Chemistry Reference
In-Depth Information
Autoxidation of fats and oils
Fixed oils and fats are naturally occurring products, usually of plant origin,
that are used extensively in pharmaceutical formulation. They are very
susceptible to oxidative decomposition (a process called
rancidity
) and
special precautions must be taken to control their stability and prevent their
decomposition. Compounds of this type exist as complex mixtures of
structurally similar oils, the composition of which can vary from year to
year depending on factors such as climate, time of harvest, etc. Chemically,
fixed oils and fats are esters of the alcohol glycerol (propane-1,2,3-triol)
with three molecules of long-chain carboxylic acids, called fatty acids,
which may all be the same or may differ depending on the oil (Figure 8.15).
O
CH
2
OC
R
1
O
CH
O
C
R
2
O
CH
2
OC
R
3
Figure 8.15
The structure of triglycerides.
Compounds of this type are called
triglycerides
and contain several
sites within the molecule where autoxidation can occur to cause breakdown
of the molecule. This is especially true if the fatty acids are
unsaturated
(i.e.
contain at least one carbon-carbon double bond; if the carbon chain
contains several double bonds, the oil is said to be
polyunsaturated
).
The stability of oils is very important in pharmaceuticals since non-
polar drugs (for example, contraceptive steroids and neuroleptic tranquil-
lisers) are often formulated in oily injection vehicles for intramuscular or
depot injection. Injections of this type can be given, for example, once a
month, and the drug exerts its pharmacological effect as it leaches out of
the injection site into the bloodstream. Oils used as injection vehicles
include arachis oil, from the peanut plant, olive oil, castor oil and ethyl
oleate, the ethyl ester of the 18-carbon fatty acid oleic acid (Figure 8.16).
These oils, if they are to be used parenterally, need to be chemically
pure and free from microbial contamination. As stated above, plant oils are
often complex mixtures of chemically similar compounds and so require
special forms of pharmaceutical assay (e.g. determination of their acid and
