Alkyl radicals hydrogen abstraction

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Equation (24) has been chosen as an example of the radical deoxygenation of secondary alcohols via thiono esters [58], whereas Eq. (25) represents an example of deamination of primary amines via isocyanides [7, 54]. The reaction mechanism of these reductions is similar to that described for tin hydride, i.e. attack of silyl radical on the C=S or N=C moieties to form a radical intermediate which undergoes -scission to form alkyl radicals. Hydrogen abstraction from the hydride gives the product and the (TMS)3Si radical, thus completing the cycle of this chain reaction. 

For both bromination and chlorination of alkanes, the rate-determining step is hydrogen abstraction to form an alkyl radical. Hydrogen abstraction is endothermic for bromination and exothermic for chlorination. 

A secondary alkyl radical is more stable than a primary radical Bromine therefore adds to C 1 of 1 butene faster than it adds to C 2 Once the bromine atom has added to the double bond the regioselectivity of addition is set The alkyl radical then abstracts a hydrogen atom from hydrogen bromide to give the alkyl bromide product as shown m... 

The reaction is formally a hydrosilylation process analogous to the homogeneous reactions described in Chapter 5. Scheme 8.11 shows the proposed H—Si(lll) surface-propagated radical chain mechanism [48]. The initially formed surface silyl radical reacts with alkene to form a secondary alkyl radical that abstracts hydrogen from a vicinal Si—H bond and creates another surface silyl radical. The best candidate for the radical translocation from the carbon atom of the alkyl chain to a silicon surface is the 1,5 hydrogen shift shown in Scheme 8.11. Hydrogen abstraction from the neat alkene, in particular from the... 

The alkyl radical may also dissociate thermally to form an alkene and a smaller alkyl radical. The mechanism that is initiated by these reactions is chain propagating rather than chain branching and for this reason the overall oxidation rate of the fuel decreases. Also there is a change from OH to HO2 as the main chain carrier, and as we have seen, the HO2 radical is much less reactive than OH. The HO2 radical is formed both from alkyl + O2 hydrogen abstraction reactions such as (R69) and from recombination of hydrogen atoms with O2, H + O2 + M HO2 + M (R5). Under lean conditions any hydrogen atoms formed will primarily react with oxygen. At intermediate temperatures the reaction H + O2 O + OH (Rl) is still too slow to compete with (R5). 

A mechanism was proposed in which the perferryl iron-oxeme, resulting from heterolytic cleavage of the 0-0 bond of the iron-peroxy intermediate, abstracts an electron from the 0=0 double bond of the carbonyl group of the aldehyde. The reduced perferryl attacks the 1-carbon of the aldehyde to form a thiyl-iron-hemiacetal diradical. The latter intermediate can fragment to form an alkyl radical and thiyl-iron-formyl radical. The alkyl radical then abstracts the formyl hydrogen to produce the hydrocarbon and C02 (Reed et al 1995). 

Another difference between bulk and monolayer oxidations was observed in the amounts of alkanes and alkenes produced. In bulk, the alkanes in ethyl palmitate heated for 1 hr at 180°C were greater than the 1-alkenes, indicating that hydrogen abstraction is the preferred route for termination of the alkyl radicals which are formed by hydroperoxide scission. The reverse was true in the ordered state. Restriction of the alkyl radicals limits abstraction of hydrogen from other substrate molecules to form alkanes thus favoring the production of 1-alkenes (Hau, L. B. Nawar, W. W. University of Massachusetts, Amherst, unpublished data). 

Olefin isomerization has been widely studied, mainly because it is a convenient tool for unravelling basic mechanisms involved in the interaction of olefins with metal atoms (10). The reaction is catalyzed by cobalt hydrocarbonyl, iron pentacarbonyl, rhodium chloride, palladium chloride, the platinum-tin complex, and by several phosphine complexes a review of this field has recently been published (12). Two types of mechanism have been visualized for this reaction. The first involves the preformation of a metal-hydrogen bond into which the olefin (probably already coordinated) inserts itself with the formation of a (j-bonded alkyl radical. On abstraction of a hydrogen atom from a diflFerent carbon atom, an isomerized olefin results. 

Nonfluorinated alkyl hypofluorites (ROF) are mostly unstable and decompose through HF elimination with formation of the corresponding carbonyl compounds. However, tert-butyl [24] and methyl hypofluorites (RO+ F ) [25] can be generated and employed in situ for alkoxy-fluorination of alkenes. The relatively higher stability of both hypofluorites comes from the nonavailability of a-hydrogens which are reactive to the radical hydrogen abstraction [26]. Both of the hypofluorites add to carbon-carbon double bonds regio- and stereoselectively as shown in Scheme 2.48. The anti-addition mode (37-39) is in sharp contrast to the syn-addition mode of electrophilic fluoroalkoxy and acetoxy reactions of fluorinated alkyl and acetyl hypofluorites (RfO F+, 40-41) [27]. 

Alkyl radicals have been used to abstract hydrogen atom from C-H bonds at secondary carbon centers. For instance, Fuchs has developed a self-immolative elimination of aryl sulfones [84]. The or/Ao-silylated sulfone gives upon treatment with tin hydride and AIBN a primary alkyl radical that abstracts a hydrogen atom in a... 

A short review of photo-oxidation of a range of polymers by Faucitano and co-workers [30] summarised the understanding at the time of PET photolysis as a splitting of C-0 bonds in the ester gronps, with formation of acyl and carboxyl radicals, which themselves can lose carbon oxides to produce phenyl or alkyl radicals, or abstract hydrogen to produce aldehydes and carboxylic acids. When oxygen is present, the authors state that the autoxidation chain reaction will lead to formation of anhydrides and aldehydes, and to hydroxy-substituted phenyl species. 

The mechanism proposed for the peroxide effect involves a radical chain reaction. The initiation step (equation 9.31) produces a bromine atom, which then attaches to the less alkyl-substituted carbon atom of a carbon-carbon double bond (equation 9.32). The resulting alkyl radical then abstracts a hydrogen atom from HBr to produce the anti-Markovnikov product and regenerate a bromine atom in the second propagation step (equation 9.33). Termination steps, not shown, interrupt the chain reaction. 

The percentage of cyclohexylation is given in Fig. 1-20. (411,412). Hydrogen abstraction from the alkyl side-chain produces, in addition, secondary products resulting from the dimerization of thiazolylalkyl radicals or from their reaction with cyclohexyl radicals (Scheme 68) (411). 

Some details of the chain-initiation step have been elucidated. With an oxygen radical-initiator such as the /-butoxyl radical, both double bond addition and hydrogen abstraction are observed. Hydrogen abstraction is observed at the ester alkyl group of methyl acrylate. Double bond addition occurs in both a head-to-head and a head-to-tail manner (80). 

The effect substitution on the phenolic ring has on activity has been the subject of several studies (11—13). Hindering the phenolic hydroxyl group with at least one bulky alkyl group ia the ortho position appears necessary for high antioxidant activity. Neatly all commercial antioxidants are hindered ia this manner. Steric hindrance decreases the ability of a phenoxyl radical to abstract a hydrogen atom from the substrate and thus produces an alkyl radical (14) capable of initiating oxidation (eq. 18). 

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