Chemistry Reference
In-Depth Information
Tutorial examples
Q
1 Predict which of the following fatty acids will undergo
oxidation most easily, and explain what precautions should be
employed to minimise the oxidation.
CH
3
(CH
2
)
6
CH
2
CH
CHCH
2
(CH
2
)
6
COOH (oleic acid)
b
CH
3
(CH
2
)
3
CH
2
CH
CHCH
2
CH
CHCH
2
(CH
2
)
6
COOH
b
b
(linoleic acid)
CH
3
CH
2
CH
CHCH
2
CH
CHCH
2
CH
CHCH
2
(CH
2
)
6
b
b
b
COOH (linolenic acid)
A
1 Compounds that contain allylic and benzylic centres are
especially prone to autoxidation, since the radicals formed on
oxidation are stabilised by resonance. Oleic acid contains two
allylic positions, linoleic acid contains two allylic positions and
one 'double allylic' position, while linolenic contains two allylic
and two 'double allylic' positions. We would therefore expect
linolenic to be the most susceptible acid to oxidation, followed
by linoleic and oleic. (The actual relative rates of autoxidation are
linolenic (25)
oleic (1)). Precautions that can be
employed to minimise oxidative deterioration are reducing the
oxygen concentration in the container by, for example, the use of an
inert atmosphere, and the use of a well-closed and well-filled
container. It would also be advisable to store the product at low
temperature and in a dark place.
linoleic (12)
Q
2 The BP monograph for Chloramphenicol Eye Drops
contains a limit test for 2-amino-1-(4-nitrophenyl)propane-
1,3-diol (Figure 8.26).
(a) Explain why this limit test is included, and show how
the diol could be formed.
(b) Both chloramphenicol and the diol absorb ultraviolet
light at the same
k
max
. Outline the principles of a stability-
indicating test that could be used to measure the amount of
diol in the eye drops.
