Page 100 - Mechanism and Theory in Organic Chemistry

Solutions 89
of solvent with solute, that determine the thermodynamic properties of the solu-
tion. Thermodynamic measurements yield the total enthalpies and entropies of
solution, or of transfer of a solute from one solvent to another, but do not reveal
the origin of the changes. Some of the data are difficult to interpret, and no really
satisfactory theory is available. For example, negative entropies of solution show
that there is a net increase in the amount of ordering upon dissolving nonpolar
solutes in water, whereas exothermic enthalpies indicate favorable energy changes.
These results are just the opposite of what one might have predicted by arguing
that the main effect of introducing a nonpolar molecule would be to break up the
water structure and hence to raise the energy while decreasing order by breaking
hydrogen bonds. Ions (except for very small ones such as Li+) cause a net
decrease in the amount of structure, even though there must be a considerable
amount of organization of water molecules around the ion.30 These phenomena
clearly require further investigation.

Protic and Dipolar Aprotic Solvents
It is useful to classify the more polar solvents (E > - 15) into two categories
depending on whether they are protic or aprotic. It is found that reactions
involving bases, as for example S,2 substitutions (Chapter 4), E, eliminations
(Chapter 7), and substitutions at carbonyl groups (Chapter 8), proceed much
faster in dipolar aprotic than in protic solvents, typically by factors of three to
four powers of ten and sometimes by as much as six powers of ten.31
The phenomenon can be explained by considering the various aspects of
solvent-solute interactions that we have discussed. The reactions typically take
place through the attack of an anionic reagent on a neutral molecule. The protic
solvents solvate the anions strongly by hydrogen bonding, whereas the aprotic
solvents cannot. Furthermore, the aprotic dipolar solvents, although they
ordinarily have large dipole moments, are relatively ineffective at solvating the
negative ions by dipole interactions because the positive ends of the dipoles are
usually buried in the middle of the molecule. The dipolar aprotic solvents, on the
other hand, are effective at solvating the positive counter ion because the nega-
'tive end of the dipole is ordinarily an exposed oxygen or nitrogen atom. The
result of these differences is that the anions are more free of encumbrance by
solvation in the dipolar aprotic solvents, and less energy is required to clear sol-
vent molecules out of the way so that reaction can occur.

Measures of Solvating Ability
Because solvent-solute interactions are so complex, relatively little progress has
been made in understanding them quantitatively from first principles. A useful,
if somewhat unsatisfying approach, is to assign parameters characterizing solvat-
ing ability on the basis of the measurement of some chemical or physical property
that, one hopes, is closely related at the molecular level to the phenomenon under
study.
One approach is to take the rates of a particular standard reaction in various

E. A. Arnett and D. R. McKelvey, in Solute-Solvent Interactions, Coetzee and Ritchie, Eds., p. 349.
31 See note 27(f), p. 84.