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RADICAL REACTIONS
stabilized by resonance delocalization; indeed, they
are even more stable than tertiary radicals. In the
presence of a suitable initiator, bromine dissociates
to bromine atoms that will selectively abstract an
allylic or a benzylic hydrogen from a suitable
substrate, generating the corresponding allyl and
benzyl radicals.
In the case of cyclohexene, this leads to a
resonance-stabilized allylic radical that then reacts
with bromine to give the allylic bromide, plus
a further bromine atom to continue the chain
propagation steps. The symmetry in cyclohexene
means that the two resonance structures are identical.
It does not matter which allylic radical picks up
bromine, we get the same product. It is not difficult to
appreciate that a mixture of brominated products must
result if we start with a non-symmetrical substrate.
Br Br
hn
Br
Br
H
Br
resonance-stabilized
allylic radical
Br Br
Br Br
Br
Br
Br
Br
HBr
cyclohexene
For example, radical allylic bromination of pent2-ene must produce a mixture of three products.
There are two allylic positions in the substrate, and
either can suffer hydrogen abstraction. If hydrogen
is abstracted from the methylene, then the two
contributing resonance structures for the allylic
radical are equivalent, and one product results
when this captures a bromine atom. Abstraction
of hydrogen from the terminal methyl gives an
allylic radical for which the resonance structures are
not equivalent, and hence two different brominated
products may be formed. The net result will be a
mixture of all three products. If we want to exploit
allylic bromination, this means we must choose
the substrate carefully if we prefer to get a single
product.
H
Br
H
Br
equivalent resonance
structures
non-equivalent resonance
structures
Br
Br
Br
Br
same product
different products
pent-2-ene