Chemistry Reference
In-Depth Information
CH
3
O
N
COOH
Cl
OCH
3
CH
3
O
N
COO
-
Cl
+ H
+
OCH
3
Figure 3.12
The ionisation of indometacin.
Phenytoin
Phenytoin is an anticonvulsant widely used in the treatment of epilepsy. The
properties of phenytoin resemble those of barbiturates. It is a cyclic imide
with a p
K
a
of 8.3. The anion is stabilised by resonance of the negative
charge onto the oxygens of the carbonyl group and the drug is usually
administered as the sodium salt to increase water solubility. See Figure 3.15.
Phenytoin and barbiturates display tautomerism of the imine-imide
type, as shown in Figure 3.16. The predominant tautomer is the imide form,
although some older textbooks list the structure of the drug as the minor
tautomer.
Sulfonamides
Sulfonamides are a class of antibacterial compounds, all of which contain
the sulfonamido group
a
SO
2
NH. Although they were widely used in the
past, their use has decreased in recent years with the advent of newer anti-
biotics such as penicillins and cephalosporins. Sulfonamides are all weakly
acidic (p
K
a
approximately 5-8) due to the powerful electron-withdrawing
