Smith rate-determining step

The interpretation of the kinetics is based on the mechanisms proposed by Schechter et al.55) and Smith 57), for the reaction of secondary amines with epoxides, extended to include the primary amine reaction. The rate-determining step is assumed to be the reaction of amine, epoxide and hydroxyl or other proton-donor species, HX, to form a termolecular complex, Eq. (3-13). The proposed reaction scheme is ... 

The hydrolysis of p-trifluoromethylphenyldimethylethoxysilane was carried out at four different acid concentrations and the rate constants were calculated (Table 8). If the reaction is first order in acid, the rate constant should not change as shown in Table 8. Each of the alkoxysilanes was hydrolyzed with a ten-fold excess of water. If the rate equation is correct, water does not react until after the rate determining step and should not affect the rate constants (Table 9).- Again, this is the case. If the mechanism proceeded through a pentavalent state such as Smith suggested, we would expect to see water involved in the rate equation. At this point, Jada s mechanism seems more likely. 

The three routes are not always kinetically distinguishable. Silversmith and Smith (1958), for example, mentioned that the second-order kinetics in the reaction of l,l-diphenyl-2-fluoroethylene with ethoxide ion fits a reaction via a carbanionic intermediate, or the formation of a fluoroether, if the latter is either formed rapidly and decomposed slowly (kikzKk-i + kz ki), or if it formed in a rate-determining step (k1k2l(k-l + k2Xki). 

The decomposition is first order in nitrobenzene and tetrahydronaphthalene and the Arrhenius parameters obtained with various solvents for phenyl azide and some derivatives are collected in Table 16. As seen from the data, meta substitution has no efiect on the rate, and it was concluded that the rate-determining step is N2 evolution and not ring closure to form azepines (in aniline solution). In indene, Smith observed that para substituents increased the rate of Nj evolution by about eight times however, with both meta and para substituents the amine yields varied widely and in some cases the Nj yields were not quantitative with respect to the amount of azide decomposed. Benzenes and substituted benzenes have been observed in small amounts and it appears that a minor decomposition pathway would involve azide radical loss . On the other hand, Waters reports that the thermolysis of phenyl azides proceeds by different mechanisms, depending on the medium ... 

G. C. Bond University of HvU) It follows from Table II of Professor Smith s paper (Lecture 73) that the activation energy for hydrogenolysis of methoxybenzenes is greater than that for the addition process hence, the slow step cannot be the same in the two processes. This conclusion seems to be at variance with Professor Smith s statement that the cleavage must occur after the rate-determining step. ... 

Substituting an atom in a reactant molecule with a heavier isotope of that atom changes the rate of reaction this is known as the kinetic isotope effect (KIE). For example, a hydrogen atom ( H) can be replaced by a deuterium atom ( H or D), and the ratio of the different reaction rates (kn/ko) indicates whether the substimted bond is involved in the rate-determining step of the reaction. The KIE is a valuable diagnostic tool for determination of mechanism in organic chemistry (Smith and March, 2006). 

The pronounced decrease of (kt) in the TD regime is associated with the occurrence of the so-called gel-effect. " Also known as the Trommsdorff, Norrish-Smith or Norrish-Trommsdorff effect, this effect can cause problems within both an industrial and scientific context ranging from a product mixture to reactor explosion, due to its exothermic nature. " " Increasing polymer content induces overlap of polymer chains and decreases the mesh-size in between the polymer chains beyond a critical limit. As a consequence, TD may become the rate-determining step in Scheme 1.21 for the majority of macroradicals, thus (kt) decreases by orders of magnitude in some cases. It is important not to confuse the gel effect with the auto-acceleration that is observed when a polymerization is carried out under non-isothermal conditions, so that the reaction temperature increases with increasing monomer conversion, due to the exothermic nature of the polymerization reaction. The gel effect is observed under isothermal reaction conditions. The cause of the gel effect has been discussed extensively and various theories have emerged which can explain all or part of the experimental data (excellent reviews on the topic can be found in ref. 150 and 151). 

In many catalytic processes and transition metal mediated reactions, a-bor-ane complexes have been shown to be intermediates. The bis(borane) complex Cp2Ti( 72-HBcat/)2 (HBcaT = HBcat-4-f-Bu) is a highly active catalyst for the hydroboration of vinylarenes [37]. A mechanism, shown in Scheme 3, has been proposed for the Ti-catalyzed hydroboration on the basis of a detailed mechanistic study [37]. Theoretical calculations provided further support to the proposed reaction mechanism and showed that the reductive elimination step, giving the product molecules, is rate-determining [38]. In the Cp2Ti(CO)2 catalyzed hydroboration of alkynes [36,37], the proposed reaction mechanism (Scheme 4) also involves a a-borane complex similar to 11 and 14. In the titanium-catalyzed decaborane-olefin hydroborations [47,48], a-borane complexes were also considered as intermediates. In the Cp2MH (M = Nb, Ta) mediated hydroboration reactions of olefins [39,41], Smith and his coworkers observed several interesting cr-borane complexes, such as 21-23 discussed above. 

The quantitative solution of the problem, i.e. simultaneous determination of both the sequence of surface chemical steps and the ratios of the rate constants of adsorption-desorption processes to the rate constants of surface reactions from experimental kinetic data, is extraordinarily difficult. The attempt made by Smith and Prater 82) in a study of cyclohexane-cyclohexene-benzene interconversion, using elegant mathematic procedures based on the previous theoretical treatment 28), has met with only partial success. Nevertheless, their work is an example of how a sophisticated approach to the quantitative solution of a coupled heterogeneous catalytic system should be employed if the system is studied as a whole. 

Olson and Smith measured the rate of oxidation of sulfur dioxide with air in a differential fixed-bed reactor. The platinum catalyst was deposited on the outer surface of the cylindrical pellets. The composition and the rates of the bulk gas were known. The objective was to determine the significance of external diffusion resistance by calculating the magnitude of — C. If this difference is significant, then the values must be used in developing a rate equation for the chemical step. 

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