Organic Chemistry

Section 10.10 The Role of the Solvent in SN 2 and SN 1 Reactions 391

Free energy

Progress of the reaction

reactants are more highly charged

transition state is less highly charged

∆G‡

∆G‡

nonpolar

solvent

polar

solvent

∆G‡

∆G‡

nonpolar

solvent

polar

solvent

Figure 10.7
Reaction coordinate diagram for a reaction in which the
charge on the reactants is greater than the charge on the
transition state.

∆G‡

∆G‡

nonpolar

solvent

polar

solvent

∆G‡

∆G‡

reactants are less highly charged

transition state is more highly charged

nonpolar

solvent

polar

Free energy solvent

Progress of the reaction

Figure 10.8

Reaction coordinate diagram for a reaction in which the

charge on the transition state is greater than the charge

on the reactants.

Increasing the polarity of the solvent

will decrease the rate of the reaction if

one or more reactants in the rate-

determining step are charged.

Increasing the polarity of the solvent

will increase the rate of the reaction if

none of the reactants in the rate-

determining step is charged.

The Effect of the Solvent on the Rate of a Reaction
One simple rule describes how a change in solvent will affect the rate of most chemi-
cal reactions:Increasing the polarity of the solvent will decrease the rate of the reac-
tion if one or more reactants in the rate-determining step are charged and will
increase the rate of the reaction if none of the reactants in the rate-determining step is
charged.
Now let’s see why this rule is true. The rate of a reaction depends on the difference
between the free energy of the reactants and the free energy of the transition state in
the rate-determining step of the reaction. We can, therefore, predict how changing the
polarity of the solvent will affect the rate of a reaction simply by looking at the charge
on the reactant(s) and the charge on the transition state of the rate-determining step, to
determine which of these species will be more stabilized by a polar solvent. The
greater the charge on the solvated molecule, the stronger the interaction with a polar
solvent and the more the charge will be stabilized.
Therefore, if the charge on the reactants is greater than the charge on the rate-deter-
mining transition state, a polar solvent will stabilize the reactants more than it will sta-
bilize the transition state, increasing the difference in energy between them.
Consequently,increasing the polarity of the solvent will decrease the rate of the reac-
tion, as shown in Figure 10.7.
On the other hand, if the charge on the rate-determining transition state is greater
than the charge on the reactants, a polar solvent will stabilize the transition state more
than it will stabilize the reactants. Therefore,increasing the polarity of the solventwill
decrease the difference in energy between them, which will increase the rate of
the reaction, as shown in Figure 10.8.

1 ¢G‡ 2

1 ¢G‡ 2

The Effect of the Solvent on the Rate of an Reaction
Now let’s see how increasing the polarity of the solvent affects the rate of an re-
action of an alkyl halide. The alkyl halide is the only reactant in the rate-determining
step of an reaction. It is a neutral molecule with a small dipole moment. The rate-
determining transition state has a greater charge because as the carbon–halogen bond
breaks, the carbon becomes more positive and the halogen becomes more negative.
Since the charge on the transition state is greater than the charge on the reactant,

SN 1

SN 1

SN 1