Organic Chemistry

1012 CHAPTER 24 Catalysis

Reaction C is faster than reaction B because the alkyl substituents of the reactant in C

decrease the available volume for rotation of the reactive groups away from each other.

Thus, there is a greater probability that the molecule will be in a conformation with the

reacting groups positioned for ring closure. This is called the gem-dialkyl effect(or

Thorpe–Ingold effect) because the two alkyl substituents are bonded to the same

(geminal) carbon. Comparing the rate when the substituents are methyl groups with the

rate when the substituents are isopropyl groups, we see that the rate is further increased

when the size of the alkyl groups is increased.

The increased rate of reaction E is due to the double bond that prevents the reacting

groups from rotating away from each other. The bicyclic compound in F reacts even

faster, because the reacting groups are locked in the proper orientation for reaction.

PROBLEM 10

The relative rate of reaction of the cis alkene (E) is given in Table 24.2. What would you

expect the relative rate of reaction of the trans isomer to be?

24.7 Intramolecular Catalysis

Just as putting two reacting groups in the same molecule increases the rate of a reac-

tion compared with having the groups in separate molecules, putting a reacting group

and a catalystin the same molecule increases the rate of a reaction compared with

having them in separate molecules. When a catalyst is part of the reacting molecule,

the catalysis is called intramolecular catalysis. Intramolecular nucleophilic catalysis,

intramolecular general-acid or general-base catalysis, and intramolecular metal-ion

catalysis all occur. Intramolecular catalysis is also known as anchimeric assistance

(anchimericmeans “adjacent parts”in Greek). Let’s now look at some examples of in-

tramolecular catalysis.

The following intramolecular general-base-catalyzed enolization reaction is con-

siderably faster than the analogous intermolecular general-base-catalyzed reaction:

When chlorocyclohexane reacts with an aqueous solution of ethanol, an alcohol

and an ether are formed. Two products are formed because there are two nucleophiles

( and ) in the solution.

CH 3 CH 2 OH

H 2 O ++HCl

Cl OH OCH 2 CH 3

H 2 O CH 3 CH 2 OH

+ RCO

intermolecular general-base catalysis

O CH 3

C CCH 3 O

H

−

+ RCOH

O

−O CH

3

C CCH 3

intramolecular general-base catalysis

O

O

CH 3

C

CO

CCH 3

H

−

−O

O

CH 3

C

COH

CCH 3