Kinetic Study of the Hydrolysis of T-Butyl Chloride

Kinetic Study of the Hydrolysis of tert-Butyl Chloride

CH 236-A8 Group 7

Authors, Reviewers, Editors:

J. Foncea, J. Messner, K. Simmons

Introduction

In organic chemistry, there are two different types of nucleophilic substitution reactions: SN1 and SN2.  SN1 signifies unimolecular reactions. Also known as the first order reaction, the reaction rate is based only on the concentration of one reactant and can be expressed as r = k[A].  For bimolecular reactions, or SN2, the reaction rate is determined by the product of two concentrations and can be expressed as r= k[A][B]. In SN1 reactions, substrate affects the reaction rate, whereas in SN2 reactions, both substrate and nucleophile affect the reaction rate.1 To find the order of a reaction, the rate law must be derived in the form of differential equations, and then can be integrated to obtain an equation where concentration is a function of time. The hydrolysis of tert-Butyl chloride is an SN1 reaction, which means only one molecule is involved as a reactant in the rate determining step.2 The mechanism in Figure 1. shows the rate determining step is the formation of a carbocation intermediate from tert-Butyl chloride. Therefore, the rate of reaction should be proportional to the concentration of tert-Butyl chloride.

[pic 1][pic 2][pic 3]

Figure 1. Mechanism for the Hydrolysis of tert-Butyl Chloride

Two factors that can affect reaction rate are concentration and temperature.2  Increasing the concentration of one or more of the reactants will more than likely increase the reaction rate. Additionally, increasing the temperature will increase kinetic energy, and ultimately increase the rate of reaction.  The Arrhenius equation is used to describe the effect of temperature on the rate of a chemical reaction, and is shown in Equation 1, where k is the rate constant, A is the pre-exponential factor, Ea is the activation energy, R is the universal gas constant, and T is the temperature in Kelvin.2

[pic 4]                                                                        Eq. 1[pic 5]

By taking the natural logarithm of the Arrhenius equation, it yields the same form as the equation for a straight line, or y= mx+b, where x is equivalent to 1/T. When plotted, Equation 2 is then used to find the Ea, or activation energy from the slope of the straight line.2 

[pic 6]                                                                Eq. 2[pic 7][pic 8][pic 9]