Hydrogen atom abstraction by radicals

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... 

The rates of hydrogen atom abstractions by radicals are subject to the same factors that control rates of alkene additions [130]. Both enthalpic and polar... 

Selective hydrogen atom abstraction by radicals. Giesc and Curran2 note that radicals such as 4 abstract hydrogen atoms in a ratio that is remarkably similar to that observed by Cram for reduction of ketones by lithium aluminum hydride. 

The high selectivity for reaction at primary C-H bonds is one of the most striking features of the oxidative addition of alkanes. This selectivity contrasts with that of hydrogen atom abstraction by radical species, which occur fastest at tertiary C-H bonds and slowest at primary C-H bonds. It also contrasts with the chemistry of metal-oxo or metal-carbene ° complexes, which tend to insert the 0x0 or carbene group into the weakest, sterically accessible C-H bond. This selectivity of oxidative addition for reaction at a primary C-H bond implies that alkanes can be converted selectively to products with functional groups in the terminal position. 

Conventional free radical chain oxidation (referred to as autoxidation) involves chain initiation, propagation, and termination steps [15-17]. Thepossibility of hydrogen atom abstraction by radical species in arenes without alkyl side chains is small because of the strong sp C—H bonds (the dissociation energies of the sp Ar—H bonds and sp ArCR —H are 112 and ca. 90kcal/mol, respectively [18]). Phenols readily react by direct abstraction of H from the hydroxyl gronp to form phenoxyl radical ArO, and this pathway is implicated in their antioxidant properties [16, 19]. Numerous literature addresses structure-reactivity relationships in the chemistry of phenols and phenoxyl radicals (see Refs. 1, 16, 19 and references therein). 

Selective chlorination of the 3-position of thietane 1,1-dioxide may be a consequence of hydrogen atom abstraction by a chlorine atom. Such reactions of chlorine atoms are believed to be influenced by polar effects, preferential hydrogen abstraction occurring remotely from an electron withdrawing group. The free radical chain reaction may be propagated by attack of the 3-thietanyl 1,1-dioxide radical on molecular chlorine. 

Chemical combustion is initiated by the oxidation or thermal decomposition of a fuel molecule, thereby producing reactive radical species by a chain-initiating mechanism. Radical initiation for a particular fuel/oxygen mixture can result from high-energy collisions with other molecules (M) in the system or from hydrogen-atom abstraction by 02or other radicals, as expressed in reactions 6.1-6.3 ... 

There are also useful intramolecular functionalization methods that involve hydrogen atom abstraction by oxygen radicals. The conditions that were originally developed involved thermal or photochemical dissociation of alkoxy derivative of Pb(IV) generated by exchange with Pb(OAc)4.374 These decompose, giving alkoxy... 

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

The reaction of hydrogen atom abstraction by the alkylhydroxyperoxyl radical from alcohol limits chain propagation in oxidized alcohol [8,9]. 

A molecule of linear alkyl ether possesses a very convenient geometry for intramolecular hydrogen atom abstraction by the peroxyl radical. Therefore, chain propagation is performed by two ways in oxidized ethers intermolecular and intramolecular. As a result, two peroxides as primary intermediates are formed from ether due to oxidation, namely, hydroperoxide and dihydroperoxide [62],... 

FIGURE 16.1 The dependence of activation energy E on reaction enthalpy A He for reaction of hydrogen atom abstraction by aminyl radical from the C—H bond of alkylperoxyl radical and O—H bond of hydroperoxyl radical calculated by IPM method (see Chapter 6). The points fix the reactions with minimum and maximum enthalpy among known aromatic aminyl radicals. 

Iron complexes or microsomal nonheme iron are undoubtedly obligatory components in the microsomal oxidation of many organic compounds mediated by hydroxyl radicals. In 1980, Cohen and Cederbaum [27] suggested that rat liver microsomes oxidized ethanol, methional, 2-keto-4-thiomethylbutyric acid, and dimethylsulfoxide via hydrogen atom abstraction by hydroxyl radicals. Then, Ingelman-Sundberg and Ekstrom [28] assumed that the hydroxylation of aniline by reconstituted microsomal cytochrome P-450 system is mediated by hydroxyl radicals formed in the superoxide-driven Fenton reaction. Similar conclusion has been made for the explanation of inhibitory effects of pyrazole and 4-methylpyrazole on the microsomal oxidation of ethanol and DMSO [29],... 

Bedard et al. [7] studied quantitatively the initiation of the peroxidation of human low-density lipoproteins (LDL) with H00702 . In accord with the above findings the initiation rate increased when pH decreased from 7.6 to 6.5. It was suggested that initiation occurred via hydrogen atom abstraction by perhydroxyl radical from endogenous a-tocopherol, which in this process exhibited prooxidant and not antioxidant properties. Neutral, positively, and negatively charged alkyl peroxyl free radicals were the more efficient initiators of LDL peroxidation compared to superoxide. 

The COs radical anion is a strong one-electron oxidant ( 7-1.7 V vs NHE [15]) that oxidizes appropriate electron donors via electron transfer mechanisms [103]. Detailed pulse radiolysis studies have shown that carbonate radicals can rapidly abstract electrons from aromatic amino acids (tyrosine and tryptophan). However, reactions of CO3 with S-containing methionine and cysteine are less efficient [104-106]. Hydrogen atom abstraction by carbonate radicals is generally very slow [103] and their reactivities with other amino acids are negligible [104-106]. 

Hydrogen atom abstraction by non-radical, metal- species, once considered unlikely, has been shown in recent years to operate in a large number of reactions (113-115) with the kinetics responding to the thermodynamic driving force and intrinsic barriers as predicted by the Marcus cross relation (116). 

The organic radicals are produced essentially by one of the following methods Aliphatic carbon-centered radicals are formed mainly from saturated aliphatic compounds via hydrogen atom abstraction by OH and H radicals ( H radicals react with solutes to produce the same radicals as OH radicals, but the rate constants are usually considerably smaller (7)) ... 

Hydroperoxides can be formed via hydrogen-atom abstraction by R02. The RO2—H bond strength is about 90 kcal. per mole consequently, many such abstractions will be endothermic. Benson (3) has estimated an activation energy of 6 kcal. per mole for exothermic abstractions and 6 + AH for endothermic reactions. The pre-exponential factors should be about 108 0 M"1 sec."1—i.e., about a factor of 2 lower than hydrogen-atom abstraction by CH3 radicals, Some typical abstraction reactions are listed in Table II. 

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