Section 24.3 Acid Catalysis 1003A proton is donated to the reactant
in an acid-catalyzed reaction.24.3 Acid Catalysis
An acid catalyst increases the rate of a reaction by donating a proton to a reactant. For ex-
ample, the rate of hydrolysis of an ester is markedly increased by an acid catalyst
How an acid catalyzes the hydrolysis of an ester can be understood by examining
the mechanism for the acid-catalyzed reaction. Donation of a proton to, and removal
of a proton from, an electronegative atom such as oxygen are fast steps. Therefore, the
reaction has two slow steps: formation of the tetrahedral intermediate and collapse of
the tetrahedral intermediate. A catalyst must increase the rate of a slow step because
increasing the rate of a fast step will not increase the rate of the overall reaction.
The acid increases the rates of both slow steps of this reaction. It increases the rate
of formation of the tetrahedral intermediate by protonating the carbonyl oxygen,
thereby increasing the reactivity of the carbonyl group. We have seen that a protonated
carbonyl group is more susceptible to nucleophilic attack than an unprotonated car-
bonyl group because the protonated carbonyl group is more electrophilic. In other
words, the protonated carbonyl group is more susceptible to nucleophilic attack
(Section 17.11). Increasing the reactivity of the carbonyl group by protonating it is an
example of providing a way to convert the reactant into a less stable (i.e., more reac-
tive) species (Figure 24.1a).
acid-catalyzed first slow step+
CH 3 OCH 3C H 2 OOH+uncatalyzed first slow stepC + H 2 OOCH 3 OCH 3Cmechanism for acid-catalyzed ester hydrolysisHB+ slowslowCH 3C
OCH 3OOCH 3 OCH 3+CH (^3) +
C
OCH 3
- H 2 O
H
OH
CH 3 C OCH 3
OH
OH
CH 3 C OCH 3
OH
OH
CH 3 C OCH 3
H
OH
OH
H
H
B+
CH 3
C + CH 3 OH
O
CH 3
C
O HB+ B
B
HB+
CH 3 COCH 3 + H 2 O
O
CH 3 COH + CH 3 OH
O
an acid
catalyst
(HB+).
A catalyst must increase the rate of a
slow step. Increasing the rate of a fast
step will not increase the rate of the
overall reaction.