I
n addition to undergoing the
nucleophilic substitution reac-
tions described in Chapter 10,
alkyl halides undergo elimination
reactions. In an elimination reac-
tion, groups are eliminated from a
reactant. For example, when an
alkyl halide undergoes an elimina-
tion reaction, the halogen (X) is re-
moved from one carbon and a proton
is removed from an adjacent carbon.
A double bond is formed between the
two carbons from which the atoms are eliminated. Therefore,the product of an elimi-
nation reaction is an alkene.
In this chapter, we will first look at the elimination reactions of alkyl halides.
Then we will examine the factors that determine whether an alkyl halide will under-
go a substitution reaction, an elimination reaction, or both a substitution and an
elimination reaction.
CH 3 CH 2 CH 2 X +
CH 3 CH 2 CH 2 Y
substitution
elimination
CH 3 CH CH 2 + HY + X−
+ X−
Y−
new double
bond
400
11
CH 3 CH 2 CHCH 2 CH 3 CH 3 O−
+
+
Cl
CC
H 3 C
CH 2 CH 3
H
H
CC
H 3 C CH 2 CH 3
H H
Elimination Reactions of
Alkyl Halides •
Competition Between
Substitution and Elimination
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