Chemistry Reference
In-Depth Information
The most common examples of rearrangements
involve an electron-deficient atom, and pre-eminent
amongst these are carbocations. Since carbocations
are a feature of the S
N
1 and E1 mechanisms, it
follows that rearrangements can be side-reactions
of these types of transformation. The driving force
in
carbocation rearrangements
is to form a more
stable carbocation.
Consider a proposed nucleophilic substitution reac-
tion on the secondary alcohol shown using aqueous
HBr. As a secondary alcohol, either S
N
2orS
N
1
mechanisms are possible (see Section 6.2.3), but S
N
1
is favoured because of the acidic environment and
the large
tert
-butyl group hindering approach of the
nucleophile. The expected S
N
1 bromide product is
formed, together with a smaller amount of the E1-
derived alkene in a competing reaction.
formation of carbocation
favoured; S
N
2 inhibited by
large tert-butyl group
OH
OH
2
H
H
HBr
H
H
3
C
H
3
C
H
3
C
CH
3
CH
3
CH
3
H
3
C
H
3
C
H
3
C
H
2
O
CH
3
CH
3
CH
3
S
N
1
E1
secondary alcohol
Br
H
H
3
C
H
3
C
CH
3
H
3
C
H
3
C
CH
3
CH
3
However, other products are also produced. These
are
rearranged carbon skeleton. Their formation is ratio-
nalized as follows:
isomers
of
the
above
products
and
have
a
migration of methyl group
and electron pair
CH
3
H
CH
3
H
H
S
N
1
H
3
C
H
2
C
H
3
C
CH
3
CH
3
CH
3
H
3
C
H
3
C
CH
3
CH
3
Br
secondary carbocation
more stable
tertiary carbocation
E1
CH
3
CH
3
H
3
C
+
H
3
C
CH
3
CH
3
CH
3
The first-formed carbocation is secondary. It is
possible for this carbocation to become a more stable
tertiary carbocation via rearrangement, in which a
methyl group with its pair of electrons migrates from
one carbon to the adjacent positive centre. Now the
rearranged tertiary carbocation can yield S
N
1- and
E1-type products in much the same manner as the
original secondary carbocation. A rearranged bromide
is formed, together with two alkenes from an E1
process, with both more-substituted Saytzeff and less-
substituted Hofmann alkenes being produced. The
formation of such rearranged products proves that this
unexpected transformation must occur.
These carbocation rearrangements are termed
Wagner - Meerwein rearrangements
. They are most
commonly encountered with secondary carbocations
where rearrangement produces a more stable ter-
tiary carbocation. They are less common with tertiary
