Organic Chemistry

644 CHAPTER 16 Reactions of Substituted Benzenes

In the synthesis of meta-chlorobenzoic acid, the methyl group is oxidized before

chlorination because a meta director is needed to obtain the desired product.

In the following synthesis of para-propylbenzenesulfonic acid, the type of reaction

employed, the order in which the substituents are put on the benzene ring, and the point at

which a substituent is chemically modified must all be considered: The straight-chain

propyl substituent must be put on the ring by a Friedel–Crafts acylation reaction because

of the carbocation rearrangement that would occur with a Friedel–Crafts alkylation

reaction. The Friedel–Crafts acylation must be carried out before sulfonation because

acylation could not be carried out on a ring with a strongly deactivating sulfonic acid sub-

stituent, and the sulfonic acid group is a meta director. Finally, the sulfonic acid group

must be put on the ring after the carbonyl group is reduced to a methylene group, so that

the sulfonic acid group will be directed primarily to the para position by the alkyl group.

PROBLEM 17

Show how each of the following compounds can be synthesized from benzene:

a.p-chloroaniline e.m-bromopropylbenzene

b. m-chloroaniline f. o-bromopropylbenzene

c.p-nitrobenzoic acid g. 1-phenyl-2-propanol

d.m-nitrobenzoic acid h. 2-phenylpropene

16.9 Synthesis of Trisubstituted Benzenes

When a disubstituted benzene undergoes an electrophilic aromatic substitution reaction,

the directing effect of both substituents has to be considered. If both substituents direct the

incoming substituent to the same position, the product of the reaction is easily predicted.

CH 3

NO 2

+ HNO 3

both the methyl and nitro substituents

direct the incoming substituent

to these positions

p-nitrotoluene

CH 3

NO 2

NO 2

2,4-dinitrotoluene

H 2 SO 4

CH 3 COOH

Cl

COOH

meta-chlorobenzoic acid

Cl 2

FeCl 3

Na 2 Cr 2 O 7 , H+

∆

CH 3 CH 3

Cl Cl

COOH

para-chlorobenzoic acid

Cl 2

FeCl 3

KMnO 4 , H+

∆

3-D Molecules:

p-Nitrotoluene;

m-Chlorotoluene;

p-Methylphenol;

p-Chlorotoluene

CCH 2 CH 3 CH 2 CH 2 CH 3 CH 2 CH 2 CH 3

HO 3 S

O

para-propylbenzenesulfonic acid

Zn(Hg)

HCl

1. CH 3 CH 2 CCl H 2 SO 4

O

AlCl 3 ∆

2. H 2 O ∆