Factors affecting reaction rate reactant structure

Let s now turn our attention to the transition state for this reaction. What is the structure of the transition state This is an important question because a better understanding of its structure will help in predicting how various factors affect its stability and therefore will aid in predicting how these same factors affect the rate of the reaction. The transition state has a structure that is intermediate between that of the reactant, tot-butyl chloride, and that of the product, the tot-butyl carbocation. It has the bond between the carbon and the chlorine partially broken and can be represented as shown in the following structures ... 

The idea that the defect structure of a solid reactant affects the rate of decomposition seems to be generally accepted but of all the factors influencing the kinetics, this is the one most difficult to characterize quantitatively. Boldyrev and his many co-workers28 have made considerable progress in elucidating the factors which affect the decomposition rate of solids however, at the level of detail required for the understanding of a particular reaction, it seems difficult to make connections to the standard kinetic equations derived by earlier workers. 

In solution, all participants in a chemical reaction are solvated the reactants and the products—and the transition state. Our examination of these must include any solvent molecules that help make up the structures and help determine their stabilities. And so, in Chapter 7, using as our examples the nucleophilic substitution reactions the students have just studied, we show how reactivity— and, with it, the course of reaction—is affected by the solvent. We show just how enormous solvent effects can be that the presence of a solvent can speed up—or slow down—a reaction by a factor of l(P that a change from one solvent to another can bring about a miUionfold change in reaction rate. 

The major carbon centered reaction intermediates in multistep reactions are carboca-tions (carbenium ions), carbanions, free radicals, and carbenes. Formation of most of these from common reactants is an endothermic process and is often rate determining. By the Hammond principle, the transition state for such a process should resemble the reactive intermediate. Thus, although it is usually difficult to assess the bonding in transition states, factors which affect the structure and stability of reactive intermediates will also be operative to a parallel extent in transition states. We examine the effect of substituents of the three kinds discussed above on the four different reactive intermediates, taking as our reference the parent systems [ ]+, [ ]-, [ ], and [ CI I21-... 

The ways in which reaction parameters affect a two phase batch reaction are similar to those considered above for the three phase systems. Since there is no gas phase, agitation only serves to keep the catalyst suspended making it more accessible to the dissolved reactants so it only has a secondary effect on mass transfer processes. Substrate concentration and catalyst quantity are the two most important reaction variables in such reactions since both have an influence on the rate of migration of the reactants through the liquid/solid interface. Also of significant importance are the factors involved in minimizing pore diffusion factors the size of the catalyst particles and their pore structure. 

We shall examine the most likely structure for the transition state, then, to see what structural features affect its stability without at the same time affecting by an equal amount the stability of the reactants that is, we shall look for factors that tend to increase or decrease the energy difference between reactants and transition state. Having decided what structural features affect the act> we shall compare the transition states for the reactions whose rates we wish to compare the more stable the transition state, the faster the reaction. 

Particle size affects the diffusion distance of the reactants and products. The longer distance of the diffusion, the bigger concentration gradient of reactant and products is. The effective diffusion coefficient of reactants and products is a complex coefficient. It is subject to influence by many factors, such as pore structure of catalyst, porosity of catalyst, and kinds of reactants and products. The reaction or formation rate of reactants and products is determined by reaction kinetics. Therefore, the research results of catalyst particle size, catalyst pore size, and diffiisivity of products should be discussed in detail. 

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