Organic Chemistry

Section 12.8 Arene Oxides 459

Because the first step is rate-determining, the rate of formation of phenol depends on

the stability of the carbocation. The more stable the carbocation, the easier it is to open

the epoxide ring and form the phenol.

Only one arene oxide can be formed from naphthalene because the “double bond”

shared by the two rings cannot be epoxidized. Recall from Section 7.11 that benzene

rings are particularly stable, so it is much easier to epoxidize a position that will leave

one of the benzene rings intact. Naphthalene oxide can rearrange to form either

1-naphthol or 2-naphthol. The carbocation leading to 1-naphthol is more stable

because its positive charge can be stabilized by electron delocalization without

destroying the aromaticity of the benzene ring on the left of the structure. In contrast,

the positive charge on the carbocation leading to 2-naphthol can be stabilized by

electron delocalization only if the aromaticity of the benzene ring is destroyed.

Consequently, rearrangement leads predominantly to 1-naphthol.

PROBLEM 20

Draw all possible resonance contributors for the two carbocations in the preceding reaction.

Use the resonance contributors to explain why 1-napthol is the major product of the reaction.

PROBLEM 21

The existence of the NIH shift was established by determining the major product obtained

from rearrangement of the following arene oxide, in which a hydrogen has been replaced

by a deuterium:

What would be the major product if the NIH shift did not occur? (Hint:Recall from

Section 11.7 that a bond is easier to break than a bond.)

PROBLEM 22

How would the major products obtained from rearrangement of the following arene oxides

differ?

O

H

DHO

D

C¬H C¬D

H 2 O

O

OH

D D

H

CH 3 CH 3

naphthalene oxide

+

more stable 1-naphthol

90%

O

OH OH

2-naphthol

10%

OH

can be stabilized by electron

delocalization without destroying the

aromaticity of the benzene ring

+

less stable

OH

can be stabilized by electron

delocalization only by destroying the

aromaticity of the benzene ring

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