Organic Chemistry

PROBLEM 27

How will the rate of each of the following reactions change if the polarity of a protic

polar solvent is increased?

PROBLEM 28

Which reaction in each of the following pairs will take place more rapidly?

DMSO

CH 3 Br + HO− CH 3 OH + Br−

EtOH

CH 3 Br + HO− CH 3 OH + Br−

CH 3 Br + NH 3 CH 3 OH + Br−

CH 3 Br + H 2 O CH 3 OH + Br−

CH 3 I + HO− CH 3 OH + I−

CH 3 Cl + HO− CH 3 OH + Cl−

CH 3 Br + HO− CH 3 OH + Br−

CH 3 Br + H 2 O CH 3 OH + HBr

a.

b.

c.

d.

CH 3

CH 3 SCH 3 + CH 3 O−

+

+

CH 3 OCH 3 + CH 3 SCH 3

CH 3 CH 3 I + NH 3 CH 3 CH 2 NH 3 I−

a. CH 3 CH 2 CH 2 CH 2 Br + HO− CH 3 CH 2 CH 2 CH 2 OH + Br−

b.

c.

SN 2

Section 10.10 The Role of the Solvent in SN 2 and SN 1 Reactions 393

ENVIRONMENTAL ADAPTATION

The microorganism Xanthobacterhas learned to

use the alkyl halides that reach the ground as in-

dustrial pollutants as a source of carbon. The microorganism

synthesizes an enzyme that uses the alkyl halide as a starting

material to produce other carbon-containing compounds that it

needs. This enzyme has a several nonpolar groups at its active

site—a pocket in the enzyme where the reaction it catalyzes

takes place. The first step of the enzyme-catalyzed reaction is

an reaction. Because the nucleophile is charged, the

reaction will be faster in a nonpolar solvent. The nonpolar

groups on the surface of the enzyme provide this nonpolar

environment.

SN 2

An reaction of an alkyl halide is

favored by a high concentration of a

good nucleophile in an aprotic polar

solvent.

SN 2

An reaction of an alkyl halide is

favored by a low concentration of a

poor nucleophile in a protic polar

solvent.

SN 1

Ideally, one would like to carry out an reaction with a negatively charged
nucleophile in a nonpolar solvent because the localized negative charge on the reactant is
greater than the dispersed negative charge on the transition state. However, a negatively
charged nucleophile generally will not dissolve in a nonpolar solvent, so an aprotic polar
solvent is used. Because aprotic polar solvents are not hydrogen bond donors, they are
less effective than polar protic solvents in solvating negative charges; indeed, we have
seen that they solvate negative charges particularly poorly (Section 10.3). Thus, the rate
of an reaction involving a negatively charged nucleophile will be greater in an apro-
tic polar solvent than in a protic polar solvent. Consequently, an aprotic polar solvent is
the solvent of choice for an reaction in which the nucleophile is negatively charged,
whereas a protic polar solvent is used if the nucleophile is a neutral molecule.
We have now seen that when an alkyl halide can undergo both and
reactions, the reaction will be favored by a high concentration of a good (nega-
tively charged) nucleophile in an aprotic polar solvent, whereas the reaction will
be favored by a poor (neutral) nucleophile in a protic polar solvent.

SN 1

SN 2

SN 2 SN 1

SN 2

SN 2

SN 2

Tutorial:

Common terms in and

reactions

SN 1 SN 2

Tutorial:

SN 2 promoting factors