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3.5
ethanol
1-propanol
3.0
methanol
2.5
2.0
0.01
0.02
0.03
0.04
1/
ε
9.8
9.6
9.4
ethanol
1-propanol
methanol
9.2
9.0
8.8
0.01
0.02
0.03
0.04
1/
ε
Figure 2.3.
Plots of the experimental values of log
K
2
(p
K
a1
) (upper) and log
K
1
(p
K
a2
)
(lower) versus the reciprocal of dielectric constant of different mixed solvents at 25°C
and an ionic strength of 0.1 mol/dm
3
NaClO
4
(adapted from Gharib et al. [70] with the
permission of the American Chemical Society).
examined in water containing organic solvents. An example of polarity effect
on dissociation is demonstrated in Figure 2.3 for Trp in mixtures of water and
alcohol [70]. The p
K
a1
of the carboxylic group of Trp increased, but the p
K
a2
of the amino group first decreased and then increased with an increasing per-
centage of alcohols in the mixed solvents. In the case of carboxyl group dis-
sociation, no net change in charge on the molecule occurred and the change
in polarity of the medium had a minor effect. Comparatively, dissociation of
the amino group involving changes in charge and polarity greatly influenced
the p
K
a2
of Trp in water-alcohol mixtures [70]. The microscopic protonation
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