Organic Chemistry

642 CHAPTER 16 Reactions of Substituted Benzenes

Aniline and N-substituted anilines also do not undergo Friedel–Crafts reactions. The

lone pair on the amino group will complex with the Lewis acid that is needed

to carry out the reaction, converting the substituent into a deactivating meta

director. As we have just seen, Friedel–Crafts reactions do not occur with benzene

rings containing meta-directing substituents.

Phenol and anisole undergo Friedel–Crafts reactions—orienting ortho and para—be-

cause oxygen, being a weaker base than nitrogen, does not complex with the Lewis acid.

Aniline also cannot be nitrated because nitric acid is an oxidizing agent and prima-

ry amines are easily oxidized. (Nitric acid and aniline can be an explosive combina-

tion.) Tertiary aromatic amines, however, can be nitrated. Because the tertiary amino

group is a strong activator, nitration is carried out with nitric acid in acetic acid, a

milder combination than nitric acid in sulfuric acid. About twice as much para isomer

is formed as ortho isomer.

PROBLEM 15

Give the products, if any, of each of the following reactions:

a. c.

b. d.

PROBLEM 16

When N,N-dimethylaniline is nitrated, some meta-nitro-N,N-dimethylaniline is formed.

Why does the meta isomer form? (Hint:The of N,N-dimethylaniline is 5.07, and more

meta isomer is formed if the reaction is carried out at than if it is carried out

at )

16.8 Designing a Synthesis III: Synthesis of

Monosubstituted and Disubstituted Benzenes

As the number of reactions with wich we are familiar increases, we have more reac-

tions to choose from when we design a synthesis. For example, we can now design two

very different routes for the synthesis of 2-phenylethanol from benzene.

Br MgBr CH 2 CH 2 OH

Br 2

FeBr 3

Mg

Et 2 O

1.

O

2. H+
   2-phenylethanol

pH=4.5.

pH=3.5

pKa

aniline+ 3 Br 2 benzene+ 2 CH 3 Cl+AlCl 3

benzonitrile+methyl chloride+AlCl 3 benzoic acid+CH 3 CH 2 Cl+AlCl 3

aniline

H 2 N

AlCl 3

+

H 2 N AlCl 3

−

a meta director

NH 2

(AlCl 3 )

3-D Molecules:

N,N-Dimethylaniline;

o-Nitro-N,N-dimethylaniline

N(CH 3 ) 2

N,N-dimethylaniline

N(CH 3 ) 2

+

NO 2

o-nitro-N,N-dimethylaniline

N(CH 3 ) 2

NO 2

p-nitro-N,N-dimethylaniline

1. HNO 3


2. HO -

CH 3 COOH