Hydrocarbons hydrogen atom abstraction from

The enthalpy of the R02 + RH reaction is determined by the strengths of disrupted and newly formed bonds AH= Z>R H—Droo—h- For the values of O—H BDEs in hydroperoxides, see the earlier discussion on page 41. The dissociation energies of the C—H bonds of hydrocarbons depend on their structure and vary in the range 300 - 440 kJ mol-1 (see Chapter 7). The approximate linear dependence (Polany-Semenov relationship) between activation energy E and enthalpy of reaction AH was observed with different E0 values for hydrogen atom abstraction from aliphatic (R1 ), olefinic (R2H), and alkylaromatic (R3H) hydrocarbons [119] ... 

Table VIII. Absolute (and Relative) Rate Constants for Hydrogen Atom Abstraction from Hydrocarbons at 30°C.

The rate constants of a-hydrogen atom abstraction from four hydrocarbons by cumylperoxy, tetralylperoxy, and 9,10-dihdroanthracyl-9-peroxy radicals and by the normal chain-carrying peroxy radicals are compared in Table VIII. The results show that the reactivities of peroxy radicals are affected by the nature of the organic group. The relatively low propagation constant for the oxidation of pure cumene may be caused by the low reactivity of the cumylperoxy radical. 

The approximate linear dependence (Polany-Semenov relationship) between activation energy E and enthalpy of reaction AH was observed with different E0 values for hydrogen atom abstraction from aliphatic (R1 ), olefinic (R2H), and alkylaromatic (R3H) hydrocarbons [119] ... 

The very high stability (10 years at room temperature) of R2M radicals is due to the very bulky R substituents at M hindering recombination, as well as the comparatively low values of the M—H bond energy. Therefore, hydrogen atom abstraction from the hydrocarbon solvent turns out to be unfavourable. 

How does structure determine organic reactivity, 35, 67 Hydrated electrons, reactions of, with organic compounds, 7,1 15 Hydration, reversible, of carbonyl compounds, 4, 1 Hydride shifts and transfers, 24, 57 Hydrocarbons, small-ring, gas-phase pyrolysis of, 4, 147 Hydrogen atom abstraction from O—H bonds, 9, 127 Hydrogen bonding and chemical reactivity, 26, 255 Hydrogen isotope effects in aromatic substitution reactions. 2, 163... 

The C H activation processes were proposed to proceed via a trimolecular pathway, involving a linear four-centered concerted breaking of the C—H bond between two Rh(II) metalloradicals [see Fig. 59 (a)]. In principle, this reaction could also proceed via two subsequent separate steps hydrogen-atom abstraction from the hydrocarbon R3C H bond, thus generating a Rh H species and a carbon-centered radical wCRs, which is rapidly followed by capture of the thus... 

The electrophilic character of perhaloalkyl radicals (and in particular perfluoroalkyl radicals) makes them particularly attractive for hydrogen atom abstraction [100], Indeed, their reactivity resembles that of alkoxyl radicals, and rates of hydrogen atom abstraction from C-H bonds are much higher than those of the corresponding hydrocarbon radicals [lOlj. Their electrophilic character facilitates abstraction at... 

Rate Constants of the Hydrogen Atom Abstraction by Peroxyl Radicals from the Hydrocarbons (R02 + RH —> ROOH + R )... 

Thus, the UV spectrum of 22, was interpreted as incompatible with a a /a configuration. At the same time, the IR data were interpreted in favor of the a-ir/a-ir configuration. In addition, the reactivity pattern of 22 differs significantly from that of 6, which has a ground-state configuration. For example, 22 does not appear to ring expand upon irradiation (as 6 is known to do) and reacts with hydrocarbons (like methane) via hydrogen-atom abstraction (rather than by C—H insertion, as is the case with 6) [86,87]. Thus, the conclusion that 22 is a aTr/a-ir biradical was reached. 

In many cases both Kolbe and non-Kolbe products are isolated from a reaction. Carboxylic acids with an a-alkyl substituent show a pronounced dual behaviour. In these cases, an increase in the acid concentration improves the yield of the Kolbe product. An example of the effect of increased substrate concentration is given in Kolbe s classical paper [47] where 2-methylbutyric acid in high concentration affords mostly a dimethylbexane whereas more recent workers [64], using more dilute solutions, obtained both this hydrocarbon and butan-2-ol. Some quantitative data is available (Table 9.2) for the products from oxidation of cyclohexanecar-boxylic acids to show the extent of Kolbe versus non-Kolbe reactions. The range of products is here increased through hydrogen atom abstraction by radical intermediates in the Kolbe reaction, which leads to some of the monomer hydrocarbon... 

Some conclusions on the reaction mechanism may be drawn from the rate constants obtained. It was shown for hydroxyl reactions with saturated compounds (propane, for example) that the main reaction of OH was the hydrogen atom abstraction in the formation of water. This is an accepted point of view. However, another route is possible for reactions with unsaturated hydrocarbons, i.e., addition at the double bond. This is the case for the H atom with saturated compounds H reacts by abstraction, and with unsaturated ones by addition. 

The photo-oxidation of n-butane has been modelled by ab initio and DFT computational methods, in which the key role of 1- and 2-butoxyl radicals was confirmed.52 These radicals, formed from the reaction of the corresponding butyl radicals with molecular oxygen, account for the formation of the major oxidation products including hydrocarbons, peroxides, aldehydes, and peroxyaldehydes. The differing behaviour of n-pentane and cyclopentane towards autoignition at 873 K has been found to depend on the relative concentrations of resonance-stabilized radicals in the reaction medium.53 The manganese-mediated oxidation of dihydroanthracene to anthracene has been reported via hydrogen atom abstraction.54 The oxidation reactions of hydrocarbon radicals and their OH adducts are reported.55... 

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