Page 379 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

360 Scheme 3.7. Effect of Solvent Polarity on Reactions of Various Charge Types
1. Neutralization of charge.
CHAPTER 3
− +
Structural Effects on − B → A----B → A −B Favored by nonpolar solvent
−
Stability and Reactivity A +
2. Separation of charge.
− +
−
A −B → A----B → A +B − Favored by polar solvent
3. Neutral reactants and products.
A +B → A----B → A −B Relatively insensitive to solvent polarity
4. Relative concentration of charge.
+ +
A −B → A----B → A +B + Slightly favored by polar solvent
+
5. Relative dispersal of charge.
+ +
A +B → A----B → A −B + Slightly favored by nonpolar solvent
+
leaves the oppositely charge reactants in a more reactive state. Arguing along these
lines, the broad relationships between reactivity and solvent type shown in Scheme 3.7
can be deduced.
Many empirical measures of solvent polarity have been developed. 150 One of the
most useful is based on shifts in the absorption spectrum of a reference dye. The
position of absorption bands is sensitive to solvent polarity because the electronic
distribution, and therefore the polarity, of the excited state is different from that of the
ground state. The shift in the absorption maxima reflects the effect of solvent on the
energy gap between the ground state and excited state molecules. An empirical solvent
polarity measure called E (30) is based on this concept. 151 Some values for common
T
solvents are given in Table 3.33 along with the dielectric constants for the solvents. It
can be seen that a quite different order of polarity is given by these two quantities.

Table 3.33. E (30), an Empirical Measure of Solvent Polarity, Compared with
T
Dielectric Constant a
E T (30) E T (30)
Water 63 1 78 DMF 43 8 37
Trifluoroethanol 59 5 Acetone 42 2 21
Methanol 55 5 32 7 Dichloromethane 41 1 8 9
80:20 Ethanol-water 53 7 Chloroform 39 1 4 8
Ethanol 51 9 24 5 Ethyl acetate 38 1 6 0
Acetic acid 51 2 6 1 THF 37 4 7 6
2-Propanol 48 6 19 9 Diethyl ether 34 6 4 3
Acetonitrile 46 7 38 Benzene 34 5 2 3
DMSO 45 0 47
From C. Reichardt, Angew. Chem. Int. Ed. Engl., 18, 98 (1979)

150 C. Reichardt, Angew. Chem. Int. Ed. Engl., 18, 98 (1979); C. Reichardt, Solvent Effects in Organic
Chemistry, Verlag Chemie, Weinheim, 1990; J. Catalan, V. Lopez, P. Perez, R. Martin-Villamil, and
J. G. Rodriguez, Liebigs Ann., 241 (1995); C. Laurance, P. Nicolet, M. T. Dalati, J. L. M. Abboud, and
R. Notario, J. Phys. Chem., 98, 5807 (1994).
151
C. Reichardt and K. Dimroth, Fortshr. Chem. Forsch., 11, 1 (1968); C. Reichardt, Justus Liebigs Ann.
Chem., 752, 64 (1971).