Multistep Chemical Reactions

High temperature (often exceeding 1000 K) drives the endothermic chemical reactions. Multistep cycles for water splitting are used because very high temperatures are required before an appreciable amount of water decomposes in single-step cycles. Thus, in one or more subsequent chemical reactions, the intermediary compounds can be recovered to the original substance, which is used repeatedly. The thermochemical water decomposition steps involve the following five principal reactions ... 

To this point we have focused on reactions with rates that depend upon one concentration only. They may or may not be elementary reactions indeed, we have seen reactions that have a simple rate law but a complex mechanism. The form of the rate law, not the complexity of the mechanism, is the key issue for the analysis of the concentration-time curves. We turn now to the consideration of rate laws with additional complications. Most of them describe more complicated reactions and we can anticipate the finding that most real chemical reactions are composites, composed of two or more elementary reactions. Three classifications of composite reactions can be recognized (1) reversible or opposing reactions that attain an equilibrium (2) parallel reactions that produce either the same or different products from one or several reactants and (3) consecutive, multistep processes that involve intermediates. In this chapter we shall consider the first two. Chapter 4 treats the third. 

Like other heterogeneous chemical reactions, electrochemical reactions are always multistep reactions. Some intermediate steps may involve the adsorption or chemisorption of reactants, intermediates, or products. Adsorption processes as a rule have decisive influence on the rates of electrochemical processes. 

Solid-supported technologies are already well established methods in medicinal chemistry and automated synthesis. Over the last couple of years new trends have evolved in this field which are of utmost importance as they have the potential to revolutionize the way chemical synthesis especially for library production is performed. Microchip-based synthesis technologies and multistep sequences with solid-supported catalysts or reagents in flow-through systems are only two spectacular examples. A new approach is the use of solid-supported systems for the scale-up of chemical reactions thereby enabling the rapid and smooth transition from discovery to development units. 

Only twenty years ago, effecting a chemical reaction catalytically and in an enantioselective manner appeared to be a difficult and relatively unattainable goal to organic chemists. Clearly, many significant advances have since been made in this area, as is evident by the total syntheses summarized in this article. These accomplishments indicate that catalytic enantioselective methods can, and should be, utilized in the planning and execution stages of multistep syntheses of target molecules. In this... 

With the availability of perturbation techniques for measuring the rates of rapid reactions (Sec. 3.4), the subject of relaxation kinetics — rates of reaction near to chemical equilibrium — has become important in the study of chemical reactions.Briefly, a chemical system at equilibrium is perturbed, for example, by a change in the temperature of the solution. The rate at which the new equilibrium position is attained is a measure of the values of the rate constants linking the equilibrium (or equilibria in a multistep process) and is controlled by these values. 

The advantages of functional polymers are best realized when used in two situations— multistep sequential reactions and automated parallel combinatorial synthesis to produce libraries of compounds. In both situations there is a large savings in the time and expense of carrying out the many chemical reactions and their corresponding handling and purification steps. 

Deseribe in less than one page each the chemical reactions and processes by which the following solids processes are carried out Each involves a multistep process with physical separation steps between ehemieal steps. 

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... 

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. 

Multistep electrode reactions can also be complicated by homogeneous chemical reactions. The most studied case is that the product of a first electron transfer undergoes a homogeneous chemical transformation with an electro-inactive species present in a large excess under these conditions, the reaction scheme is that corresponding to a pseudo-first-order ECE mechanism given by... 

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. 

Stoichiometric number— The concept of stoichiometric number, introduced by Horiuti and Ikusima in 1939, was initially applied to reactions associated with the synthesis of NH3. In electrochemistry, stoichiometric number was used in the kinetics of hydrogen and oxygen evolution. Generally, stoichiometric number of a substance B is the z/p coefficient in the stoichiometric equation of the multistep -> chemical reaction... 

A theory of KIE for multistep enzymatic reactions was developed by Cleland and Northrop (1999). It is obvious that when the barrier of the chemical reaction step is at least several kcal/mole above all others, the step is essentially step- limiting. The isotope in this step is fully expressed in the experimental ratio V/K, where V and K are the reaction maximum rate and Michaelis constant, respectively. If the chemical step does not have the highest barrier, the isotope effect can be partially or fully suppressed. For the mechanism ... 

Dry reagent chemistries have been described for the analysis of a variety of blood constituents. These include metabolites, enzymes, electrolytes, hormones, and therapeutic drugs. A partial list is presented in Table 3. With the exception of electrolytes, nearly all analyses depend on enzyme-mediated chemistries and that includes immunochemical assays. A brief survey of element structures will illustrate how physical functions and chemical reactions used in conventional multistep procedures are integrated in the construction of dry reagent test devices. These examples will illustrate how reactions in dry reagent elements can be compartmentalized and how end produas are shunted to other compartments for further reaction. In its final form, each element provides a complete analytical procedure. 

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