Chemistry Reference
In-Depth Information
8.1.4 Carbocation rearrangements
Carbocations are highly reactive intermediates, and
are notorious for their ability to rearrange into more
stable variants. We have already met this concept
when we considered
carbocation rearrangements
as competing reactions during S
N
1 nucleophilic sub-
stitutions (see Section 6.4.2). Since carbocations are
also involved in electrophilic reactions, we must
expect that analogous rearrangements might occur
in these. This is indeed the case. In Section 6.4.2,
we included examples of rearrangements in carboca-
tions formed during electrophilic additions, because
identical processes are involved, and it was more
appropriate to consider these topics together, rather
than separately.
As a simple example, note that the major products
obtained as a result of addition of HBr to the alkenes
shown below are not always those initially expected.
For the first alkene, protonation produces a particu-
larly favourable carbocation that is both tertiary and
benzylic (see Section 6.2.1); this then accepts the bro-
mide nucleophile. In the second alkene, protonation
produces a secondary alkene, but hydride migration
then leads to a more favourable benzylic carbocation.
As a result, the nucleophile becomes attached to a
carbon that was not part of the original double bond.
Further examples of carbocation rearrangements will
be met under electrophilic aromatic substitution (see
Section 8.4.1).
HBr
Br
2-phenylbut-1-ene
formation of tertiary benzylic
carbocation favoured
HBr
Br
H
H
3-phenylprop-1-ene
hydride migration produces
more favourable benzylic
carbocation
formation of secondary
carbocation favoured
Rearrangements are an unexpected complication,
and it is sometimes difficult to predict when they
might occur. We need to look carefully at the
structure of any proposed carbocation intermediate
and consider whether any such rearrangements are
probable.
on a terminal carbon atom; protonation of either
C-2 or C-3 would produce an unfavourable primary
carbocation.
resonance-stabilized
allylic cation
In
most
cases
we
shall
only
need
to
H
H
H
HBr
rationalize
such
transformations,
and
will
not
be
trying to predict their possible occurrence.
1
3
2
4
buta-1,3-diene
8.2 Electrophilic addition to
conjugated systems
Br
−
Br
−
H
H
The first step in the reaction of HX with an alkene
is protonation to yield the more stable cation. If
we extend this principle to a conjugated diene, e.g.
buta-1,3-diene, then we can see that the preferred
carbocation will be produced if protonation occurs
Br
Br
1,2-addition
1,4-addition
conjugate addition
