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Reaction mechanism The sequence

Thus far, our discussion of chemical kinetics has centered on reaction rates. We ve seen that the rate of a reaction usually depends on both reactant concentrations and the value of the rate constant. An equally important issue in chemical kinetics is the reaction mechanism, the sequence of molecular events, or reaction steps, that defines the pathway from reactants to products. Chemists want to know the sequence in which the various reaction steps take place so they can better control known reactions and predict new ones. [Pg.492]

Reaction Mechanism The sequence of reaction steps that defines the pathway from reactants to products. [Pg.492]

In the absence of adequate kinetic data, these proposed reaction sequences are speculative in nature and will not be dignified by the term reaction mechanism. The sequences proposed Will be found valuable in both correlating many nitric oxide reaction and in suggesting additional experiments. [Pg.145]

Reaction mechanism The sequence of steps by which reactants are converted into products. [Pg.698]

The aim of biosynthetic experiments with fungal metabolites is to establish the structure of the building blocks, the order in which they are assembled, the way in which chains are folded to form the carbon skeleton and the sequence interrelating precursors with the final metabolite. Biosynthesis is concerned with both sequences and reaction mechanisms. The sequence of the biosynthetic events, the role of intermediates and the stereochemistry of enzymatic reactions can be studied with appropriately isotopically-labelled substrates and with structural analogues of the natural intermediates. The chemical enzymology of individual steps and the role of key components and structures of the enzyme may be studied with isolated enzyme systems obtained from fungi. The features that determine the function of the enzyme and which control its activity may be determined by genetic studies in which mutants play an important role. [Pg.29]

Reaction mechanism. The sequence of elementary steps that leads to product formation. (13.5)... [Pg.1049]

The identification of a rate law provides valuable insight into the reaction mechanism, the sequence of elementary steps by which a reaction takes place. The aim is to identify the reaction mechanism by constructing the rate law that it implies. This procedure may be simplified by identifying the rate-determining step of a reaction, the slowest step in a sequence that determines the overall rate. Thus, if the proposed mechanism is A B followed by B C, and the former is much faster than the latter, then the overall rate of the reaction will be equal to the rate of A B, for once B is formed, it immediately converts into C. [Pg.705]

Raoult s law. The vapor pressure of the solvent over a solution is given by the product of the vapor pressure of the pure solvent and the mole fraction of the solvent in the solution. (12.6) rare earth series. See lanthanide series, rate constant (it). Constant of proportionality between the reaction rate and the concentrations of reactants. (13.1) rate law. An expression relating the rate of a reaction to the rate constant and the concentrations of the reactants. (13.2) rate-determining step. The slowest step in the sequence of steps leading to the formation of products. (13.5) reactants. The starting substances in a chemical reaction. (3.7) reaction mechanism. The sequence of elementary steps that leads to product formation. (13.5)... [Pg.1107]

The relationship between a kinetic expression and a reaction mechanism can be appreciated by considering the several individual steps that constitute the overall reaction mechanism. The expression for the rate of any single step in a reaction mechanism will contain a concentration term for each reacting species. Thus, for the reaction sequence... [Pg.193]

The proposed mechanism indicates that all the foregoing reactions in the sequence are now more or less rapidly established equilibria and the complete reaction scheme can be written as shown in Eq. (11). [Pg.111]

Reaction mechanism A sequence of steps by which a reaction occurs at the molecular level, 307,318-319q elementary steps, 307 intermediates, elimination of, 309-311 rate expression for, deducing, 308-309 slow steps, 307... [Pg.695]

Each of these variables will be considered in this book. We start with concentrations, because they determine the form of the rate law when other variables are held constant. The concentration dependences reveal possibilities for the reaction scheme the sequence of elementary reactions showing the progression of steps and intermediates. Some authors, particularly biochemists, term this a kinetic mechanism, as distinct from the chemical mechanism. The latter describes the stereochemistry, electron flow (commonly represented by curved arrows on the Lewis structure), etc. [Pg.9]

Some of the most conclusive studies of the mechanisms of chain reactions come from experiments in which some of the propagating steps have been independently measured directly. This measurement can sometimes be done by the use of flash photolysis and pulse radiolysis (Chapter 11). Such determinations can verify the occurrence of a certain reaction in the sequence and provide its rate constant. [Pg.189]

We stressed in Section 13.3 that we cannot in general write a rate law from a chemical equation. The reason is that all but the simplest reactions are the outcome of several, and sometimes many, steps called elementary reactions. Each elementary reaction describes a distinct event, often a collision of particles. To understand how a reaction takes place, we have to propose a reaction mechanism, a sequence of elementary reactions describing the changes that we believe take place as reactants are transformed into products. [Pg.667]

Mechanism. The sequence of steps connecting reactants and products in an overall chemical reaction. Each step starts from an equilibrium form (reactant or intermediate) and ends in an equilibrium form (intermediate or product). [Pg.764]

Kinetics Third The dynamic nature of chemistry becomes fully evident through kinetics. Our approach shows how insight and model building are critical to the indentification of reaction mechanisms. The full chapter on kinetics follows coverage of equilibrium however, qualitative kinetic arguments are used early to develop an understanding of equilibrium, and the full treatment can be used anywhere in the sequence. [Pg.24]

In this connection there may be an objection that if an expression is given, e.g., for time as a function of the degree of advancement, and this contains a number of constants, one can always choose the constants so as to make the theoretically predicted curve fit the experimental curve. This would be possible if we could choose at will certain families of functions and if we were free to dispose of many constants. But the functions appearing in the equations cannot be chosen arbitrarily since they follow as logical consequences of the assumed mechanism, and the number of constants is restricted by the finite number of partial reactions in the sequence. Only if the range of the degree of advancement is small and if the precaution to make experiments with different initial concentrations is not taken, may it be possible to obtain agreement between the same set of experiments and different mechanisms, but not otherwise. [Pg.313]

Trypsin was named more than 100 years ago. It and chymotrypsin were among the first enzymes to be crystallized, have their amino acid sequences determined, and have their three-dimensional structure outlined by x-ray diffraction. Furthermore, both enzymes hydrolyze not only proteins and peptides but a variety of synthetic esters, amides, and anhydrides whose hydrolysis rates can be measured conveniently, precisely and, in some instances, extremely rapidly. As a result, few enzymes have received more attention from those concerned with enzyme kinetics and reaction mechanisms. The techniques developed by the pioneers in these various fields have enabled other serine proteases to be characterized rapidly, and the literature on this group of enzymes has become immense. It might be concluded that knowledge of serine proteases is approaching completeness and that little remains but to fill in minor details. [Pg.187]

When a reaction rate is measured in a chemical reactor, the reaction is generally a composite reaction comprised of a sequence of elementary reactions. An elementary reaction is a reaction that occurs at the molecular level exactly as written (Laidler, 1987). The mechanism of the reaction is the sequence of elementary reactions that comprise the overall or composite reaction. For example, mineral dissolution reactions generally include transport of reactant to the surface, adsorption of reactant, surface dilfusion of the adsorbate, reaction of the surface complex and release into solution, and transport of product species away from the surface. These reactions occur as sequential steps. Reaction of surface complexes and release to solution may happen simultaneously at many sites on a surface, and each site can react at a different rate depending upon its free energy (e.g., Schott et al., 1989). Simultaneous reactions occurring at different rates are known as parallel reactions. In a series of sequential reactions, the ratedetermining step is the step which occurs most slowly at the onset of the reaction, whereas for parallel steps, the rate-determining step is the fastest reaction. [Pg.2334]


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