Steps competing

Kinetic examination of the methane yield shows behavior quite similar to that of methyl radical a pressure dependent yield of 0.406 molecule/100 e.v., a pressure independent yield of 0.126 molecule/100 e.v., and a rate constant ratio of kq/kf = 1.5 X 106 mole-1 cc. for the competing steps. 

The value of Fj depends on the rate ratio of competing steps (79) and (63). The smaller this ratio, the greater is the value of Fj. Therefore, any factor... 

Although reaction (3.61) is endothermic and its reverse step reaction (-3.61) is faster, the competing step reaction (3.63) can be faster still thus the isomerization [reaction (3.61)] step controls the overall rate of formation of ROO and subsequent chain branching. This sequence essentially negates the extent of reaction (-3.48). Thus the competition between ROO and olefin production becomes more severe and it is more likely that ROO would form at the higher temperatures. 

That additives affect the rich limit more than the lean limit can be explained by the important competing steps for possible chain branching. When the system is rich [reaction (3.23)],... 

An example of such a third competing step is the reaction of the intermediate aryl radical with H-atom donors present in the reaction medium, possibly the solvent itself. Indeed, aryl radicals are good H-atom scavengers and reduction of aryl halides is often carried out in organic solvents such as acetonitrile (ACN), lV,Af-dimethylformamide (DMF), dimethyl sulphoxide (DMSO), and ethers, that are good H-atom donors... 

If two competing steps in multiple reactions have rate constants and /c2, then the relative rates of these steps are given by... 

Tables I and II include data for the co-oxidations of styrene and butadiene in chlorobenzene and ferf-butylbenzene solutions, as well as with no added solvent. These solvents were chosen because the rate of oxidation of cyclohexene varies significantly in them at the the same rate of initiation (6). There is a variation in the over-all rate of oxidation under these solvent conditions, but there appears to be no significant difference in the measured ra and rb (Table II). If the solvent does affect the propagation reaction in autoxidation reactions, it affects the competing steps to the same degree.

B. E. Conway and J. O M. Bockris, Proc. Roy. Soc. London 248A 394 (1958). Calculations on competing steps in electrochemical metal deposition. 

From my estimates on the thermodynamic properties of peroxy and polyoxide molecules and radicals, we can estimate that the bond dissociation energy of the tetroxide is about 5 kcal. Thus, at room temperature, or even at dry ice temperature, the tetroxide is extremely unstable and should redissociate into the more stable (from a thermodynamic point of view) peroxy radicals. The competing step would be a concerted decomposition into an RO and an R03 (Step 14) radical, which would be uphill by 20 kcal., or else a concerted decomposition into 2 RO radicals and 02 (Step 14 ). The latter is almost thermoneutral. If we take the current data at face value, it provides, from the reported activation energy at least, strong evidence that the propagating interaction of two alkylperoxy radicals proceeds in a concerted fashion. 

When the results of Sections III-A and III-B are considered, i.e., (a) all the ground states are on the same surface, and (b) the ground states of some isomers interact with the lowest excited state of the parent molecule, Figure 1 can be redrawn as Figure 2 from which the competing steps can be deduced. 

The initiation step is laser photoexcitation of the molecule to yield a caged geminate pair of atomic iodine radicals. The two competing steps open to the caged pair are 1) separative diffusion to yield atomic iodine radicals and 2) recombination to give molecular iodine. 

The term competing steps is used if one and the same component participates as reactant in more than one step of the pathway or network. [The idea is that such steps "compete" with one another for the reactant they have in common an alternative term is series-parallel steps.] The simplest such case is [21]... 

In such a pathway, aldehyde is a reactant in both the first and second steps. Accordingly, this is a typical case of competing steps, discussed in Section 5.5. With Xf at trace level, the general formula for simple pathways (eqns 6.4 to 6.6.) gives... 

Other options lor the —50 -retention limit pathway have drawbacks, (a) l or a triplet or mixed singlet-triplet version of pathway 1-3-4, it is not be clear why spin factors and intersyslem crossing would operate in the same way for reductions by metals or electrodes ax homogeneous reductions by Naph-. yet the presumed limit is found for these types of reactions, (b) Diffusion control of competing steps 2 and 3 could describe reductions by MNaph and solvated electrons but, again, it is not clear how- this would apply to reductions by metals and electrodes. If this competition were not diffusion controlled, [hen a signilicunt step 2 would introduce halogen effects on the limit. 

Thus, data for reductions of I halo-1-methyl 2.2-diphcnyleyelopropancs appear to conform rationally to a mechanism with competing initial steps I and 6 and competing steps 2 and X subsequent to 1 (Scheme 7.1). More reducing power favors I relative to 6 and 3 relative to X. When the halogen effect is determined by the 3 X competition, retention varies in the older I Ur > Cl > I-. and w hen it is determined by the l-b competition, it varies oppositely. 

The same arguments can be applied to other energetically facile interconversions of two potential reactants. For example, some organic molecules can undergo rapid proton shifts (tautomerism) and the chemical reactivity of the two isomers may be quite different. However, it is not valid to deduce the ratio of two tautomers on the basis of subsequent reactions that have activation energies greater than that of the tautomerism. Just as in the case of conformational isomerism, the ratio of products formed in subsequent reactions is not controlled by the position of the facile equilibrium, but by the of the competing steps. 

The evaluation of absolute rate coefficients of elementary reactions (hereafter referred to only as rate coefficients) is one of the most important steps in the kinetic analysis. Comparison of such values with our general chemical knowledge of radical reactions serves first as a check on the kinetic analysis and second, if shown to be reliable, they may be used in the kinetic analysis of other systems. It is often possible and useful to evaluate the rate coefficients directly in oxidation reactions as well as in much more simplified systems where many of the competing steps have been eliminated. 

In a crystalline host, the potential curves as drawn in Figure 5 describe the total energy, complex and environment. If vibrational relaxation within an electronic state is faster than other competing steps, then photophysical and photochemical processes occur in thermally equilibrated populations. Figure 5 is also applicable for a rigid, noncrystalline medium, but as the solvent melts and solvent relaxation takes place during the excited-state lifetime, a more complex representation is required. 

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