Handbook of Solvents - George Wypych - ChemTech - Ventech!

10.3 Solvent effects based on pure solvent scales 611
Figure 10.3.9. Plot of the fluorescence maxima, v max(em),
for the Neutral Red in different solvents vs. the predicted,
v max(em), values according to eq. [10.3.26].
10.3.7.2.2 Kinetics
Kinetics so closely related to the solvent effect as those of the Menschutkin reaction between
triethylamine and ethyl iodide [eq. (10.3.27)], the solvolysis of tert-butyl chloride
[eq. (10.3.28)] or the decarboxylation of 3-carboxybenzisoxazole [eq. (10.3.29)], are acceptably
described by our scales. 15,92,93
The equation for the Menschutkin kinetics is
log k s/k Hex = 8.84(±0.66)SPP + 1.90(±1.37)SA - 4.07(±0.53) [10.3.27]
with n = 27, r = 0.947 and sd = 0.41.
The rate of this reaction between triethylamine and ethyl iodide, which varies by five
orders of magnitude from n-hexane (1.35×10 -8 l mol -1 s -1 ) to DMSO (8.78×10 -4 l mol -1 s -1 ), is
accurately described by solvent polarity and acidity -the sensitivity to the latter is somewhat
imprecise. The equation for the kinetics of solvolysis of tert-butyl chloride is:
log k = 10.02(±1.14)SPP + 1.84(±0.99)SB + 8.03(±0.69)SA - 19.85(±0.70) [10.3.28]
with n = 19, r = 0.985 and sd = 0.80.
Based on eq. [10.3.28], all solvent effects increase the rate of solvolysis. However, the
strongest contribution is that of polarity and the weakest one that of acidity. Although much
less significant, the contribution of solvent basicity is especially interesting as it confirms
that nucleophilicity also assists in the solvolytic process. Taking into account that it encompasses
data spanning 18 orders of magnitude (from log k = -1.54 for water to log k = -19.3
for the gas phase), the fit is very good (see Figure 10.3.10). The equation for the kinetics of
decarboxylation of 3-carboxybenzisoxazole is:
log k = 10.37(±1.47)SPP + 2.59(±0.74)SB - 5.93(±0.58)SA - 9.74(±1.17) [10.3.29]
with n = 24, r = 0.951 and sd = 0.73.
Figure 10.3.10. Plot of Grunwald and Wistein’s log
k tBuCl values vs. the predicted log k tBuCl values according
to eq. [10.3.29].