Organic Chemistry

694 CHAPTER 17 Carbonyl Compounds I

Because and have approximately the same basicity, it will be as likely for

tetrahedral intermediate I to collapse to reform the ester as it will for tetrahedral

intermediate III to collapse to form the carboxylic acid. Consequently, when the reaction

has reached equilibrium, both ester and carboxylic acid will be present in approximately

equal amounts.

Excess water will force the equilibrium to the right (Le Châtelier’s principle; Sec-

tion 10.4). Or, if the boiling point of the product alcohol is significantly lower than the

boiling points of the other components of the reaction, the reaction can be driven to the

right by distilling off the product alcohol as it is formed.

The mechanism for the acid-catalyzed reaction of a carboxylic acid and an alcohol to

form an ester and water is the exact reverse of the mechanism for the acid-catalyzed hy-

drolysis of an ester to form a carboxylic acid and an alcohol. If the ester is the desired

product, the reaction should be carried out under conditions that will drive the equilibri-

um to the left—using excess alcohol or removing water as it is formed (Section 17.14).

PROBLEM 19

Referring to the mechanism for the acid-catalyzed hydrolysis of methyl acetate:

a. What species could be represented by

b. What species could be represented by

c. What species is most likely to be in a hydrolysis reaction?

d. What species is most likely to be in the reverse reaction?

PROBLEM 20

Referring to the mechanism for the acid-catalyzed hydrolysis of methyl acetate, write the

mechanism—showing all the curved arrows—for the acid-catalyzed reaction of acetic acid

and methanol to form methyl acetate. Use and to represent proton-donating and

proton-removing species, respectively.

Now let’s see how the acid increases the rate of ester hydrolysis. The acid is a cata-

lyst. Recall that catalystis a substance that increases the rate of a reaction without

being consumed or changed in the overall reaction (Section 4.5). For a catalyst to in-

crease the rate of a reaction, it must increase the rate of the slow step of the reaction.

Changing the rate of a fast step will not affect the rate of the overall reaction. Four of

the six steps in the mechanism for acid-catalyzed ester hydrolysis are proton transfer

steps. Proton transfer to or from an electronegative atom such as oxygen or nitrogen is

a fast step. So there are two relatively slow steps in the mechanism: formation of a

HB+ ≠B

HB+

HB+

≠B?

HB+?

OCH 3 excess

+ CH 3 OH

HCl

+ H 2 O

OH

C

O

CH 3 CH 3

C

O

OCH 3

CH 3 OH

CH 3 CH 3

+

HCl

excess

+ H 2 O

OH

C

O

C

O

CH 3 OCH 3 CH 3

+ H 2 O

OH

+ CH 3 OH

HCl

both ester and carboxylic acid will be

present in approximately equal amounts

when the reaction has reached equilibrium

C

O

C

O

H 2 O CH 3 OH

Tutorial:

Manipulating the equilibrium