Reactions of radicals. 41. Reactivity patterns of the methyl radical

Abstract

The relative reactivities of a series of hydrogen donors toward the methyl radical were measured in solution at 110°. Methyl radicals were generated by the thermal decomposition of ferf-butyl peracetate in mixtures of the hydrogen donor RH and the standard substrate tert-butyl mercaptan-S-t/, and the ratio [CHJ [CH3D] was determined by mass spectrometry: CH3• + RH CH4 + R• (¿HH) and CH3• + tm-BuSD -CH3D + tert-BuS• (¿sd)• A wide range of substrates was studied, including compounds containing alkyl, allylic, benzylic, and cycloalkyl hydrogens, and the data are combined to give a scale of rate constants for reaction of methyl radicals with any given type of hydrogen. Our scale of reactivities for hydrogen-abstraction reactions and Szwarc's scale3 for addition reactions can be put on a common basis using the reactivity of isooctane as the bridge: CH3• + (olefinic or aromatic compound) -» CH3-C-C • (¿add) and CH3• + C8H18 -CH4 -\-C8H17• (¿¡so). This permits us to tabulate the relative reactivity toward methyl radicals of virtually every type of hydrocarbon-saturated, olefinic, or aromatic; from these data the relative reactivity of any hydrocarbon of interest can be predicted. The selectivity of the methyl radical toward various types of hydrogens is compared with that of a number of other radicals.The methyl radical is found to be quite similar to the phenyl radical in selectivity. Calculations show the relative reactivity of a series of donors toward the methyl radical is the same in the gas phase and in solution.(1) Reactions of Radicals. Part 41. This work was supported in part by NIH Grant GM-11908. (2) (a)

Modification of carbon surfaces with methyl groups by using ferrocene derivatives as redox catalysts of the oxidation of acetate ions

“…These results reveal the formation of multilayers whose formation can be attributed to the high population of methyl radicals generated during the mediated oxidation of acetate. This proposal is consistent with the fact that methyl radicals can extract hydrogen atoms from hydrocarbons to afford methane and alkyl radicals (CH Å 3 + RH = CH 4 + R Å ) [29]. In this way, the formation of aggregates is possible when methyl radicals subtract hydrogen atoms from the attached groups, giving rise to transient grafted radicals, which next undergo subsequent coupling with the methyl radicals coming from the mediated oxidation of acetate.…”

Advanced Organic Chemistry

“…Interaction between the hydrogens and the 1r system favor the eclipsed conformation to this extent. 44 Let us examine the reason for the preference for the eclipsed conformation. This issue can be approached 44.…”

“…Whereas the free energy of activation for ring inversion in cyclohexane is 10.3 kcal/mol, the barrier is reduced to 7.7 kcal/mol in methylenecyclohexane, 43 and to 4.9 kcal/mol in cyclohexanone. 44 The decrease in activation energy is related to the lower torsional barriers for rotation about sp 2 -sp 3 bonds, and to the decreased steric requirements of a carbonyl or methylene group. 41.…”

Radiation‐induced dechlorination of carbon tetrachloride in cyclohexane Solutions. The kinetics of liquid‐phase reactions of trichloromethyl radicals

“…Hydrogen atom abstraction from various substrates by C1 atoms [3-71 and methyl radicals [8,9], and from CHCl, by CHClz radicals [lo] represent series of metathetical reactions, the kinetics of which have been studied in both the liquid and gas phases. The C1 atom studies indicate that the activation parameters are different for reactions in the gas and liquid phases [4,6,7].…”

“…The C1 atom studies indicate that the activation parameters are different for reactions in the gas and liquid phases [4,6,7]. However, comparative studies on C H 3 radicals, carried out by Mayo [8] and Pryor and coworkers [9], show that the relative reactivities of methyl radicals toward different substrates are the same in the gas phase as in solutions. The experimental data which served as the basis for the latter studies do not make it possible to ascertain whether this lack of phase effect is due to the equality of activation parameters under both sets of conditions, or to the fact that phase effects on E values are counterbalanced by appropriate variations in ,4 factors.…”