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Chapter 19 657
Copyright © 2017 Pearson Education, Inc.
14.
There are three possible sites for electrophilic substitution: C-2, C-4, and C-5. To determine the major
product, compare the relative stabilities of the carbocations formed in the first step of the reaction.
Substitution at C-2
N NCH
3
N NCH
3
Br
N NCH
3
Br
+
N NCH
3
Br
+
+
Br Br FeBr
3
+
_
Substitution at C-2 forms an intermediate with three resonance contributors; all the atoms in one contribu-
tor have complete octets and a positive charge on N; one contributor has a carbon with an incomplete octet,
and one has a nitrogen with an incomplete octet.
Substitution at C-4
N NCH
3
N NCH
3
N NCH
3
+
Br
Br
+
Br Br FeBr
3
+
_
Substitution at C-4 forms an intermediate with two resonance contributors; all the atoms in one contributor
have complete octets and a positive charge on N; one contributor has a carbon with an incomplete octet.
Substitution at C-5
NCH
3
N
NCH
3
N
NCH
3
N
Br
+
Br
+
NCH
3
N
Br
+
Br Br FeBr
3
+
_
Substitution at C-5 forms an intermediate with three resonance contributors; all the atoms in one con-
tributor have complete octets and a positive charge on N; two contributors have a carbon with an
incomplete octet.
Substitution at C-4 forms the least stable intermediate because it has only two of the three resonance con-
tributors that the others have. Substitution at C-5 forms the most stable intermediate because a positively
charged carbon with an incomplete octet is more stable than a positively charged nitrogen with an incom-
plete octet.
Therefore, the major product of the reaction is 5-bromo-
N
-methylimidazole.
NCH
3
N
Br
5-bromo-
N
-methylimidazole