anions, indicating that only the reaction of phenoxide anions will... 

The rate constants for oxidation of alkanes and alkenes with peroxynitrous acid in aqueous-gas phase are bell-shaped functions of the volume ratio between the liquid and gas phases. The kinetics of the generation of OH radicals and its importance in understanding the mechanism of lipid membrane oxidation has been stressed.236... 

Bimolecular rate constants for oxidation by oxyradicals and singlet oxygen have been measured in numerous studies and have been compiled and used to develop reasonably accurate SARs for photooxidations. Atkinson (1987) developed an additivity SAR to estimate kHO(air) for many kinds of organic structures. Mill (1989) has summarized information on SARs for photooxidants in water and air. See Chapter 14 for a detailed discussion of the procedures used for estimating kHO(air) for most classes of organic compounds. 

The most difficult aspect of estimating indirect photoreaction rates is finding a measured value of koX or estimating koX for the oxidant and compound of interest. Measured values of koX are usually much preferred to estimated values, but measured values are available only for a small proportion of organic compounds likely to be found in surface waters. Critical compilations of rate constants for oxidant reactions with organic compounds in water appear in Hendry et al. (RO, R02) (1974) Wilkinson et al. (J02) (1995) Buxton et al. (HO and e Aq) (1988) Hendry and Schuetzle (HOz) (1976) Neta et al. (R02) (1990) and Haag and Yao (HO) (1992). 

Rate Constants for Oxidation of Major Classes of Organic Compounds3 in Insolated Surface Waters... 

Hammett coefficients for oxidations of 2,6-di-t-butyl-4-X-phenols by t-BuOz in isopentane and Me2(CN)C02 in water are very similar over a temperature range of -40°C to 40°C, suggesting that SARs for other phenols which Table 15.7 lists can be used with moderate reliability to estimate rate constants for oxidations in water. 

Anthracene is a polycyclic conjugated structure that cannot be fragmented into smaller units. The oxidants each have preferred points of oxidation on the conjugated rings, but the 9 and 10 positions are favored because of symmetry and stabilization of benzylic radicals. Rate constants for oxidation of anthracene by ROz, 102, and HO have been measured in solution or in vapor kox(R02) is 60 M 1 s 1 in chlorobenzene at 60°C (Mahoney, 1965) ... 

Selected Rate Constants for Oxidations of Environmental Contaminants. 

Forster and Keyes prepared the tetrazine containing [Os(II)(BL)Os(III)] dimer (see below) and examined light induced intramolecular electron transfer in the system. They also measured heterogeneous electron transfer rate constants for oxidation of the complex at a Pt electrode [38]. The work is... 

The rate constants for oxidation of a series of cycloalkenes with ozone have been determined using a relative rate method. The effect of methyl substitution on the oxidation of cycloalkenes and formation of secondary organic aerosols has been analysed.155 Butadiene, styrene, cyclohexene, allyl acetate, methyl methacrylate, and allyl alcohol were epoxidized in a gas-phase reaction with ozone in the absence of a catalyst. With the exception of allyl alcohol, the yield of the corresponding epoxide ranged from 88 to 97%.156 Kinetic control of distereoselection in ozonolytic lactonization has been (g) reported in the reaction of prochiral alkenes.157... 

If the substrate is an insulator Js — 0. If both tip and substrate reactions are electrochemical processes, the rate constants for oxidation (kf) and reduction (kb) are given by the Butler-Volmer relations... 

Entry no. 2 of Table 16 introduces the most remarkable aspect of Co(III) chemistry, namely its ability to oxidize non-activated C—H bonds (for a recent study, see Jones and Mellor, 1977) and we immediately see that Marcus theory here completely rules out the possibility of an initial electron-transfer step. This is predicted to be ca. 1016 times slower than what is actually observed. On the other hand, the theory correctly predicts the rate constant for oxidation of naphthalene under the same conditions, and the postulated direct abstraction of a 7t-electron is thus feasible (Cooper and Waters, 1967). The Co(III) trifluoroacetate study of entry no. 3, including only substrates without an alkyl side-chain, however, cannot be fitted to any physically realistic set of E°, A parameters. With E° = 1.83 V (value in 1.0 M HC104) A ought to be ca. 40 kcal mol-1 and the slope of the log k/AG0 regression line ca. —0.6 the A value is then in reasonable agreement with the estimated one but not the slope. With E° = 3.2 V (clearly not a physically very realistic standard potential) a A value of ca. 80 kcal mol-1 is required. This seems to be far too large for such a system. 

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