Organic Chemistry

Section 17.11 Acid-Catalyzed Ester Hydrolysis 695

tetrahedral intermediate and collapse of a tetrahedral intermediate. The acid increases
the rates of both of these steps.
The acid increases the rate of formation of the tetrahedral intermediate by pro-
tonating the carbonyl oxygen. Protonated carbonyl groups are more susceptible than
nonprotonated carbonyl groups to nucleophilic attack because a positively charged
oxygen is more electron withdrawing than a neutral oxygen. Increased electron with-
drawal by the oxygen increases the electron deficiency of the carbonyl carbon, which
increases its attractiveness to nucleophiles.

The acid increases the rate of collapse of the tetrahedral intermediate by decreasing
the basicity of the leaving group, which makes it easier to eliminate. In the acid-
catalyzed hydrolysis of an ester, the leaving group is ROH, a weaker base than the
leaving group in the uncatalyzed reaction.

Hydrolysis of Esters with Tertiary Alkyl Groups
Esters with tertiary alkyl groups undergo hydrolysis much more rapidly than do other
esters because they hydrolyze by a completely different mechanism—one that does
not involve formation of a tetrahedral intermediate. The hydrolysis of an ester with a
tertiary alkyl group is an reaction because when the carboxylic acid leaves, it
leaves behind a relatively stable tertiary carbocation.

Transesterification
Transesterification—the reaction of an ester with an alcohol—is also catalyzed by
acid. The mechanism for transesterification is identical to the mechanism for normal
ester hydrolysis, except that the nucleophile is ROH rather than H 2 O.As in hydrolysis,

SN 1

OH

CH 3 C

OH

OCH 3

tetrahedral intermediate in

uncatalyzed ester hydrolysis

OH

CH 3 C

OH

OCH 3

+

tetrahedral intermediate in

acid-catalyzed ester hydrolysis

H

(RO-)

CH 3 OCH 3 CH 3 OCH 3

C

+O

more susceptible

to attack by a

nucleophile

C

O

less susceptible

to attack by a

nucleophile

protonation of the carbonyl oxygen increases the susceptibility of

the carbonyl carbon to nucleophilic attack

H

An acid catalyst increases the

electrophilicity of the carbonyl

carbon atom and decreases the

basicity of the leaving group.

+

CH 3 HCH 3

CH 3 CH 3

HO CCH 3

CH 3

CH 3

HO CCH 3

CH 3

departure of the

leaving group to form

a tertiary carbocation

reaction of

the carbocation

with a nucleophile

H

H

B+ B HB+

+

+

+

CH 3 CH 3

CH 3

CH 3

CH 3

CH 3

SN 1

R O H 2 O

C

O

R O

C

OH

R O

CC C

O

C