Rate-determining step chain reactions

However, while it is generally accepted that the rate of radical-radical reaction is dependent on how fast the radical centers of the propagating chains (Pp and Pj ) come together, there remains some controversy as to the diffusion mechanism(s) and/or what constitutes the rate-determining step in the diffusion process. The steps in the process as postulated by North and coworkers30 3" arc shown conceptually in Scheme 5.5. 

To answer the question whether the ds-transisomerization of the bridged polypeptides with a Ala-Gly-Pro sequence represents the rate-determining step, the following experiment was carried out The polypeptide with a chain length n = 8 was denaturated in a rapid reaction with a temperature jump from 9.2 to 30 °C and subjected to renatura-tion at 9.2 °C after an incubation time of 25 s. In a second and a third experiment, the incubation in the coiled state was prolonged respectively to 75 and 125 s. It could be observed that the amplitude of the rapid phase depends on the time that lapses between the denaturation and renaturation (Fig. 32). 

Reactions with molecular species above the arrow e.g. RIO) involve subsequent reactions with these species to produce the indicated products. In most cases the reactants shown to the left of the arrow participate in the slowest or rate-determining step]. The CH3O radical formed in Rll then follows reaction R7. The H02 radical formed in RIO is the other member of the family and is linked with HO in a variety of chain reactions. These radicals are produced following HO attack on hydrocarbons or by photodissociation of oxygenated hydrocarbons such as formaldehyde (RIO) and acetaldehyde ... 

However, the kinetics of the reduction are often simplified in that the first step (production of H02 ) is rate-determining the ensuing reactions, however, may confer radical-chain behaviour on the system. Hydrolysis of the reductant can also modify the kinetics. 

The apparent lack of dependence of the propagation reaction on the surface area of the sodium suggests that the reaction of a chlorine ended chain with sodium is probably fast and not the rate determining step. The rate determining process is probably the reaction of the sodium ended chain with the dichloride. This latter reaction is presumably not on the sodium surface because of the lack of dependence on the surface area. This is supported by the observation that if the sodium is allowed to settle part way through the reaction most of the polymer appears to be in the solution and not absorbed on the sodium surface via the longlived active chain ends. 

The rate of dissociation has been measured by oxygen uptake in the presence of an inhibitor of chain reactions as in the case of hexaaryl-ethanes. Since the uptake of oxygen obeys the same kinetic law, it is a reasonable extrapolation to suppose that here too the rate-determining step is a dissociation into radicals. When one of the phenyl groups in triphenylmethyl is replaced by a cyclohexyl group, the rate of dissociation of the ethane is reduced by a factor of 170.38 Some dissociation rate parameters are given in Tables III A and B. 

Radicals are also formed in solution by the decomposition of other radicals, which are not always carbon free radicals, and by removal of hydrogen atoms from solvent molecules. Because radicals are usually uncharged, the rates and equilibria of radical reactions are usually less affected by changes in solvent than are those of polar reactions. If new radicals are being made from the solvent by hydrogen abstraction, and if the new radicals participate in chain reactions, this may not be true of course. But even in cases of non-chain radical reactions in which no radicals actually derived from the solvent take part in a rate-determining step, the indifference of the solvent has perhaps been overemphasized. This will be discussed more fully when radical and polar reactions are compared in Chapter XII. 

Although UGTs catalyze only glucuronic acid conjugation, CYPs catalyze a variety of oxidative reactions. Oxidative biotransformations include aromatic and side chain hydroxylation, N-, O-, S-dealkylation, N-oxidation, sulfoxidation, N-hydroxylation, deamination, dehalogenation and desulfation. The majority of these reactions require the formation of radical species this is usually the rate-determining step for the reactivity process [28]. Hence, reactivity contributions are computed for CYPs, but a different computation is performed with the UGT enzyme (as described in Section 12.4.2). 

In the above chain reaction, the first step is the slow step and the second is the rapid step. The equilibrium constant for Reaction 2-8 is iCs = 0.00287, and the forward rate constant is lcgf = 0.043 s. The equilibrium constant for Reaction 2-132 is iCi32 = 10 and the rate constant is Ci32f = 10 s. Hence, the first step is the rate-determining step. Using the steady-state concept, the reaction rate law is (ignoring the backward reaction of Reaction 2-132) ... 

In Reaction Scheme 2 the acceleration of co-oxidation by DSBPD arises from >NO radical production by direct interaction of the amine with oxygen in Reaction 8, together with the introduction of a pair of fast steps (Reactions 9 and 10) which bypass the rate-determining step of the main propagation chain (Reaction 4 of Scheme 1). The main propagation chain has now become a four-step chain consisting of Reactions 5, 6, 9, and 10. 

Hence, (V111-58) is the rate-determining step for the ClO-Cl catalytic cycle. The chain-terminating step is the reaction with methane to form HCI... 

In the reaction of I in the steady stage, when the concentration of olefin is low, the slow step involves oxygen. In effect there must be a chain reaction because formation of a complex, addition and elimination reactions, and other possible steps must precede the rate-determining step. Thus the isolatable complexes may be involved only in initiation steps that produce a more reactive palladium species. 

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