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Nucleophilic Aliphatic Substitution Reactions

An Overview

Introduction

In our discussion of acid-base chemistry we alluded to the similarities between the reactions of NaOH with HCl on the one hand and CH3Cl on the other. Scheme 1 compares the two.

Scheme 1

One and the Same

The reaction of HCl with NaOH is a specific example of a more general type of reaction known as a nucleophilic substitution. The reaction of CH3Cl with NaOH exemplifies a type of nucleophilic substitution called nucleophilic aliphatic substitution, where the word aliphatic indicates that the substitution occurs at an sp3 hybridized carbon atom. The study of nucleophilic aliphatic substitution reactions has provided chemists with detailed insights into the nature of chemical reactivity. Scheme 2 presents a generalized description of nucleophilic aliphatic substitution reactions.

Scheme 2

Nucleophilic Aliphatic Substitution

Before we begin our discussion of the details of Scheme 2, let's clarify some terms.

Definitions

There are two features of Scheme 2 that merit further comment.

First, notice that the reaction is depicted as an equilibrium process. In most nucleophilic aliphatic substitution reactions, the value of the equilibrium constant is very large, i.e. the reaction is essentially irreversible. The rules that we developed for assessing the equilibrium constant of any acid-base reaction will serve as a useful guide in estimating the equilibrium constant for nucleophilic aliphatic substitution reactions as well.

Second, since the reaction is an equilibrium process, there is no fundamental difference between a nucleophile and a leaving group. It all depends on your perspective; the reagent that acts as a nucleophile in the forward direction assumes the role of a leaving group in the reverse direction.


Exercise 1 Calculate the approximate equilibrium constant for each of the following nucleophilic aliphatic substitution reactions.

Exercise 2 In the first reaction above, which ion is more accurately described as the nucleophile? iodide ionbromide ion


Having seen the features of nucleophilic aliphatic substitution reactions in broad outline, we will now examine the process in more detail. Specifically, we will look at the effect of each of the following parameters on the rates of nucleophilic aliphatic substitution reactions:

Before we examine each of these variables however, we first need to develop a basic understanding one of the most useful experimental methods there is for gaining insight into the nature of chemical reactions, chemical kinetics.

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