Organic Chemistry

658 CHAPTER 16 Reactions of Substituted Benzenes

Also like benzene, naphthalene undergoes electrophilic aromatic substitution reac-

tions. Substitution occurs preferentially at the 1-position. In common nomenclature,

the 1-position is called the and the 2-position is called the

Naphthalene is more reactive than benzene toward electrophilic substitution be-

cause the carbocation intermediate is more stable and, therefore, easier to form than

the analogous carbocation intermediate formed from benzene. Because of naphtha-

lene’s increased reactivity, a Lewis acid is not needed for bromination or chlorination.

PROBLEM 33

Use the resonance contributors of naphthalene to predict whether all the carbon–carbon

bonds have the same length, as they do in benzene.

PROBLEM 34

Draw the resonance contributors for the carbocation intermediates obtained from elec-

trophilic aromatic substitution at the 1-position and the 2-position of naphthalene. Use the

resonance contributors to explain why substitution at the 1-position is preferred.

Sulfonation of naphthalene does not always lead to substitution at the 1-position. If

the reaction is carried out under conditions which cause it to be irreversible (80°C),

substitution occurs at the 1-position.

If, however, the reaction is carried out under conditions which cause it to be readily

reversible (160°C), substitution occurs predominantly at the 2-position.

This is another example of a reaction whose product composition depends on whether

the conditions used in the experiment cause the reaction to be irreversible (under kinetic

control) or reversible (under thermodynamic control). (See Section 8.7.) The 1-substi-

tuted product is the kinetic product because it is easier to form. It is the predominant

product, therefore, when the reaction is carried out under conditions that cause it to be

SO 3 H

naphthalene-2-sulfonic acid

thermodynamic product

+ H 2 SO 4

160 °C

naphthalene-1-sulfonic acid

kinetic product

SO 3 H

naphthalene-1-sulfonic acid

++H 2 SO 4 H 2 O

SO 3 H

80 °C

1-bromonaphthalene

++Br 2 HBr

Br

H 2 SO 4

++HNO 3 H 2 O

NO 2

1-nitronaphthalene

α-nitronaphthalene

8 1

5 4

2

6 3

7

a-position b-position.

3-D Molecule:

Naphthalene

3-D Molecules:

Naphthalene-1-sulfonic acid;

Naphthalene-2-sulfonic acid