RADICAL SUBSTITUTION REACTIONS: HALOGENATION
325
relative stability of the radical produced, though this
is an oversimplification and we ought to consider
relative energies of transition states.
9.3.1 Stereochemistry of radical reactions
The planarity of a radical (see Section 9.2) means
that, when it reacts with a reagent, there is an equal
probability that it can form a new bond to either
side of the radical. In many cases this is of no
consequence; but, should the formation of the product
generate a chiral centre, we are going to get an
equimolar mixture of both possible configurations,
i.e. formation of a racemic mixture. This outcome
has already been noted when a carbocation, another
planar system, reacts to produce a chiral centre (see
Section 6.2).
Thus, if we consider radical chlorination of butane,
we expect to get a mixture of products, including
the monochlorinated compounds 1-chlorobutane and
2-chlorobutane.
H 3 C
CH 3
Cl 2
H 3 C
+
H 3 C
CH 3
Cl
hn
H 3 C
CH 2
H 3 C
CH 3
H
achiral
racemic product
Et
Me
H
radical can abstract
chlorine atom from either
side of planar structure
Cl Cl
Cl Cl
Et
Me
H
Et
Cl
Me
H
Et
Cl
Me
H
butane
Cl
Cl 2
Cl 2
In the formation of 1-chlorobutane, an intermediate
primary radical is involved, and there are no stereochemical consequences. However, the secondary radical involved in 2-chlorobutane formation is planar,
and when it abstracts a chlorine atom from a chlorine molecule it can do so from either side with equal
probability. The result is formation of a racemic product, (±)-2-chlorobutane.
9.3.2 Allylic and benzylic substitution:
halogenation reactions
The selectivity of radical bromination reactions
depends, in part, on the increased stability of
secondary or tertiary radical intermediates compared
with primary radicals. In Section 9.2 we noted
that allyl and benzyl radicals were especially
H
Br
allylic position
H
benzylic position
Br
allylic radical
benzylic radical
325
relative stability of the radical produced, though this
is an oversimplification and we ought to consider
relative energies of transition states.
9.3.1 Stereochemistry of radical reactions
The planarity of a radical (see Section 9.2) means
that, when it reacts with a reagent, there is an equal
probability that it can form a new bond to either
side of the radical. In many cases this is of no
consequence; but, should the formation of the product
generate a chiral centre, we are going to get an
equimolar mixture of both possible configurations,
i.e. formation of a racemic mixture. This outcome
has already been noted when a carbocation, another
planar system, reacts to produce a chiral centre (see
Section 6.2).
Thus, if we consider radical chlorination of butane,
we expect to get a mixture of products, including
the monochlorinated compounds 1-chlorobutane and
2-chlorobutane.
H 3 C
CH 3
Cl 2
H 3 C
+
H 3 C
CH 3
Cl
hn
H 3 C
CH 2
H 3 C
CH 3
H
achiral
racemic product
Et
Me
H
radical can abstract
chlorine atom from either
side of planar structure
Cl Cl
Cl Cl
Et
Me
H
Et
Cl
Me
H
Et
Cl
Me
H
butane
Cl
Cl 2
Cl 2
In the formation of 1-chlorobutane, an intermediate
primary radical is involved, and there are no stereochemical consequences. However, the secondary radical involved in 2-chlorobutane formation is planar,
and when it abstracts a chlorine atom from a chlorine molecule it can do so from either side with equal
probability. The result is formation of a racemic product, (±)-2-chlorobutane.
9.3.2 Allylic and benzylic substitution:
halogenation reactions
The selectivity of radical bromination reactions
depends, in part, on the increased stability of
secondary or tertiary radical intermediates compared
with primary radicals. In Section 9.2 we noted
that allyl and benzyl radicals were especially
H
Br
allylic position
H
benzylic position
Br
allylic radical
benzylic radical