RADICAL ADDITION OF OXYGEN: AUTOXIDATION REACTIONS
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low molecular weight carboxylic acids. These are the cause of rancidity, the unpleasant odour and taste associated
with badly stored fats. Linoleic acid is a typical unsaturated fatty acid component, and hydrogen abstraction will
occur from the methylene between the two non-conjugated double bonds. The radical thus produced benefits from
extensive delocalization, as shown by the resonance forms that can be drawn.
CO 2 R´
ester of linoleic acid
H
H
R
CO 2 R´
H
H
H
resonance-stabilized
radical
hydrogen abstraction occurs at
methylene between double bonds
O O
non-conjugated
double bonds
conjugated
double bonds
conjugated
double bonds
H
O O
CO 2 R´
H
H
CO 2 R´
O OH
hydroperoxide
However, the resonance forms in which the double bonds are conjugated are inherently more stable than
that with the unconjugated double bonds (see Section 9.2). Accordingly, the hydroperoxide subsequently formed
upon reaction with oxygen will have conjugated double bonds. Abstraction of a hydrogen atom to form the
hydroperoxide is part of the chain propagation process.
Fragmentation of the hydroperoxide can then lead to chain shortening, as illustrated.
O OH
O
OH
O
H
O
HO
oxidation
Acidic products result from further oxidation of aldehydes (or ketones), again by a radical process. Oxidation
of an aldehyde to a carboxylic acid in the presence of air involves a peroxy acid (compare peroxyacetic acid,
Section 8.1.2). Finally, a reaction between the peroxy acid and a molecule of aldehyde yields two carboxylic
acid molecules; this is not a radical reaction, but is an example of a Baeyer–Villiger oxidation. Baeyer–Villiger