Chemistry Reference
In-Depth Information
Thus, the radical from the initiation reaction
abstracts hydrogen from the allylic position of
cyclohexene, as we have seen previously, to give the
resonance-stabilized radical (see Section 9.2).
In the propagation steps, this radical then reacts
with oxygen, producing a peroxyl radical, which then
abstracts hydrogen from a further molecule of the
substrate. The product is thus the hydroperoxide,
reaction having occurred at the allylic position of the
alkene. Two possible chain-termination steps might
O
OH
O
2
initiator
3-cyclohexenyl
hydroperoxide
cyclohexene
The processes that occur when cyclohexene reacts
with oxygen in the presence of an initiator to give
the allylic hydroperoxide exemplify this nicely.
RH
R
H
resonance-stabilized
radical
O
O
OO
hydroperoxides easily dissociate
further to generate radicals
H
O
OH
O
O
RO
OH
RO
OH
hydroperoxide
termination steps
O
O
O
O
peroxide
be the combination of two cyclohexenyl radicals
or the formation of a peroxide, as shown. The
hydroperoxide itself can easily dissociate to produce
radicals that may then initiate other chain reactions.
Peroxyl radicals are not particularly reactive, and thus
tend to be highly selective. They tend to abstract
hydrogen atoms most readily from tertiary, allylic
and benzylic C-H bonds. These are systems with the
weakest bonds and that have maximum stabilization
in the radical produced.
Box 9.1
Autoxidation in fats and oils: the origins of rancidity
Oxygen-mediated
autoxidation
can occur with unsaturated acid components of fats and oils, which are esters of
fatty acids with glycerol (see Box 7.16). This leads initially to hydroperoxides that decompose further to produce
