Chemistry Reference
In-Depth Information
to nucleophilic attack. The structures of a penicillin (ampicillin) and the
decomposition product, penicilloic acid, are shown in Figure 8.22.
HNH
2
HH
S
H
N
CH
3
O
N
CH
3
COOH
O
H
Ampicillin
H
2
O
HNH
2
HH
S
H
N
CH
3
O
N
H
CH
3
COOH
O
OH
H
Penicilloic acid derivative
Figure 8.22
The structures of ampicillin and penicilloic acid.
Penicillin and cephalosporin antibiotics are insufficiently stable to be
supplied dissolved in aqueous solutions. Instead, they are supplied as a dry
powder, which is reconstituted immediately prior to dispensing by the
pharmacist. The solution (or, more accurately, suspension) dispensed must
be stored in a refrigerator and discarded after 7 days. The ring-opened prod-
uct (penicilloic acid) is inactive as an antibiotic.
Other mechanisms of degradation
Rarely, some other forms of decomposition may be encountered. These
include hydration (found in some alkaloids of ergot), polymerisation
(which can affect solutions of the antibiotic ampicillin) and dimerisation
reactions (which can be seen as a result of free radical attack on
morphine). While these methods of decomposition are important and
should be borne in mind, the majority of chemical deterioration can be
explained by consideration of the few mechanisms outlined above.
