Dominant reaction pathway

In simple chemical systems, it is often possible to make a good first guess at the dominant reaction pathway [25-28]. An example of such a reaction is the chair-to-boat isomerization in cyclohexane. In that pathway, a clever combination of two torsion angles provides an excellent reaction coordinate for the isomerization reaction [29,30]. 

NMHC. A large number of hydrocarbons are present in petroleum deposits, and their release during refining or use of fuels and solvents, or during the combustion of fuels, results in the presence of more than a hundred different hydrocarbons in polluted air (43,44). These unnatural hydrocarbons join the natural terpenes such as isoprene and the pinenes in their reactions with tropospheric hydroxyl radical. In saturated hydrocarbons (containing all single carbon-carbon bonds) abstraction of a hydrogen (e,g, R4) is the sole tropospheric reaction, but in unsaturated hydrocarbons HO-addition to a carbon-carbon double bond is usually the dominant reaction pathway. 

The shape of the TPD spectra are different than they were for Ni(100), and the peak occurs at slightly higher temperatures, about 245 K. Also, in contrast to the trend observed on Ni(KW), on Ni(l 11) the peak in the methane TPD curve shifts to lower tenqjcratures as the surface coverage is increased. This trend is frequently associated with a reaction step that is higher than first order in reactant coverage. It is an important indication that the dominant reaction pathway is probably diffoent than it is on the (100) surface. 

Unlike the behavior over 0.2% platinum/alumina, the main features of the labeled product distributions obtained over 10% platinum/alumina and over platinum film catalysts (Tables VI and VII respectively) cannot be explained in terms of a single dominant reaction pathway via an adsorbed C6 cyclic intermediate. Again, parallel, multiple-step reaction pathways are involved. The results from 2-methylpentane-2-13C have been qualitatively accounted for (84) by the pathways... 

It can be observed that the initial rate of polymerization decreases and the autoacceleration peak is suppressed as the TED concentration is increased. The TED molecules generate dithiocarbamyl (DTC) radicals upon initiation. As a result, termination may occur by carbon-carbon combination which leads to a dead polymer and by carbon-DTC radical reaction which produces a reinitiatable ( living ) polymer. The cross-termination of carbon-DTC radicals occurs early in the reaction (with the carbon-carbon radical termination), and this feature is observed by the suppression of the initial rate of polymerization. As the conversion increases, the viscosity of the system poses mass transfer limitations to the bimolecular termination of carbon radicals. As has been observed in Figure 3, this effect results in a decrease in the ktCC. However, as the DTC radicals are small and mobile, the crosstermination does not become diffusion limited, i.e., the kinetic constant for termination of carbon-DTC radicals, ktCS, does not decrease. Therefore, the crosstermination becomes the dominant reaction pathway. This leads to a suppression of the autoacceleration peak as the carbon-DTC radical termination limits the carbon radical concentration to a low value, thus limiting the rate of polymerization. This observation is in accordance with results of previous studies (10) with XDT and TED, where it was found that when there was an excess of DTC radicals, the carbon radical concentration was lower and the cross-termination reaction was the dominant termination pathway. 

Ejection of dinitrogen from the triazoline adducts to form the related aziridines was promoted by ultraviolet irradiation (300 nm, benzene) and usually proceeded in excellent yield. An exception was found in the irradiation of the triazoline substrate 59, where cleavage of the cyclobutane ring occurred as the dominant reaction pathway to form the pyridazino norbomadiene 61 (and secondary photoproducts derived therefrom), together with the triazole-4,5-diester 62. A role for the pyridazine ring and the 2-pyridyl substituents in stabilising the diradical intermediate 60 has been proposed for this abnormal outcome (Scheme 8). 

Nonstationary behavior Platinum on YSZ exhibits pronounced hysteretic effects, suggesting that passage of current can alter either the kinetics of the reaction or the dominant reaction pathway itself. As we saw in section 3.6 (and will again in... 

Hammett s p = 1.07 in MeCN. The Ad -E route is the dominant reaction pathway, as revealed by the effects of the changes in the substituent, solvent, nucleophile and nucleofuge no competitive 5n1 reaction was observed. ... 

This development is a good example for the fine-tuning of a tailor-made catalyst based on mechanistic considerations and theoretical calculations The mechanistic scheme for 52 postulates the dissociation of a phosphine ligand as the key step in the dominant reaction pathway (Scheme 11). ... 

As discussed in Section 6.9 1, 3-dienes and dienophiles in which multiple bonds are not activated by electron-withdrawing or electron-releasing substituents fail to undergo cycloaddition except under the most severe conditions. Particular difficulty is encountered in the cycloaddition of two unactivated species since homodimerization can be a competitive and dominant reaction pathway. The use of transition-metal catalysts, however, has proved to be a valuable solution. Complexation of unactivated substrates to such catalysts promotes both inter- and intramolecular cycloadditions. Consequently, the cycloaddition of such unactivated compounds, that is, simple unsubstituted dienes and alkenes, catalyzed by transition metals is a major, important area of study.655 In addition, theoretical problems of the transformation have frequently been addressed in the more recent literature. 

Figure 10 shows the v3 region of C02 spectra from a single crystal of UP photolyzed at 20 K and warmed in stages through the temperatures at which EPR spectra showed the intermediates depicted in Figure 9. The final spectrum (lOd) was taken after warming to 140 K to destroy the radical pairs and then recooling to 20 K. The correspondence between EPR and IR spectral changes demonstrates that both techniques reveal the dominant reaction pathway.

As with allylsilanes, substitution is found to be the dominant reaction pathway (equation 102)169. 

Subsequent efforts to inhibit photochemical yellowing of lignin-containing paper were based on what was believed to be the dominant reaction pathways to yellow chromophores the formation of phenoxy free-radicals by direct excitation of phenolic groups and the abstraction of phenolic hydrogen by the aromatic carbonyl triplet excited state, as shown by reactions 1 and 2 in Scheme 1. The approaches used by researchers were modification of lignin by ... 

Anodic treatment of 3,5-lutidine (35) on BDD electrodes also turned out to be challenging. Only traces of the desired pyridine-3,5-dicarboxylic acid (36) could be detected. As electrolyte a dilute NaOH solution was employed. The mineralization and decomposition seem to be the dominant reaction pathways (Scheme 16). 

The observed overabundance of deuterated species in molecular clouds and outer disks compared to the measured interstellar D/H ratio of 10 5 is well established. A classical isotopic deuterium fractionation is possible at low temperatures of 10-20 K owing to disbalance between forward and reversed reaction efficiencies H+ + HD 5 H2D+ + H2 + 232K (e.g. Millar et al. 1989 Gerlich et al. 2002). The temperature dependency in an isotope exchange reaction is a consequence of the zero-point vibrational energy difference for the isotopically substituted molecules (Bigeleisen Mayer 1947 Urey 1947). This leads to an elevated ratio ol H2t)+/H compared to HD/H2, which is quickly transferred into other molecules by ion-molecule reactions (see e.g. Roberts Millar 2000 Roberts et al. 2003). For example, the dominant reaction pathway to produce DCO+ is via ion-molecule reactions of CO with H2D+. In disks it results in a DCO+ to HCO+ ratio that increases with radius owing to the outward decrease of temperature (Aikawa Herbst 2001 Willacy 2007 Qi et al. 2008). 

In the absence of such functionality, the formation of the fused 7-azabicyclo[4.2.0]octa-2,4-dienes, for example, 1065, becomes the dominant reaction pathway. A reasonable mechanism to explain its formation is shown in Scheme 207 <1996JOC2305>. 

Taylor et al. [104] investigated the reaction of hydroxyl radicals with acetaldehyde in a wide temperature range using a quantum RRK model to describe the competition between addition and abstraction. They conclude that different reaction mechanisms occur, depending on the temperature, and that OH addition followed by CH3 elimination is the dominant reaction pathway between 295 and 600 K. Moreover, they claimed that the H-atom elimination pathway is largely insignificant, except possibly at the lowest temperatures. 

For NO t > 0-5 ppb (typical of urban and polluted rural sites in the eastern USA and Europe) Equations (3) and (4) represent the dominant reaction pathways for HO2 and RO2 radicals. In this case the rate of ozone formation is controlled largely by the rate of the initial reaction with hydrocarbons or CO (Equations (1) and (2)). Analogous reaction sequences lead to the formation of various other gas-phase components of photochemical smog (e.g., formaldehyde (HCHO) and PAN) and to the formation of organic aerosols. 

The mechanism of the reaction CH2C(CH3)0 + NO is very similar to that discussed for CH2CHO + NO. The formation of 0N-CH2C(CH3)0 is the dominant reaction pathway. The fall-off behavior of the reaction system was also analyzed within the Troe formalism.17"23 The fall-off curves of Delbos et al.m were constructed on the basis of their experimental results. The best fit leads to results corresponding to the fall-off parameters... 

Despite these significant advances, allylic C H insertion under these conditions remains problematic, with competing olefin aziridination often observed as the dominant reaction pathway (Scheme 12.5) [29]. 

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