Organic Chemistry

methyl acetate

690 CHAPTER 17 Carbonyl Compounds I

PROBLEM 14

a. Propose a mechanism for the reaction of acetic anhydride with water.

b. How does this mechanism differ from the mechanism for the reaction of acetic anhy-

dride with an alcohol?

PROBLEM 15

We have seen that acid anhydrides react with alcohols, water, and amines. In which one of

these three reactions does the tetrahedral intermediate not have to lose a proton before it

eliminates the carboxylate ion? Explain.

17.10 Reactions of Esters

Esters do not react with halide ions or with carboxylate ions because these nucleo-

philes are much weaker bases than the leaving group of the ester (Table 17.1).

An ester reacts with water to form a carboxylic acid and an alcohol. This is an

example of a hydrolysisreaction—a reaction with water that converts one compound

into two compounds (lysisis Greek for “breaking down”).

An ester reacts with an alcohol to form a new ester and a new alcohol. This is an ex-

ample of an alcoholysisreaction. This particular alcoholysis reaction is also called a

transesterification reactionbecause one ester is converted to another ester.

Both the hydrolysis and the alcoholysis of an ester are very slow reactions because water

and alcohols are poor nucleophiles and esters have very basic leaving groups. These reac-

tions, therefore, are always catalyzed when carried out in the laboratory. Both hydrolysis

and alcoholysis of an ester can be catalyzed by acids. (See Section 17.11.) The rate of hy-

drolysis can also be increased by hydroxide ion, and the rate of alcoholysis can be in-

creased by the conjugate base of the reactant alcohol (Section 17.12).

Esters also react with amines to form amides. A reaction with an amine that converts

one compound into two compounds is called aminolysis. Notice that the aminolysis of

an ester requires only one equivalent of amine, unlike the aminolysis of an acyl halide

or an acid anhydride, which requires two equivalents (Sections 17.8 and 17.9). This is

because the leaving group of an ester is more basic than the amine, so the alkox-

ide ion—rather than unreacted amine—picks up the proton generated in the reaction.

+

CH 3 CH 2

CH 3 CH 2 OH

ethyl propionate

an aminolysis reaction

N-methylpropionamide

+

CH 3 CH 2

CH 3 NH 2

OCH 2 CH 3 NHCH 3

C

O

C

O

(RO-)

(RO-)

+ CH 3 OH

HCl

OCH 2 CH 3

methyl benzoate

a transesterification reaction

ethyl benzoate

+ CH 3 CH 2 OH

OCH 3

C

O

C

O

+

CH 3

+ CH 3 OH

CH 3

H 2 O

HCl

OCH 3 OH

methyl acetate

a hydrolysis reaction

acetic acid

C

O

C

O

RO-