Alkyl radical dissociations

As the temperature of the system rises it is noted that the decays of the atomic species in the presence of excess alkene are no longer exponential, but show a biexponential decay. At high temperatures the rate of alkyl radical dissociation occurs on the same timescale as the addition reaction and equilibrium is established. The two portions of the biexponential decay are related to the rapid approach to equilibrium, which depends on the sum (A 34[C2H4] -I- k 34), followed by the slower diffusive loss of the equilibrated species from the observation region. Analysis of such data leads directly to third law determinations with the advantage of reduced systematic errors as only one experimental technique is used [71, 72]. 

EFFECT OF POTENTIAL ENERGY SURFACE PROPERTIES ON UNIMOLECULAR DYNAMICS FOR A MODEL ALKYL RADICAL DISSOCIATION REACTION H-C-C H + C=C... 

Dynamic features are manifested in the simplest alkyl radical dissociation... 

Energy distributions of alkyl radical dissociation products, as measured by crossed molecular beam and infrared chemiluminescence experiments, show deviations from the predictions of statistical theories such as the RRKM or phase space theory. In these experiments an atom X is added to a C=C double bond of an olefin RY to form... 

The degree to which allylic radicals are stabilized by delocalization of the unpaired electron causes reactions that generate them to proceed more readily than those that give simple alkyl radicals Compare for example the bond dissociation energies of the pri mary C—H bonds of propane and propene... 

The reaction rate of molecular oxygen with alkyl radicals to form peroxy radicals (eq. 5) is much higher than the reaction rate of peroxy radicals with a hydrogen atom of the substrate (eq. 6). The rate of the latter depends on the dissociation energies (Table 1) and the steric accessibiUty of the various carbon—hydrogen bonds it is an important factor in determining oxidative stabiUty. 

It was first suggested that the reaction of an alkyl halide with a nickel(I) Schiff base complex yields an alkylnickel(III) intermediate (Equation (56)). Homolytic cleavage of RBr to give an alkyl radical R and a nickel(II) complex (Equation (57)) or, alternatively, one-electron dissociative reduction leading to R (Equation (58)) are possible pathways.254 A mechanism based on the formation of R via dissociative electron transfer of Ni -salen to RX (Equation (59)) has also been proposed.255... 

The active enzyme abstracts a hydrogen atom stereospecifically from the intervening methylene group of a PUFA in a rate-limiting step, with the iron being reduced to Fe(II). The enzyme-alkyl radical complex is then oxidized by molecular oxygen to an enzyme-peroxy radical complex under aerobic conditions, before the electron is transferred from the ferrous atom to the peroxy group. Protonation and dissociation from... 

To perform the dissociation of the hydrocarbon to alkyl radicals with C—C bond scission, a hydrocarbon molecule should absorb light with the wavelength 270-370 nm. However, alkanes do not absorb light with such wavelength. Therefore, photosensitizers are used for free radical initiation in hydrocarbons. Mercury vapor has been used as a sensitizer for the generation of free radicals in the oxidized hydrocarbon [206-212], Nalbandyan [212-214] was the first to study the photooxidation of methane, ethane, and propane using Hg vapor as photosensitizer. Hydroperoxide was isolated as the product of propane oxidation at room temperature. The quantum yield of hydroperoxide was found to be >2, that is, oxidation occurs with short chains. The following scheme of propane photoxidation was proposed [117] ... 

The sulfonyl radical is unstable and dissociates via C—S bond back to the alkyl radical and sulfur dioxide. The rate constant of this reaction for the cyclohexylsulfonyl radical was calculated from the kinetic data on the chain decomposition of cyclohexylsulfonyl chloride [2]. This decay of cyclohexylsulfonyl chloride initiated by DCHP occurs according to the following chain mechanism [29,31] ... 

The phenoxyl radical has an increased electron density in the ortho- and pura-positions and adds dioxygen similar to alkyl radicals. However, the C—00 bond is weak in this peroxyl radical and back dissociation occurs rapidly. Therefore, the formation of quinolide peroxide occurs in two steps, which was studied for the 2,4,6-tris(l,l-dimethylethyl)phenoxyl radical [100,101],... 

An alkyl radical and a nitroxide radical exist in an equlibrium with the corresponding alkoxyamine as their coupling product (Eq. 57). Moad and Rizzardo [213] and Kazmaier et al. [214] independently estimated the effects of the structure of the alkyl group and the nitroxide on the dissociation energy of various alkoxyamines into the radicals by semiempirical molecular orbital calculations. The bond dissociation energies determined are summarized in Table 5 ... 

---