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226 Physical Organic Chemistry

74 is stabilized relative to 69 by the interaction between of acetic acid is 4.75 in water, the standard solvent for pK a ,
the C Cl dipole and the negative charge that is distributed but is 9.65 in methanol. The dielectric constant of water
over the O and the OH. is 81, but methanol is a less polar solvent, with a dielec-
tric constant of only 33. Therefore, according to Eq. (27),
+
methanol provides less stabilization of the ions H and
CH 3 CO . In terms of Fig. 10, the change from water to
−
2
methanol represents a relative destabilization of the prod-
ucts of ionization and therefore a decreased equilibrium
constant (from 10 −4.75 to 10 −9.65 ).
F. Solvent Effects Similarly, the solvolysis of tert-butyl chloride (SOH =
5
H 2 O or C 2 H 5 OH) is 3 × 10 -fold faster in water than in
Solvent effects also arise from electrostatic interactions,
ethanol as solvent:
but between solvent and solute. The energy of interaction
of an ion or dipole is given approximately as follows, (CH 3 ) 3 C Cl → (CH 3 ) 3 C δ+ ··· Cl δ− ‡

where q is the charge, µ is the dipole moment, r is its
+
→ (CH 3 ) 3 C + Cl − (29)
radius (the closest distance of approach between solvent
and solute), and ε is the dielectric constant of the solvent: (CH 3 ) 3 C + SOH → (CH 3 ) 3 CO(H)S +
+
2
q ε − 1 → (CH 3 ) 3 COS + H . (30)
+
E solv,ion = , (27)
r 2ε
In this three-step reaction the first step is rate-limiting and
2
µ ε − 1 its transition state has a large dipole moment, which can
E solv,dipole = 3 . (28)
r 2ε + 1 be stabilized by a polar solvent [Eq. (28)]. In terms of
The dielectric constant is an empirical measure of the po- Fig. 12, the change from methanol to water represents a
larity of a solvent, or of how well it stabilizes ions or stabilization of the transition state and therefore a faster
dipoles. The second factor in each equation corresponds reaction.
to a greater stabilization with increasing ε. Much current research involves the investigation of
A polar solvent achieves that stabilization by clustering ions in the gas phase, free of the influence of solvation.
its own dipoles around the solute, as illustrated in Fig. 14. A dominant influence is the size of the ion because
An anion or the negative end of a dipole is especially well a positive or negative charge constrained to a small
volume repels itself strongly. Thus the order of gas-
stabilized by water because the H is so small that its δ+
can approach quite close to the negative charge. The small phase acidities of alcohols is CH 3 OH < CH 3 CH 2 OH <
r in Eqs. (27) and (28) then allows a large stabilization E. (CH 3 ) 2 CHOH < (CH 3 ) 3 COH because the negative charge
−
This is a general aspect of protic solvents, those with OH is distributed over a larger volume in (CH 3 ) 3 CO . This is
or NH groups. It is often attributed to hydrogen bonding, exactly the opposite order from solution, where solvation
−
as though there were a bond between the H and the solute, of (CH 3 ) 3 CO is most hindered.
but the interaction is largely electrostatic. In contrast, the
δ+ of polar aprotic solvents, such as dimethyl sulfoxide,
G. Delocalization Effects
(CH 3 ) 2 S O, is buried in the center of the molecule and
cannot stabilize anions as well as the exposed δ− on the Delocalization of electrons generally leads to a stabiliza-
oxygen stabilizes cations. tion. This is certainly true with both resonance and aro-
Solvents have a large effect on reactions that create or maticity, and the only exception is with antiaromaticity.
destroy ions. For example, the acidity constant (Table III) Then, according to Fig. 10 or Fig. 12, delocalization can
affect the position of equilibrium or the rate of reaction
through stabilizing (or destabilizing) reactant, product, or
transition state.
Table IV lists C H bond-dissociation energies of some
hydrocarbons. All the other bonds are weaker than the
C H of methane and require less energy to break. This
weakening does not arise from differences in the hydro-
carbons since all these C H bonds are the same in that
3
they are formed from an sp orbital on carbon and an s or-
FIGURE 14 Solvation stabilization of solute cations, anions, or bital on hydrogen. Instead the weakening must come from
dipoles by water molecules. stabilization of the radical that results from removing the