Reaction mechanisms competing

One also obtains analogous findings with trace-crossing effects for the electropolymerization of thiophene and pyrrole. This cannot be explained by a simple linear reaction sequence, as presented in Scheme I, because it indicates competing homogeneous and heterogeneous electron transfer processes. Measurements carried out in a diluted solution of JV-phenylcarbazole provide a more accurate insight into the reaction mechanism (Fig. 2). 

Protonation of the enolate ion is chiefly at the oxygen, which is more negative than the carbon, but this produces the enol, which tautomerizes. So, although the net result of the reaction is addition to a carbon-carbon double bond, the mechanism is 1,4 nucleophilic addition to the C=C—C=0 (or similar) system and is thus very similar to the mechanism of addition to carbon-oxygen double and similar bonds (see Chapter 16). When Z is CN or a C=0 group, it is also possible for Y to attack at this carbon, and this reaction sometimes competes. When it happens, it is called 1,2 addition. 1,4 Addition to these substrates is also known as conjugate addition. The Y ion almost never attacks at the 3 position, since the resulting carbanion would have no resonance stabilization " ... 

The reaction is similar to the Diels-Alder (in action, not in scope), and if dienes are involved, the latter reaction may compete, though most alkenes react with a diene either entirely by 1,2 or entirely by 1,4 addition. Three mechanisms can be proposed analogous to those proposed for the Diels-Alder reaction. Mechanism... 

Fig. 13. Proposed reaction mechanism for ACS. The reaction involves the sequential assembly of acetyl-CoA from a carbonyl, methyl, and CoA. We favor a Ni(l) nucleophile to form a catEdytically competent paramagnetic M-CO complex, but see text for discussion of Em alternative mechanism.

Frankcombe, K. E., Cavell, K. J., Knott, R. B., Yates, B. F., 1997, Competing Reaction Mechanisms for the Carbonylation of Neutral Palladium(II) Complexes Containing Bidentate Ligands a Theoretical Study ,... 

Aliphatic substrates that contain a good leaving group and an a-CH group may undergo reactions by competing nucleophilic substitution and elimination pathways (Scheme l).1 A stepwise mechanism is favored for these reactions... 

Benzoyl peroxide appears to decompose entirely by the radical mechanism, the reaction being rather insensitive either to solvent changes or to the addition of acid catalysts. The unsymmetrical peroxide, -methoxy-/> -nitrobenzoyl peroxide, behaves quite differently. It will decompose either by the polar mechanism or by the radical mechanism.821 The radical mechanism prevails in benzene and the acids produced are -nitrobenzoic and anisic in equal amounts. In the more polar solvents anisic acid is formed to a lesser extent than is >-nitrobenzoic acid, because the carboxy inversion reaction (rearrangement) competes successfully. The reaction is subject to acid catalysis... 

The interpretation given for Ir Cl9) involves two competing reaction mechanisms — one involves CO adsorbed on Ir and the other involves CO adsorbed over oxygen on Ir. 

Other common approaches to control the stereoselectivity of glycosylation will be discussed in the subsequent sections. In addition to the apparent complexity of the glycosidation process, there are other competing processes that cannot be disregarded. These reactions often cause the compromised yields of the glycosylation products and further complicate the studies of the reaction mechanism. 

The production of ozone in power-plant plumes has been suggested to explain ozone spatial distributions in nonurban areas.Comparison of oxidation mechanisms competing for sulfur dioxide suggests that three reactions—... 

The knowledge of thermochemistry is also important in early parts of mechanism development. For example, by examining the heats of reaction of competing elementary processes, unlikely reaction paths can be identified a priori and eliminated from further consideration. To illustrate this, consider the following simple unimolecular decomposition processes for CH3CI ... 

As pointed out earlier, CVD is a steady-state, but rarely equilibrium, process. It can thus be rate-limited by either mass transport (steps 2, 4, and 7) or chemical kinetics (steps 1 and 5 also steps 3 and 6, which can be described with kinetic-like expressions). What we seek from this model is an expression for the deposition rate, or growth rate of the thin film, on the substrate. The ideal deposition expression would be derived via analysis of all possible sequential and competing reactions in the reaction mechanism. This is typically not possible, however, due to the lack of activation or adsorption energies and preexponential factors. The most practical approach is to obtain deposition rate data as a function of deposition conditions such as temperature, concentration, and flow rate and fit these to suspected rate-limiting reactions. 

Reactions of powerful alkyllithiums with halo pyridines, quinolines, and diazines may lead to nucleophilic substitution (by addition-elimination or hetaryne mechanisms), ring opening, halogen-scrambling, and coupling reactions, which compete with the desired DoM process. 

As indicated in Table 13.7,1,2-dibromoethane (BrCH2-CH2Br) and 1,1,1-trichloro-ethane (CH3-CC13) are examples in which both hydrolysis and elimination are important. If in such cases the reactions occur by SN2 and E2 mechanisms, respectively, the ratio of the hydrolysis versus elimination products should vary with varying pH and temperature, since the two competing reactions likely exhibit different pH and temperature dependencies. On the other hand, if the reaction mechanisms were more SN1- and El-like, a much less pronounced effect of temperature or pH on product formation would be expected, since the rate-determining step in aqueous solution may be considered to be identical for both reactions ... 

The assumption of fast chemistry is useful in systems with diffusion limitation. However, if the chemistry is kinetically limited or if the chemistry involves competing product channels, only finite rate chemistry provides a good representation. Finite rate chemistry can be represented on different levels of complexity, ranging from a single global reaction to a detailed reaction mechanism involving perhaps hundreds of species. These approaches are described in more detail in the following sections. 

Reaction order treatment of complex and competing reaction mechanisms... 

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