Chemistry Reference
In-Depth Information
A
2 Both drugs are sulfonamides and ionise with the release of a
proton. The anion formed is stabilised by resonance as discussed
previously. The p
K
a
value for sulfanilamide is 10.4, while the p
K
a
of sulfacetamide is 5.4. Clearly, since the value of p
K
a
is lower,
sulfacetamide is a much stronger acid than sulfanilamide (five
units difference on a log scale
100 000 times stronger). This
difference in acidity is caused by the presence of a carbonyl group
adjacent to the sulfonamido hydrogen. This allows additional
resonance to take place in sulfacetamide. Upon ionisation, the
negative charge on the anion can delocalise onto the carbonyl
oxygen as shown in Figure 3.23. This delocalisation further
stabilises the anion and is in addition to the normal resonance
present in the sulfonamide group.
H
O
O
-
H
2
N
N
H
2
N
S
N
CH
3
S
CH
3
O
O
O
O
+ H
+
O
H
2
N
SN
CH
3
O
O
-
-
O
O
H
2
N
SN
H
2
N
SN
CH
3
CH
3
O
O
O
O
-
Figure 3.23
Resonance effects in sulfacetamide.
