Organic Chemistry

Notice that three positions are activated in the following reaction, but the new sub-
stituent ends up on only two of the three positions. Steric hindrance makes the position
between the substituents less accessible.

If the two substituents direct the new substituent to different positions, a strongly
activating substituent will win out over a weakly activating substituent or a deactivat-
ing substituent.

If the two substituents have similar activating properties, neither will dominate and
a mixture of products will be obtained.

PROBLEM 18

Give the major product(s) of each of the following reactions:

a. nitration of p-fluoroanisole

b. chlorination of o-benzenedicarboxylic acid

c. bromination of p-chlorobenzoic acid

PROBLEM 19 SOLVED

When phenol is treated with a mixture of monobromo-, dibromo-, and tribromophe-

nols is obtained. Design a synthesis that would convert phenol primarily to ortho-

bromophenol.

SOLUTION The bulky sulfonic acid group will add preferentially to the para position.

Both the OH and SO 3 Hgroups will direct bromine to the position ortho to the OH group.

Br 2 ,

H 2 SO 4

CH 3

Cl

++HNO 3

p-chlorotoluene

Cl Cl

CH 3

NO 2

4-chloro-

2-nitrotoluene

CH 3

NO 2

4-chloro-

3-nitrotoluene

CH 3 directs here

Cl directs here

OH

CH 3

+ Br 2

p-methylphenol 2-bromo-

4-methylphenol

major product

OH

CH 3

Br

OH directs here

CH 3 directs here

CH 3

Cl

++HNO 3

m-chlorotoluene

Cl Cl

CH 3

O 2 N

5-chloro-

2-nitrotoluene

CH 3

NO 2

3-chloro-

4-nitrotoluene

H 2 SO 4

both the methyl and chloro substituents

direct the incoming substituent to these

indicated positions

Section 16.9 Synthesis of Trisubstituted Benzenes 645