Chemical Kinetics Evidence for Nucleophilic Substitution Mechanisms

6 CHEMICAL KINETICS EVIDENCE FOR NUCLEOPHILIC SUBSTITUTION MECHANISMS 

Continuous changes in the concentrations of reactants and products are observed during the course of chemical reactions. Thus, as an irreversible reaction proceeds, we find that the concentration of each reactant decreases until that of the limiting reagent becomes zero, at which point the reaction stops. Simultaneously, the concentration of the product increases from zero to its maximum value when the reaction is complete. How fast these concentrations change as a function of time is determined by the reaction rate for a chemical transformation. 

The field of chemical kinetics involves investigation of the interplay of factors and variables influencing the rates of reactions. Because chemical kinetics provide great insight into the nature and details of reaction mechanisms, it is hard to overstate the importance of this area of research. Indeed, kinetic studies have provided some of the most important evidence in support of the Sjvjl and Sj j2 mechanisms (Sec. 14.2). 

To simplify our discussion of kinetics, let s first assume that the reactions are irreversible and then consider the mathematical expressions or rate laws governing these classes of reaction. To do so, we need only examine the rate-determining step for the particular type of substitution. In the case of an Sjvjl reaction, this step is the formation of a carbocation from the precursor R-L (Eq. 14.2). The rate of the overall reaction is then proportional only to the concentration of substrate, as expressed in Equation 14.24. We see that the rate is first order in the concentration of R-L, expressed as [R-L], and zeroeth order in that of Nu , that is, [Nu ] , which means that the rate is independent of its concentration. Adding the two exponents for the concentration gives the overall order of the Sj l reaction, which is seen to be first order. A simplified version of the rate law is seen in Equation 14.25, and this is the form in which it is normally written. 

For Sjvj2 reactions, the rate-determining step involves attack by the nucleophile, Nu , on the substrate R-L (Eq. 14.3). The corresponding rate law is shown in Equation 14.26, wherein the rate is seen to be proportional to the concentrations of R-L and Nu . Both concentrations are taken to the first power, so the rate of the reaction is said to be second order overall. 

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