Competing reaction channels

A second feature of the Y + CH3OH reaction that is common to many metal reactions is the presence of competing reaction channels, in this case YH2+H2CO and YOCH3+H. Time-of-flight spectra for both these products are shown in Fig. 11. The corresponding lab angular distributions and CM distributions used to fit the TOF spectra are shown in Fig. 12. 

The initial photochemical processes for CH4 are complex, with five competing reaction channels. The amount of each possible reaction, i.e. the number of products appearing in each reaction channel, is called the branching ratio and is a matter of... 

The extended (two-term) Grunwald-Winstein equation has been applied to the solvolyses of ethyl chloroformate (117) and ethyl chlorothioformate (118). For each substrate, there is evidence for two competing reaction channels. " Solvolysis... 

Finlayson-Pitts, B. J., M. J. Ezell, and C. E. Grant, Temperature Dependence of the OH + CINO Reaction Evidence for Two Competing Reaction Channels, J. Phys. Chem., 90, 17-19 (1986). 

Scheme 40 exemplifies a common situation in these systems which display competing reaction channels, i.e. fluoride displacement (a) as well as a rather unusual reaction (b and c) thought to occur via attack of the nascent F onto the neutral product276. 

As stated in Sect. 4.1.1, the critical energy, E0, is reached in an appearance energy measurement only if the inequality (21) is met. The term competitive shift refers to the fact that the number of ions available for reaction to give a particular product is depleted by competing reaction channels. In eqn. (21), this possibility is represented by the exponential term. If a reaction is the lowest energy pathway of an ion, competitive shifts should not be a significant factor. 

Selectivity on metal oxide catalysts is ultimately determined by complex intermolecular and surface-adsorbate interactions. Competing reaction channels are facilitated or hindered by the coordination geometry around metal cations, the ease of reduction of the surface, and the resulting stabilization of surface intermediates. The decomposition of relatively simple organic molecules like methanol and formic acid can be surprisingly complex, but attention to a few concepts may help to understand the reaction processes ... 

Results of early experiments on the kinetics of the reaction F + NO were analyzed under the assumption that FN02 is the sole reaction product.247 A possible occurrence of FONO isomeric reaction products was discussed in more recent studies.253,254 In a recent investigation by Pagsberg et al.254 the kinetics of the reaction F + N02 was studied with SF6 pressures of 5 - 1000 mbar. No evidence of the formation of the isomeric species FONO was found. The fluorine atoms (produced by pulse radiolysis of a SF6/NO2 mixture) are consumed by the competing reaction channels... 

We examined the reactions of C6H5 N with the series of carbonyl-containing molecules listed in Table II (10). Because H+ transfer is a competing reaction channel in some of these reactions, we factored out of the total rate constants that part due to carbonyl addition and radical fragmentation, and these rate constants were made relative to that for acetone these kreic=0 values are given in the middle column of Table II. 

Reaction mechanisms in the cyclobutane system were established in the usual manner using the ARL tandem mass spectrometer. For quasithermal-energy reactant ions impacted on cyclobutane at 190°C, the relative cross sections observed for competing reaction channels were as follows ... 

As mentioned above, in cyclotron production of radionuclides, the reaction cross section data play a very important role (for detailed discussion c Qaim 1982, 2001a). One needs the fiiU excitation function of the nuclear process to be able to calculate the yield with reasonable accuracy. Another important point is the number of competing reaction channels. At an incident projectile energy of 20 MeV, for example, about six reaction channels with significant cross sections may occur. It is imperative to know the cross sections of aU those processes. The demands on the data may thus be extensive. At small-sized cyclotrons, low energy reactions Kke (p,n), (p,a), (d,n), (d,a), etc., are used. At higher energies, on the other hand, (p,xn) reactions are commonly utilized. In some special cases, the (p,spall) process is applied. 

In attempts to establish information about the relative heats of formation of various fluorinated methyl ions, studies (33,34, 38,39) have been carried out examining the directions of fluoride and hydride transfer reactions between fluoromethyl ions and various fluorinated methanes. Some studies (38,39) have also included ions and molecules containing chlorine atoms. Although the more quantitative aspects of this work will be covered in detail later in our discussion of the thermochemistry of fluorinated ions, it is interesting to examine here the relative Importances of competing reaction channels, shown in Table III. It should be pointed out that at sufficiently high pressures, stabilization of the dimethyl fluoronium Ion intermediates of the reactions is observed ( ). 

For these aromatic compounds, the major competing reaction channel Is charge transfer, whl h accounts for 8 and 15% of the reactive encounters between CF3 and benzene or toluene, respectively. In toluene, the highly exothermic hydride transfer reaction ... 

Competing reaction channels can be prevented if an organometallic compound serves as the carbonucleophile and is irreversibly generated under aprotic conditions, i.e., generally in an ethereal medium. In this way, fluorocyclooctatetraene (99), a highlight in early dynamic nmr studies, was prepared from bromocyclooctatetraene through the lithium species, albeit in a yield of not more than 10% (Scheme 1-68). ... 

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