Hydrogen atom abstraction

About: Hydrogen atom abstraction is a research topic. Over the lifetime, 7059 publications have been published within this topic receiving 151781 citations.

Effects of diffusion on the self‐termination kinetics of isopropylol radicals in solution

Abstract: Rate constants for the bimolecular self-reaction of isopropylol radicals [(CH3)2ĊOH] in various solvents are determined as functions of temperature by kinetic electron spin resonance. For hydrocarbon solvents they are well described by theoretical equations for reactions controlled by translational diffusion if diffusion coefficients of 2-propanol, a constant reaction distance, and a spin statistical factor of 1/4 are applied. Deviations from 2kt ∼ D at high diffusion constants agree with trends expected from recent theoretical models. For hydrogen-bonding solvents large negative deviations are observed. They are attributed to steric constraints and slower rotational diffusion of radical–solvent aggregates. The disproportionation-to-combination ratio of isopropylol increases with solvent viscosity. As previously for tert-butyl, this is explained by anisotropic reorientation during encounters. Further, rate data are given for the decarbonylation of the 2-hydroxy-2-methylpropanoyl radical and for several hydrogen abstraction reactions of isopropylol.

UDFT and MCSCF descriptions of the photochemical Bergman cyclization of enediynes

Abstract: Several singlet and triplet potential energy surfaces (PES) for the Bergman cyclization of cis-1,5-hexadiyne-3-ene (1a) have been computed by UDFT, CI, CASCI, CASSCF, and CASMP2 methods. It is found that the first six excited states of 1a can be qualitatively described as linear combinations of the configurations of weakly interacting ethylene and acetylene units. Although the symmetry relaxation from C2ν to C2 makes cyclization of the 13B state Woodward−Hoffmann allowed, it also increases the probability of competing cis−trans isomerization. Hydrogen atom abstraction is another plausible pathway because the terminal alkyne carbons possess a large radical character. In view of the competing processes, we conclude that the Bergman cyclization along the 13B path is unlikely despite its exothermicity (Δ = −42 kcal/mol). Calculations on cyclic analogues of 1a lead to similar conclusions. A less exothermic, but more plausible pathway for photochemical cyclization lies on the 21A PES (Δ = −18 kcal/mol). Compare...

Reactions of CF3 radicals with aromatics. Part 4.—Hydrogen abstraction from benzene at high temperatures and the bond dissociation energy D(C6H5—H)

Abstract: The reactions of CF3 radicals with benzene in the range 100–400°C were studied using the photolysis of hexafluoroacetone as a source of radicals. The addition of CF3 radicals to the benzene ring becomes reversible above ∼140°C. Below 140°C, pesudo H-abstraction occurs via the reactions CF3+ C6H6→CF3C6H6, CF3+ CF3C6H6→CF3H + C6H5CF3, but at higher temperatures, genuine H-abstraction by reaction (3) occurs CF3+ C6H6→CF3H + C6H5(3), and we obtain log k3(cm3 mol–1 s–1)=(11.31±0.28)–(9110±760)/θ, where θ= 2.303 RT cal mol–1. Combination of E3 and published data on E–3, (for the reverse of reaction (3)), gives ΔH°3 from which ΔH°f(C6H5)= 77.9±2.0, D(C6H5–H)= 110.2±2.0 kcal mol–1, both at 298 K. These values are compared with published data and the following are recommended: ΔH°f(C6H5)= 77.7±2.0 and D(C6H5—H)= 110.0±2.0, both in kcal mol–1.

Ortho Effect in the Bergman Cyclization: Interception of p-Benzyne Intermediate by Intramolecular Hydrogen Abstraction

Abstract: Intramolecular hydrogen atom (H-atom) abstraction from the o-OCH3 group effectively intercepts the p-benzyne intermediate in the Bergman cycloaromatization of 2,3-diethynyl-1-methoxybenzene (1) before this intermediate undergoes either retro-Bergman ring opening or external H-atom abstraction. This process leads to the formation of a new diradical and renders the cyclization step essentially irreversible. Chemical and kinetic consequences of this phenomenon were investigated through the combination of computational and experimental studies.