Different Reaction Rates

Ohashi, Crystalline State Photoreactions Direct Observation of Reaction Processes and Metastable Intermediates, DOI 10.1007/978-4-431-54373-2 6, Springer Japan 2014 

A KBr disc which contained 1 % of the powdered sample of I, II, or III was exposed to a xenon lamp. The change of the infrared spectra was measured at a constant interval of 10 min. The decrease of the absorption band at 2,250 cm due to the 2-ce group within 40 min was explained by first-order kinetics for each sample as shown in Fig. 6.1. The rate constants were obtained by least-squares fitting. They are 1.88, 1.33, and 2.76 x 10 s for I, II, and III, respectively. 

In order to explain the differences in reactivity among the three crystals, the reaction cavities for the 2-ce groups were drawn and the volumes were calculated. The cavity volumes are 10.6, 8.6, and 15.3 A, for I, II, and III, respectively. The most reactive crystal III has the largest cavity, whereas the least reactive crystal of II has the smallest cavity. Since the 2-ce groups in II and III are surrounded by the host amines, the difference in reactivity is caused by the packing of the host amine. This suggests that the reactivity in the solid-state reactions can be controlled by designing the host amine, even if the control of the crystal structure may be impossible. 

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Selectivity The analysis of closely related compounds, as we have seen in earlier chapters, is often complicated by their tendency to interfere with one another. To overcome this problem, the analyte and interferent must first be separated. An advantage of chemical kinetic methods is that conditions can often be adjusted so that the analyte and interferent have different reaction rates. If the difference in rates is large enough, one species may react completely before the other species has a chance to react. For example, many enzymes selectively cat-... 

Another means of resolution depends on the difference in rates of reaction of two enantiomers with a chiral reagent. The transition-state energies for reaction of each enantiomer with one enantiomer of a chiral reagent will be different. This is because the transition states and intermediates (f -substrate... f -reactant) and (5-substrate... R-reactant) are diastereomeric. Kinetic resolution is the term used to describe the separation of enantiomers based on different reaction rates with an enantiomerically pure reagent. 

KR is the total or partial separation of two enantiomers from a racemic mixture [5]. KR is based on the different reaction rates of the enantiomers with a chiral molecule (a reagent, a catalyst, etc). In the ideal case, the difference in reactivity is large, and one of the enantiomers reacts very fast to give the product, whereas the other does not react at all (Figure 4.1). 

Figure 8 gives the calculated isocyanate decrease during curing at different reaction rate constants and at a water concentration of 1.4Z (m/m). 

In order to achieve a true comparison between both catalytic systems, colloidal and molecular, which display very different reaction rates, a series of experiments were carried out with the homogeneous molecular system, decreasing the catalyst concentration in the studied allylic alkylation reaction. The reaction evolution is monitored taking samples at different reaction times and analysing each of them by NMR spectroscopy (to determine the conversion) and HPLC chromatography with chiral column (to determine the enantioselectivity of I and II). For molecular catalyst systems, the Pd/substrate ratio was varied between 1/100 and 1/10,000. For the latter ratio, the initial reaction rate was found comparable to that of the colloidal system (Figure 2a), but interestingly the conversion of the substrate is quasi complete after ca. 100 h in... 

The decline of the DeNO. curve for N02 fractions above 50% is much stronger than the incline below 50% due to the different reaction rates of standard- and N02-SCR. The latter is much slower than the fast-SCR reaction and even slower than the standard-SCR reaction. The promoting effect of N02 levels off above 350°C, because the rate constants of standard-, fast- and N02-SCR reactions are converging at higher temperatures. 

In the case of other parallel reactions with different reaction rate expressions, similar analyses can be used to determine the influence of various reactant concentrations on the selectivity of a proposed process. Such analyses would lead to the following generalization, which is useful in considerations of parallel reactions where the reactant concentration level influences the product distribution. 

In the presence of 10 pM peroxide, the yields of H2, H202, and of H + eh are about the same in neutral and 0.4 M acid solutions. Since H atoms produced by the reaction of acid with hydrated electrons have different reaction rates and sequences of reaction, a much greater difference of the... 

Atienza et al. [657] reviewed the applications of flow injection analysis coupled to spectrophotometry in the analysis of seawater. The method is based on the differing reaction rates of the metal complexes with 1,2-diaminocycl-ohexane-N, N, N, A/Metra-acetate at 25 °C. A slight excess of EDTA is added to the sample solution, the pH is adjusted to ensure complete formation of the complexes, and a large excess of 0.3 mM to 6 mM-Pb2+ in 0.5 M sodium acetate is then added. The rate of appearance of the Pbn-EDTA complex is followed spectrophotometrically, 3 to 6 stopped-flow reactions being run in succession. Because each of the alkaline-earth-metal complexes reacts at a different rate, variations of the time-scan indicates which ions are present. 

As indicated in Figure 3.4, the covalent bond, i.e., two common shared electrons, between two carbon atoms in the complex molecule is cleaved when initiated by the exoenzymes. The highly reactive intermediates that are formed react and produce new and stable bonds resulting in two new molecules that may undergo further hydrolysis. Hydrolysis is, thus, an important initial step in the transformation of complex organic matter present in a form that cannot directly be used at substrate. Hydrolysis is a process that—with different reaction rates — proceeds under aerobic, anoxic and anaerobic conditions. It is important to note that hydrolysis takes place without use of an electron acceptor. 

Considering more species that are involved in equilibrium reactions makes th number of unknowns larger, therefore, but an equation is added to the system along with each new unknown. That is not the case for species that participate only in irreversible reactions. Consider another structural form of the solid, S, with a different reaction rate. 

Several conclusions can be reached from these data. The first is that on a per mole basis, the quaternary ammonium salt is the most favorable catalyst for this reaction. Among the other compounds, the catalytic activity of 1.5 mole-% of crown, PEG, or PEG-MME are similar. If equal weights of PEG-400 and PEG-3400 are used, quite different reaction rates are observed. This is because each polymer chain is capable of transporting one cation across the phase boundary at a time. The ratio of molecular weights is 8.5, so there are 8.5 more catalysts available in the PEG-400 catalyzed reaction than in the one involving the higher molecular weight compound. The actual ratio of rates for these two processes is 12.5, or nearly the expected value. 

To convert geologic samples to a suitable form for analysis, many different chemical preparation techniques must be used. These diverse techniques all have one general feature in common any preparation procedure providing a yield of less than 100% may produce a reaction product that is isotopically different from the original specimen because the different isotopic species have different reaction rates. 

However, copper alkoxides with longer chains appear to be more soluble in their parent alcohol. S. Shibata et al. (20) have used the n-butoxides of Y, Ba and Cu dissolved in n-butanol and hydrolyzed with water. They obtain a precipitate of oxides that is composed of a very fine submicron powder that readily sinters starting above 250°C. However, the different reaction rates for the hydrolysis and the precipitation of the three different cations lead to cationic segregation. 

Since the neutralization reaction of an amine with nitrous acid may be presumed to be instantaneous while the nitrosation of an amine proceeds at a slower rate, further investigations would be of interest to elucidate the problem of whether nitrite salt formation is a necessary preliminary step to the formation of iV-nitrosoamines or whether the covalent product forms independently. In the latter case, there would be competitive reactions of significantly different reaction rates and mechanisms. The yield of A-nitrosoamines may be influenced by the extent to which the more water-soluble nitrite salts may be present in the course of a preparation. 

Metal ion catalysis of salicyl phosphate hydrolysis is much more complicated than that of Sarin, since the former substrate can combine with metal ions to give stable complexes, and some of the complexes formed do not constitute pathways for the reaction. In addition the substrate undergoes intramolecular acid-base-catalyzed hydrolysis which is dependent on pH because of its conversion to a succession of ionic species having different reaction rates. Therefore a careful and detailed equilibrium study of proton and metal ion interactions of salicyl phosphate would be required before any mechanistic considerations of the kinetic behavior in the absence and presence of metal ions can be undertaken. 

Moreover, each of the chemical and electrochemical reactions can have different reaction rates and reversibilities. All of them are reflected in cyclic voltammograms. If we measure cyclic voltammograms of an electrode reaction, changing parameters such as potential range, voltage scan rate, temperature, electrode material and solution composition, and analyze the voltammograms appropriately, we can obtain information about the electrode reaction. However, except for cases where the electrode process is very simple, it is not easy to analyze the cyclic voltammograms appropriately. 

II(S)) and/or to a different reaction rate of the two diastereomeric 7r-olefin complexes to the corresponding diastereomeric alkyl-rhodium complexes (VI(s) and VI(R)). For diastereomeric cis- or trans-[a-methylbenzyl]-[vinyl olefin] -dichloroplatinum( II) complexes, the diastereomeric equilibrium is very rapidly achieved in the presence of an excess of olefin even at room temperature (40). Therefore, it seems probable that asymmetric induction in 7r-olefin complexes formation (I — II) cannot play a relevant role in determining the optical purity of the reaction products. On the other hand, both the free energy difference between the two 7r-olefin complexes (AG°II(S) — AG°n(R) = AG°) and the difference between the two free energies of activation for the isomerization of 7r-com-plexes II(S) and II(R) to the corresponding alkyl-rhodium complexes VI(s) and VI(R) (AG II(R) — AG n(S) = AAG ) can control the overall difference in activation energy for the formation of the diastereomeric rhodium-alkyl complexes and hence the sign and extent of asymmetric induction. 

In summary, it is apparent that the application of dielectric heating to chemical reactions may result in different reaction rates and product distributions, and chemists... 

It is reasonable to consider that in titanium silicate-catalyzed reactions the oxidizing species also acts as an electrophile. The different order of reactivity of the C4 olefins in the presence of titanium silicates relative to that observed with soluble catalysts must therefore arise from the fact that alkyl substitution at the double bond is responsible not only for inductive effects, but also for increases in the size and the steric requirements of the molecules. Since the rates of diffusion of the different butenes cannot be the cause of the different reaction rates, a restricted transition-state selectivity must be operating. 

Absorbance studies show considerable overlap of the different spectral bands in metal-ethylenediamine solutions. It appears likely that the simultaneous first-order processes in reducing water and other solutes result from different reaction rates of the various species. With this assumption, and using Beer s Law for the individual species, one obtains... 

Effect of surfactants on stability. Many organic reactions have been found to be accelerated or inhibited in the presence of micellar media. The apparent reaction rates are altered in micellar solutions because of the distribution of substrate between the micellar and aqueous bulk phases in which different reaction rates occur (Fendler and Fendler, 1975). 

Consider a mixture of species, Xi, X2,. .., X , which are undergoing common chemical transformation to products P, but with different reaction rates. 

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