Reaction mechanisms multistep

Identification of the intermediates in a multistep reaction is a major objective of studies of reaction mechanisms. When the nature of each intermediate is fairly well understood, a great deal is known about the reaction mechanism. The amount of an intermediate present in a reacting system at any instant of time will depend on the rates of the steps by which it is formed and the rate of its subsequent reaction. A qualitative indication of the relationship between intermediate concentration and the kinetics of the reaction can be gained by considering a simple two-step reaction mechanism ... 

Lefebvre, M. C. Establishing the Link Between Multistep Electrochemical Reaction Mechanisms and Experimental Tafel Slopes 32... 

The mechanism of the POCL reaction is a complex multistep process and it has proved to be difficult to elucidate. Side reactions as well as light-generating reactions are fast and overlapping in time, and many of the intermediates are unstable. Because of this complexity, the complete reaction mechanism has still not been fully resolved, despite numerous investigations since its discovery. 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

Homogeneous catalysis is an area of chemistry where computational modeling can have a substantial impact [6-9], Reaction cycles are usually multistep complicated processes, and difficult to characterize experimentally [10-12], An efficient catalytic process should proceed fastly and smoothly and, precisely because of this, the involved intermediates are difficult to characterize, when possible at all. Computational chemistry can be the only way to access to a detailed knowledge of the reaction mechanism, which can be a fundamental piece of information in the optimization and design of new processes and catalysts. 

The Boltzmann distribution helps us understand how intermediates can become trapped in energy wells between successive transition states in a multistep reaction mechanism. This behavior forms the basis for a special field of enzymology known as cryoenzymology. By appropriate choice of water-miscible solvents, enzymes can be studied at ultra-low temperatures where the rates of interconversion of enzyme species can be greatly retarded. See Cryoenzymology... 

As-described compounds have also been proposed to be formed as intermediates in the gas phase in the traditional two-component MOCVD process (pre-reactions). For instance, the deposition of AlN from AlMe3 and NH3 [11] most likely proceeds through a multistep-reaction mechanism including both the adduct Me3Al-NH3 and the heterocycle [Me2AlNH2]3, that is formed after elimination of one equivalent of methane, as more or less stable reaction intermediates. This is supported by the fact that both compounds have been successfully used for the deposition of AIN in the absence of any additional NH3 [12]. The same was found for the deposition of InP from InMe3 and PH3 [13]. 

Rate determining step (cont.) electrocatalysis and, 1276 methanol oxidation, 1270 in multistep reactions, 1180 overpotential and, 1175 places where it can occur, 1260 pseudo-equilibrium, 1260 quasi equilibrium and, 1176 reaction mechanism and, 1260 steady state and, 1176 surface chemical reactions and, 1261 Real impedance, 1128, 1135 Reciprocal relation, the, 1250 Recombination reaction, 1168 Receiver states, 1494 Reddy, 1163... 

Also, in complex electrode reactions involving multistep proton and electron transfer steps, the electrochemical reaction order with respect to the H+ or HO may also vary with pH, indicating a change of mechanism with pH. In this respect, the use of schemes of squares outlined in Sect. 2.2 is very useful in the analysis of these complex kinetics [13]. 

If the concerted four-center mechanism for formation of chloromethane and hydrogen chloride from chlorine and methane is discarded, all the remaining possibilities are stepwise reaction mechanisms. A slow stepwise reaction is dynamically analogous to the flow of sand through a succession of funnels with different stem diameters. The funnel with the smallest stem will be the most important bottleneck and, if its stem diameter is much smaller than the others, it alone will determine the flow rate. Generally, a multistep chemical reaction will have a slow rate-determining step (analogous to the funnel with the small stem) and other relatively fast steps, which may occur either before or after the slow step. 

The transfer coefficients are the ones determining how the electrode potential influences the electrochemical reaction rate or, in other words, the inclination of the relation between log I and the over-potential, also called the Tafel slope, of a multistep reaction. The coefficients are an important aid when unravelling the electrochemical reaction mechanisms, because the experimentally determined Tafel slope should correspond to the value that is calculated for the postulated sub-step sequence and RDS. 

We have also recently examined the electrooxidation of iodide at gold using the combined SERS-RDV approach.(22)The system was chosen as a simple example of a multistep process where the reaction products (iodine and/or triodide) as well as the reactant and any intermediates should be strongly adsorbed. This reaction has been studied extensively using conventional electrochemical techniques, yet the reaction mechanism remains in doubt.(23) At potentials well negative of the I /I2 formal potential, iodide yields a pair of SERS bands at gold at 124 and 158 cm-1, associated with adsorbed I -surface vibrations. 

RAIN is a computer program that finds the reaction pathways for interconverting EM(B) and EM(E). These pathways may correspond to the mechanistic pathways of chemical reactions, or to multistep sequences of chemical reactions, depending on the nature of the valence schemes that are considered. If the valence schemes are confined to those of stable compounds, a program like RAIN will generate sequences of chemical reactions, such as bilaterally generated synthetic pathways (ref. 24), networks of reaction mechanisms are obtained, when the valence schemes of transient intermediates (e.g. carbenes, radicals, carbocations, carbanions) are also included. 

Studies of reaction mechanisms and enzymic reactions rely to a great extent on labeled adducts. The anticipated synthesis of the labeled precursors often are achieved only by multistep procedures performed chemically or enzymatically. Isotope-labeled dihydroneopterin 3 -triphosphate (51), with 3H at positions C-T and C-2, respectively, has been prepared from isotope-labeled glucose as starting material. 

Reaction Mechanism. The proposed mechanism for the ozone-hydrosilane reaction (7) shown in Equation 2, as deduced by analyzing and correlating data on relative rates, substituent effects, deuterium isotope effects, low temperature NMR, and ultraviolet spectroscopy for a range of hydrosilanes, is a multistep one as follows ... 

In order to understand the manner in which the interfacial region influences the observed kinetics, especially in terms of the theoretical models discussed below, it is clearly important to gain detailed information on the spatial location of the reaction site as well as a knowledge of the mechanistic pathway. Information on the latter for multistep processes can often be obtained by the use of electrochemical perturbation techniques in order to detect reaction intermediates, especially adsorbed species [13]. Various in-situ spectroscopic techniques, especially those that can detect interfacial species such as infrared and Raman spectroscopies, are beginning to be used for this purpose and will undoubtedly contribute greatly to the elucidation of electrochemical reaction mechanisms in the future. 

Given that electrochemical rate constants are usually extremely sensitive to the electrode potential, there has been longstanding interest in examining the nature of the rate-potential dependence. Broadly speaking, these examinations are of two types. Firstly, for multistep (especially multielectron) processes, the slope of the log kob-E plots (so-called "Tafel slopes ) can yield information on the reaction mechanism. Such treatments, although beyond the scope of the present discussion, are detailed elsewhere [13, 72]. Secondly, for single-electron processes, the functional form of log k-E plots has come under detailed scrutiny in connection with the prediction of electron-transfer models that the activation free energy should depend non-linearly upon the overpotential (Sect. 3.2). 

In the context of the present discussion, it is worth noting that virtually all the experimental systems that exhibit such "anomalous temperature-dependent transfer coefficients are multistep inner-sphere processes, such as proton and oxygen reduction in aqueous media [84]. It is therefore extremely difficult to extract the theoretically relevant "true transfer coefficient for the electron-transfer step, ocet [eqn. (6)], from the observed value [eqn. (2)] besides a knowledge of the reaction mechanism, this requires information on the potential-dependent work terms for the precursor and successor state [eqn. (7b)]. Therefore the observed behavior may be accountable partly in terms of work terms that have large potential-dependent entropic components. Examinations of temperature-dependent transfer coefficients for one-electron outer-sphere reactions are unfortunately quite limited. However, most systems examined (transition-metal redox couples [2c], some post-transition metal reductions [85], and nitrobenzene reduction in non-aqueous media [86]) yield essentially temperature-independent transfer coefficients, and hence potential-independent AS orr values, within the uncertainty of the double-layer corrections. 

In the process of catalyst development, the key to efficient reaction optimization is frequently rapid access to catalysts with diverse chiral environments. Therefore, it is important to design ligands that can be generated from common intermediates in a few steps. While it is often useful to synthesize elaborate ligands to probe asymmetric induction and reaction mechanisms, few researchers are willing to perform multistep ligand syntheses in the application of catalysts. 

In the simplest reaction mechanisms one particular step is usually rate determining. However, it is not unusual in complex, multistep reaction mechanisms for different steps to be rate determining under different sets of conditions. 

While concentrating on methods, the book uses a number of reactions of industrial importance for illustration. However, no comprehensive review of multistep homogeneous reactions is attempted, simply because there are far too many reactions and reaction mechanisms to present them all. Instead, the book aims at providing the tools with which the practical engineer or chemist can handle his specific reaction-kinetic problems in an efficient manner, and examples of how problems unique to a specific reaction at hand can be overcome. Some examples drawn from my own laboratory experience have been construed or details have been left out, in order to protect former employers or clients proprietary interests. In particular, the omission of information on exact structure and composition of catalysts is intentional. 

One can consider that the Halpern mechanism and the cis mechanism are two extremes. The Halpern mechanism is most widely accepted and our ab initio MO calculations support this from the point of view of intrinsic electronic energy. However, there may be cases where the steric effect overshadows the electronic effect. There may also be cases where both effects are important. It may be, as Collman et al. said, that "this multistep reaction is very complicated. Like a chameleon, the dominant reaction mechanism changes when the nature of the catalyst, the ligands, or the substrate is altered" (8). 

As catalyzed reactions are multistep processes, kinetic studies must be concerned with the exploration of the mechanism and the evaluation of the rate-determining step. The methods of finding the rate-determining step are particularly well developed for acid-base catalyzed reactions [3, 4]. They will be extensively discussed in this article. 

Reaction mechanisms with palladium compounds are often multistep. During the course of a reaction, the identity of some groups bonded to Pd will be known with certainty, while the identity of other ligands might not be known. Consequently, only the crucial reacting groups around a metal are usually drawn and the other ligands are not specified. 

Rate-determining step (Section 6.8) In a multistep reaction mechanism, the step with the highest energy transition state. 

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