Organic Chemistry

1004 CHAPTER 24 Catalysis

The acid increases the rate of the second slow step by changing the basicity of the

group that is eliminated when the tetrahedral intermediate collapses. In the presence

of an acid, methanol of is eliminated; in the absence of an

acid, methoxide ion of is eliminated. Methanol is a weaker

base than methoxide ion, so it is more easily eliminated.

The mechanism for the acid-catalyzed hydrolysis of an ester shows that the reaction

can be divided into two distinct phases: formation of a tetrahedral intermediate and

collapse of a tetrahedral intermediate. There are three steps in each phase. Notice that

in each phase, the first step is a fast protonation step, the second step is a slow cat-

alyzed step that involves either breaking a bond or forming a bond, and the last

step is a fast deprotonation step (to regenerate the catalyst).

PROBLEM 2

Compare each of the following mechanisms with the mechanism for each phase of the

acid-catalyzed hydrolysis of an ester, indicating

a. similarities b. differences

1. acid-catalyzed formation of a hydrate (Section 18.7)


2. acid-catalyzed conversion of an aldehyde into a hemiacetal (Section 18.7)


3. acid-catalyzed conversion of a hemiacetal into an acetal (Section 18.7)


4. acid-catalyzed hydrolysis of an amide (Section 17.16)

There are two types of acid catalysis:specific-acid catalysis and general-acid

catalysis. In specific-acid catalysis, the proton is fully transferred to the reactant before

the slow step of the reaction begins (Figure 24.3a). In general-acid catalysis, the

proton is transferred to the reactant duringthe slow step of the reaction (Figure 24.3b).

The mechanism for acid-catalyzed hydrolysis on p. 1003 shows that the slow steps of

the reaction are specific-acid catalyzed.

p p

CH 3 OCH 3

H

C

OH

OH

+

CH 3 C OCH 3

OH

OH

acid-catalyzed second slow step uncatalyzed second slow step

1 pKa CH 3 OH=15.7 2

CH 3 O

+

1 pKa H 2 =-2.5 2

a. specific-acid catalysis b. general-acid catalysis

Progress of the reaction

R

P P

H

Progress of the reaction

A

R + HA

RH+

R + H+

Free energy Free energy

Figure 24.3

(a) Reaction coordinate diagram for a specific-acid-catalyzed reaction. (The proton is

completely transferred to the reactant before the slow step of the reaction begins.)

(b) Reaction coordinate diagram for a general-acid-catalyzed reaction. (The proton is

partially transferred to the reactant in the transition state of the slow step of the reaction.)