Organic Chemistry

1002 CHAPTER 24 Catalysis

If iodide ion is present in the reaction mixture, the reaction takes place by two succes-

sive reactions.

The first reaction in the catalyzed reaction is faster than the uncatalyzed reac-

tion because in a protic solvent, iodide ion is a better nucleophile than hydroxide ion,

which is the nucleophile in the uncatalyzed reaction (Section 10.3). The second

reaction in the catalyzed reaction is also faster than the uncatalyzed reaction because

iodide ion is a weaker base and therefore a better leaving group than chloride ion, the

leaving group in the uncatalyzed reaction. Thus, iodide ion increases the rate of for-

mation of ethanol by changing a relatively slow one-step reaction into a reaction with

two relatively fast steps (Figure 24.2).

Iodide ion is a nucleophilic catalyst because it reacts as a nucleophile, forming a co-

valent bond with the reactant. The iodide ion that is consumed in the first reaction is

regenerated in the second, so it comes out of the reaction unchanged.

Another reaction in which a nucleophilic catalyst provides a more favorable path-

way by changing the mechanism of the reaction is the imidazole-catalyzed hydrolysis

of an ester.

Imidazole is a better nucleophile than water, so imidazole reacts faster with the ester

than water does. The acyl imidazole that is formed is particularly reactive because the

positively charged nitrogen makes imidazole a very good leaving group. Therefore, it

is hydrolyzed much more rapidly than the ester would have been. Because formation

of the acyl imidazole and its subsequent hydrolysis are both faster than ester hydroly-

sis, imidazole increases the rate of ester hydrolysis.

+

imidazole

phenyl acetate acetic acid phenol

NNH

CH 3 CO H 2 O

O

CH 3 COH + HO

O

a nucleophilic

catalyst

SN 2

SN 2

mechanism of the iodide-ion-catalyzed reaction

ICl+ CH 3 CH 2 CH 3 CH 2 ICl+ −

−

HO + CH 3 CH 2 I CH 3 CH 2 OH + I−

−

SN 2

mechanism of the uncatalyzed reaction

HO + CH 3 CH 2 Cl CH 3 CH 2 OH + Cl−

−

The 2001 Nobel Prize in chemistry
was awarded to K. Barry Sharpless,
William S. Knowles, and Royji
Noyroifor their work in the field of
catalysis.

William Knowles, was born in 1917.
He received a Ph.D. from Columbia
University in 1942 and is a scientist
at Monsanto Company, St. Louis,
Missouri.

Royji Noyroiwas born in 1938 in
Kobe, Japan. He received a Ph.D.
from Kyoto University and is a
Professor of materials science at
Nagoya University in Nagoya, Japan.
Knowles and Noyroi were cited for
their work on chirally catalyzed
hydrogenation.

K. Barry Sharplesswas cited for his
work on chirally catalyzed oxidation
reactions (Section 20.5).

O

H 2 O

an acyl imidazole

+

+

−

+

+

an ester

CH 3 CO N NH

O

CH 3 C

O

CH 3 CO + −O

N

N

H

CH 3 CO− ++HN NH HO

O

NNH