Organic Chemistry

(Dana P.) #1

1006 CHAPTER 24 Catalysis


PROBLEM 4 SOLVED

An alcohol will not react with aziridine unless the reaction is catalyzed by an acid. Why is
a catalyst necessary?

SOLUTION Although relief of ring strain is sufficient by itself to cause an epoxide to
undergo a ring-opening reaction (Section 12.7), it is not sufficient to cause an aziridine to
undergo a ring-opening reaction. A negatively charged nitrogen is a stronger base, and
therefore a poorer leaving group, than a negatively charged oxygen. An acid, therefore, is
needed to make the ring nitrogen a better leaving group by protonating it.

24.4 Base Catalysis


A base catalystincreases the rate of a reaction by removing a proton from the reac-
tant. For example, dehydration of a hydrate in the presence of hydroxide ion is a
base-catalyzed reaction. Hydroxide ion (the base) increases the rate of the reaction
by removing a proton from the neutral hydrate.

Removing a proton from the hydrate increases the rate of dehydration by providing
a pathway with a more stable transition state. The transition state for elimination of
from a negatively charged tetrahedral intermediate is more stable because a pos-
itive charge does not develop on the electronegative oxygen atom, as it does in the
transition state for elimination of from a neutral tetrahedral intermediate.

The foregoing base-catalyzed dehydration of a hydrate is an example of specific-
base catalysis. In specific-base catalysis, the proton is completely removed from the
reactant beforethe slow step of the reaction begins. In general-base catalysis, on the
other hand, the proton is removed from the reactant duringthe slow step of the reac-
tion. Compare the extent of proton transfer in the slow step of the preceding specific-
base-catalyzed dehydration with the extent of proton transfer in the slow step of the
following general-base-catalyzed dehydration:


  • OH

  • OH


+

+ −OH

slow
ClCH 2 CCH 2 Cl

O

OH

H

ClCH 2 CCH 2 Cl

O

C

O

OH H 2 O

OH

a hydrate



ClCH 2 CH 2 Cl

specific-base catalyzed dehydration

HCl

aziridine

+ CH 3 OH

H
N H 3 NCH 2 CH 2 OCH 3
−Cl

+

A proton is removed from the reactant
in a base-catalyzed reaction.


C

OH

OH

+


C

OH

OH



transition state for elimination of −OH from
a negatively charged tetrahedral intermediate

transition state for elimination of −OH from
a neutral tetrahedral intermediate
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