Simultaneous reactions slow" steps

One B molecule and two A molecules are needed to give the species for the slow step. The three molecules do not collide simultaneously since we are disregarding the very rare termolecular steps. There must then be some number of prior fast steps to furnish at least one intermediate needed for the slow step. The second B molecule which appears in the reaction equation must be consumed in a fast step following the slow step. 

The breaking of the carbon-oxygen bond and the attachment of the 7- carbon atom to the ortho position must be simultaneous, and this step, rather than the enolization of the hydrogen, must be the rate-determining step. If the latter were the slow step, the reaction would be speeded up by dimethylaniline, and this is not observed. The cyclic mechanism accounts for the occurrence of inversion. 

For competitive adsorption of benzene and hydrogen, simultaneous reaction of two hydrogen atoms with adsorbed benzene is assumed the slow step ... 

The anodic polarization behavior of graphite is shown in Fig. 8 along with the dependence of chlorine current efficiency on current density (c.d.). Thus, at current densities beyond 10 A/dm2, the potential-log c.d. variation is nonlinear, and the chlorine current efficiency decreases with increase in current density. The deviation of the Tafel behavior at high current densities is attributed to the oxide layer on the anode and to the simultaneous discharge of oxygen.39 Comparison of the kinetic parameters evaluated from the experimental data (see Table 1 A) with the theoretical values presented in Table IB for various reaction pathways suggests the slow-step to be electrochemical desorption in the low current density region ... 

The use of multicomponent reactions (MCRs) constitutes an attractive synthetic strategy for rapid and efficient library generation because diverse products are formed in a single step. Usually, MCR transformations do not involve the simultaneous reaction of all components. Instead, they are undertaken in a sequence of steps that are determined by the synthetic design. A drawback of many MCR processes is that they can be slow and inefficient, but microwave heating can be used as a tool to overcome these problems, as illustrated here with selected examples. 

This ideaof one rate controlling step may be correct for a series of chemical and diffusion processes, but it could give the wrong interpretation in systems with simultaneous and parallel diffusion and reactions, where the slow step is not really important. When both chemical reactions and film diffusion occur at comparable... 

The ionic mechanism accommodates the data in the following respects The slow step involves electron attack there is simultaneous dissociation of the X entity so that the R—X bond strength must be involved in the potential required for reaction. 

Arguments against this alternative are the high strain of 4 and the solvent isotope effect (AH2O/AD2O = 2) that clearly indicates a proton transfer involving the water molecule in the transition state of the slow step of the reaction. In the mechanism depicted in Scheme 23.3 the nucleophilic attack of H2O (D2O) to intermediate 4 does not require any previous or simultaneous proton transfer. 

The equation indicates that one MnO ion, five Fe+2 ions, and eight H+ ions (a total of fourteen ions) must react with each other. If this reaction were to take place in a single step, these fourteen ions would have to collide with each other simultaneously. The probability of such an event occurring is extremely small—so small that a reaction which depended upon such a collision would proceed at a rate immeasurably slow. Since the reaction occurs at an easily measured rate, it must proceed by some sequence of steps, none of which involves such an improbable collision. 

The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

The shapes of the curves in Fig. 6 are consistent with a two-step pathway, analogous to that of a hydrolytic enzyme such as a-chymotrypsin,30 in which an initial acylation burst is followed by a slow deacylation reaction. Following a fast preequilibrium binding, the first kinetic step can be attributed to acylation by substrate of the polymer imidazole residue, accompanied by simultaneous release of nit-rophenol(ate). The succeeding kinetic step would then be ascribed to hydrolysis of the acylimidazole leading to carboxylate ion and regenerated imidazole. 

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