Organic Chemistry

Section 24.5 Metal-Ion Catalysis 1007

In specific-base catalysis, the base has to be strong enough to remove a proton from
the reactant completely before the slow step begins. In general-base catalysis, the base
can be weaker because the proton is only partially transferred to the base in the transi-
tion state of the slow step. We will see that enzymes catalyze reactions using general-
acid and general-base catalytic groups because at physiological pH too small a
concentration of for specific-acid catalysis or for specific-
base catalysis is available.

PROBLEM 5

The mechanism for hydroxide-ion-promoted ester hydrolysis is shown in Section 17.12.

What catalytic role does hydroxide ion play in this mechanism?

PROBLEM 6

The following reaction occurs by a mechanism involving general-base catalysis:

Propose a mechanism for this reaction.

24.5 Metal-Ion Catalysis

Metal ions exert their catalytic effect by coordinating (i.e., complexing) with atoms
that have lone-pair electrons. In other words, metal ions are Lewis acids (Section 1.21).
A metal ioncan increase the rate of a reaction in several ways.

• It can make a reaction center more susceptible to receiving electrons, as in A in
  the following diagram:


• It can make a leaving group a weaker base, and therefore a better leaving group,
  as in B.


• It can increase the rate of a hydrolysis reaction by increasing the nucleophilicity
  of water, as in C.

Nu

Metal

Metal

A

+ H+

C

O

Metal OH 2 Metal OH

B C

C

O

C OCH 3

OH

metal-bound

water

metal-bound

hydroxide ion

+

+

+

+

+

+ + −

base

C OCH 2 CH 3

CH 2 OH

+ CH 3 CH 2 OH

O

O

O

1 ' 1 * 10 -^7 M 2 H+ HO-

(7.3),

+++−OH HB

slow

ClCH 2 CCH 2 Cl

O

OH

H

C

O

H 2 O

a hydrate

B

−

B

−

ClCH 2 CH 2 Cl

general-base-catalyzed dehydration

Tutorial:

Categorizing catalytic

pathways