Oxidation rates, substituent-reactivity

The propagation step, Eq. (4), is much slower than Eq. (3) as an example, its rate constant kp is 0.18 M 1 sec-1 for cumene at 303K. Values of kp can vary considerably for different substrates, as shown by the oxidation rates of substituted toluenes (8). With respect to toluene, taken as 1.0, the reactivity of 4-nitrotoluene toward ROO is 0.33 and that of / -xylene is 1.6. A homolytic process like the fission of the C-H bond should be essentially apolar, but data for substituted toluenes correctly suggest that the hydrogen radical abstraction is favored by electron-donor substituents and that in the transition state the carbon atom involved has a partial positive charge. The difference in kp between different molecules or different groups of the same molecule is the reason of the selectivity of autoxidation. 

Oxidation of Hydroxystilbene Derivatives with Nitrobenzene and Cop-per(II). To serve as / -l lignin model compounds, some 1-(4-hydroxyphenyl)-2-(4,-substituted phenyl)ethanols, (4), (Figure 2) were synthesized ( ). The / -l lignin model was chosen, since it is the only common lignin unit structurally suitable for substituent-reactivity studies on oxidation rates. In an effort to study... 

The general reactivity of oxidized Ni anodes in various f-butanol/H20 mixtures was followed by cyclic voltammetry. " The coulombic and organic product yields of aldehyde and acid were determined for various primary alcohol derivatives. Substituent effect on the anodic oxidation rates of a series of benzyl alcohols were evaluated. Attempts were made to relate the oxidation rates to the Hammett cr parameter for substituent properties. 

The effect of conformation on reactivity is intimately associated with the details of the mechanism of a reaction. The examples of Scheme 3.2 illustrate some of the w s in which substituent orientation can affect reactivity. It has been shown that oxidation of cis-A-t-butylcyclohexanol is faster than oxidation of the trans isomer, but the rates of acetylation are in the opposite order. Let us consider the acetylation first. The rate of the reaction will depend on the fiee energy of activation for the rate-determining step. For acetylation, this step involves nucleophilic attack by the hydroxyl group on the acetic anhydride carbonyl... 

The rate of epoxidation of alkenes is increased by alkyl groups and other ERG substituents and the reactivity of the peroxy acids is increased by EWG substituents.72 These structure-reactivity relationships demonstrate that the peroxyacid acts as an electrophile in the reaction. Decreased reactivity is exhibited by double bonds that are conjugated with strongly electron-attracting substituents, and more reactive peroxyacids, such as trifluoroperoxyacetic acid, are required for oxidation of such compounds.73 Electron-poor alkenes can also be epoxidized by alkaline solutions of... 

The effectiveness of the antioxidant depends not only on its reactivity, but also on its molecular weight that affects the rate of the antioxidant loss due to evaporation. The following example illustrates this dependence. Antioxidants of the structure 2,6-bis (1, l-dimethylethyl)phenols with para-substituents of the general structure ROCOCH2CH2 were introduced into decalin and polypropylene films that were oxidized by dioxygen at... 

The influence of substituents on the rates of degradation of arylazo reactive dyes based on H acid, caused by the action of hydrogen peroxide in aqueous solution and on cellulose, has been investigated [43]. The results suggested that the oxidative mechanism involves attack of the dissociated form of the o-hydroxyazo grouping by the perhydroxyl radical ion [ OOH]. The mechanism of oxidation of sulphonated amino- and hydroxyarylazo dyes in sodium percarbonate solution at pH 10.6 and various temperatures has also been examined. The initial rate and apparent activation energy of these reactions were determined. The ketohydrazone form of such dyes is more susceptible to attack than the hydroxyazo tautomer [44]. 

The influence of steric effects on the rates of oxidative addition to Rh(I) and migratory CO insertion on Rh(III) was probed in a study of the reactivity of a series of [Rh(CO)(a-diimine)I] complexes with Mel (Scheme 9) [46]. For a-diimine ligands of low steric bulk (e.g. bpy, L1, L4, L5) fast oxidative addition of Mel was observed (103-104 times faster than [Rh(CO)2l2] ) and stable Rh(III) methyl complexes resulted. For more bulky a-diimine ligands (e.g. L2, L3, L6) containing ortho-alkyl groups on the N-aryl substituents, oxidative addition is inhibited but methyl migration is promoted, leading to Rh(III) acetyl products. The results obtained from this model system demonstrate that steric effects can be used to tune the relative rates of two key steps in the carbonylation cycle. 

Tphe original objectives of this work were to determine how much the relative reactivity of two hydrocarbons toward alkylperoxy radicals, R02, depends on the substituent R—, and whether there are any important abnormalities in co-oxidations of hydrocarbons other than the retardation effect first described by Russell (30). Two papers by Russell and Williamson (31, 32) have since answered the first objective qualitatively, but their work is unsatisfactory quantitatively. The several papers by Howard, Ingold, and co-workers (20, 21, 23, 24, 29) which appeared since this manuscript was first prepared have culminated (24) in a new and excellent method for a quantitative treatment of the first objective. The present paper has therefore been modified to compare, experimentally and theoretically, the different methods of measuring relative reactivities of hydrocarbons in autoxidations. It shows that large deviations from linear rate relations are unusual in oxidations of mixtures of hydrocarbons. 

I-Ph, or LNiIH-0-NiIUL) have been proposed as the active oxidant (92). In the reaction, E olefins are more reactive than the corresponding Z isomers, and a strong correlation was observed between the electron-donating effect of the para substituents in styrene and the initial reaction rate (91). Isotope labeling studies have shown that the epoxide oxygen is derived from PhIO. 

Pathway III of Fig. 26 has been demonstrated for thiophene and benzo-thiophene with Ir complexes (4) and for all thiophenes, including dibenzo-thiophene, with Rh complexes (94, 95). These oxidative additions appear to be influenced by substituents present on the carbon atoms adjacent to the sulfur atom. Insertion between sulfur and the unsubstituted carbon is highly preferred. For 2-methylthiophene the exclusive product is the 1-5 bond insertion product, whereas for 3-methylthiophene, no preference for insertion was observed (1-2 and 1-5 bond insertion products were equal). In competitive studies, thiophene was found to be about twice as reactive as 2,5-dimethylthiophene. This behavior is similar to that observed for relative reaction rates of substituted thiophenes observed with conventional HDS catalysts. Thus steric limitations can occur, even with monomeric, homogeneous catalysts. 

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