Organic Chemistry

Section 17.11 Acid-Catalyzed Ester Hydrolysis 693

Normal Ester Hydrolysis
The first step in the mechanism for acid-catalyzed ester hydrolysis is protonation of
the carbonyl oxygen by the acid. Recall that represents any species in the solu-
tion that is capable of donating a proton and represents any species in the solution
that is capable of removing a proton.

The carbonyl oxygen is protonated because it is the atom with the greatest electron
density, as shown by the resonance contributors.

In the second step of the mechanism, the nucleophile attacks the protonated
carbonyl group. The resulting protonated tetrahedral intermediate (tetrahedral intermedi-
ate I) is in equilibrium with its nonprotonated form (tetrahedral intermediate II). Either
the OH or the OR group of tetrahedral intermediate II (in this case, ) can be
protonated. Because the OH and OR groups have approximately the same basicity, both
tetrahedral intermediate I (OH is protonated) and tetrahedral intermediate III (OR is pro-
tonated) are formed. (From Section 1.20, we know that the relative amounts of the three
tetrahedral intermediates depend on the pH of the solution and the values of the pro-
tonated intermediates.) When tetrahedral intermediate I collapses, it expels in pref-
erence to because is a weaker base, thereby reforming the ester. When
tetrahedral intermediate III collapses, it expels rather than because
is a weaker base, thereby forming the carboxylic acid. (Tetrahedral intermediate
II is less likely to collapse, because both and are strong bases.) Acid-
catalyzed ester hydrolysis is discussed in greater detail in Section 24.3.

HO- CH 3 O-

CH 3 OH

CH 3 OH HO-

CH 3 O- H 2 O

H 2 O

pKa

OR=OCH 3

(H 2 O)

CH 3 OCH 3 CH 3 OCH 3

+

−

resonance contributors of an ester

C

O

C

O

OCH 3 CH 3 OCH 3

O

+ H

B

HB+

CH 3

O

C C

≠B

HB+

represents any species in the

solution that is capable of donating a

proton and represents any species in

the solution that is capable of removing

a proton.

≠B

HB

mechanism for acid-catalyzed ester hydrolysis

HB+

CH 3 OCH 3

+

CH 3 OCH 3 + H 2 O +

O

+

CH 3 C

tetrahedral intermediate I

tetrahedral intermediate II

tetrahedral intermediate III

OCH 3

OH

OH

B

CH 3 C OCH 3

OH

OH HB+

+

CH 3 C OCH 3

H

OH

OH

CH 3 OH

HB+

CH 3 OH

+ CH 3 OH

O

protonation of

the carbonyl

group

formation of

a tetrahedral

intermediate

elimination of

the weaker base

equilibration of the

three tetrahedral

intermediates

removal of a

proton from

the carbonyl group

O

C C

H

H

C

O

C

H B