Chemistry Reference
In-Depth Information
a very small extent, it is acceptable to consider the ionisation of
the
compound in terms of the enol, and this is shown in Figure 3.9.
Table 3.1
Comparison of resonance and tautomerism
Resonance forms of a drug
a
Tautomeric forms of a drug
b
Same compound
Different compounds
Differ only in position of electrons
Differ in position of atoms (usually
hydrogen)
Each canonical form contributes to a single
Each form exists in equilibrium
resonance structure
Canonical forms cannot be isolated
Each tautomer may be isolated
a
Represented by a double-headed arrow
2
.
b
Represented by an equilibrium arrow
3
.
O
O
O
O
OH
O
H
H
-
CH
3
O
CH
3
O
+ H
+
O
-
O
O
H
O
CH
3
Figure 3.9
The ionisation of the enol form of warfarin.
There is a popular misconception that, because amines are basic and
amines contain a nitrogen atom, then all drugs that contain nitrogen will be
basic. This is not true, as a moment's thought will confirm.
Amides contain nitrogen and are neutral, and quite a few drugs
containing nitrogen atoms are actually acidic. Compounds are basic only if
