Chemistry Reference
In-Depth Information
O
O
+ H
+
R
C
R
C
O
-
OH
Figure 3.1
The ionisation of a carboxylic acid.
O
-
O
O
R
C
R
C
R
C
-
O
-
O
O
(a)
(b)
Figure 3.2
Resonance stabilisation of the carboxylate anion.
The effect of resonance may be seen when the acidity of a simple
carboxylic acid such as acetic acid is compared with the acidity of an
alcohol such as ethanol. Both compounds can ionise to liberate a proton,
but while the anion formed on ionisation of acetic acid is resonance-
stabilised, the ethoxide anion formed on ionisation of ethanol is not so
stabilised and the negative charge resides wholly on the oxygen atom (see
Figure 3.3).
O
O
+ H
+
CH
3
C
CH
3
C
OH
O
-
p
K
a
= 4
.
7
+ H
+
CH
3
CH
2
O
-
CH
3
CH
2
OH
p
K
a
~ 16
Figure 3.3
The ionisations of acetic acid and ethanol.
The p
K
a
of acetic acid is 4.7, while the p
K
a
of ethanol is approxi-
mately 16. This means that acetic acid is almost a hundred thousand million
(or 10
11
) times more acidic than ethanol. Alcohols are much weaker acids
than water and in biological systems are considered to be neutral. To bring
about the ionisation of an alcohol requires the use of a very strong base
such as metallic sodium.
