Homolytic dissociation, rate constant

By studying a series of complexes in which the various substituents on the alpha-carbon atom are varied, we can look at the change in the magnitude of the rate constant for homolytic dissociation as a function of these substituents. The values... 

The LFP studies of the reaction of the A-methyl-A-4-biphenylylnitrenium ion with a series of arenes showed that no detectable intermediate formed in these reactions. The rate constants of these reactions correlated neither with the oxidation potentials of the traps (as would be expected were the initial step electron transfer) nor with the basicity of these traps (a proxy for their susceptibility toward direct formation of the sigma complex). Instead, a good correlation of these rate constants was found with the ability of the traps to form n complexes with picric acid (Fig. 13.68). On this basis, it was concluded the initial step in these reactions was the rapid formation of a ti complex (140) between the nitrenium ion (138) and the arene (139). This was followed by a-complex formation and tautomerization to give adducts, or a relatively slow homolytic dissociation to give (ultimately) the parent amine. 

Cobalt-Carbon Bond Homolysis Studies with coenzyme Bi2 model compounds and TEMPO as a radical trap have allowed the determination of rate constants and activation parameters, which in turn led to estimates of the Co-C bond dissociation energies (BDEs). The key role of the homolytic Co-C cleaving step in these reactions was established by the following observations (Co11) is produced addition of external... 

The initiation step consists of two reactions in series. The first is the production of free radicals, which can be accomplished in many ways. The most common method, however, involves the use of a thermolabile compound, called an initiator (or catalyst), which decomposes to yield fre Radicals when heated. Thus, the homolytic dissociation of an iiiitiator I yields a pair of radicals R, as shown by Eq. (6.3), where kd is the rate constant for initiator dissociation at the particular temperature. Its magnitude is usually of the order of 10 -10 s (Being derived from the initiator, R is referred to as an initiator radical and often as a primary radical.) The second step of the initiation process is the addition of the radical R to a monomer molecule as shown in Eq. (6.4), where RM is the monomer-ended radical containing one monomer unit and an end group R. The rate constant for the reaction is ki. For a vinyl monomer, this second step involves opening the r-bond to form a new radical ... 

A more detailed analysis of the radical mechanisms has been presented . Generally, all three processes show first-order kinetics but Ej reactions do not exhibit an induction period and are unaffected by radical inhibitors such as nitric oxide, propene, cyclohexene or toluene. For the non-chain mechanism, the activation energy should be equivalent to the homolytic bond dissociation energy of the C-X bond and within experimental error this requirement is satisfied for the thermolysis of allyl bromide For the chain mechanism, a lower activation energy is postulated, hence its more frequent occurrence, as the observed rate coefficient is now a function of the rate coefficients for the individual steps. Most alkyl halides react by a mixture of chain and E, mechanisms, but the former can be suppressed by increasing the addition of an inhibitor until a constant rate is observed. Under these conditions a mass of reliable reproducible data has been compiled for Ej processes. Necessary conditions for this unimolecular mechanism are (a) first-order kinetics at high pressures, (b) Lindemann fall-off at low pressures, (c) the absence of induction periods and the lack of effect of inhibitors and d) the absence of stimulation of the reaction in the presence of atoms or radicals. 

The hydrogenolysis reaction proceeds at a near collisional rate, but the activation of methane, the reverse reaction, does not proceed at a measurable rate in the FTICR. Thus the equilibrium constant (=kf/kj.) for Equation 1 is > 1, and the product Zr-H homolytic bond dissociation enthalpy, D(Cp2Zr -H), is therefore greater than D(Cp2Zr -CH3) since D(H-H) = D(H3C-H) =... 

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