Chemistry Reference
In-Depth Information
Chapter
4
S
N
2 Substitution Reactions
As alluded to in previous chapters, the study of organic chemistry requires an understanding
of the mechanistic types that drive reactions. While the detailed mechanisms associated
with some complex reactions may lie beyond the scope of an introductory organic chemi-
stry course, the fundamental components are easily recognized and applied to the reactions
contained within generally presented curricula. As stated in earlier discussions, this topic
presents the concept of arrow pushing with a focus on heterolytic reaction mechanisms.
However, it is important to remember that the lessons presented herein are applicable to
organic chemistry regardless of the mechanistic type.
While the fundamental mechanistic components of organic chemistry can be combined
to describe complex mechanisms associated with complex reactions, the individual
mechanistic components fall into a relatively small and well-defined group of four.
These are S
N
1, S
N
2, E1, and E2 reactions. In this chapter, the fundamentals associated
with S
N
2 reactions are presented.
4.1 WHAT IS AN S
N
2 REACTION?
Among the mechanistic types relevant to organic chemistry, the S
N
2 reaction
mechanisms are the simplest. In progressing from starting materials to products, these
reactions generally consist of a nucleophile displacing a leaving group. As illustrated in
Scheme 4.1, consider a molecule where L
2
is a leaving group. As shown, a nucleophile,
Nu
2
, can be introduced with displacement of the leaving group,
thus generating a
new molecule.
