The Foundations of Chemistry

Substitution is the most common kind of reaction of an aromatic ring. Halogenation,
with chlorine or bromine, occurs readily in the presence of iron or anhydrous iron(III)
chloride (a Lewis acid) catalyst.

The equation is usually written in condensed form as

Aromatic rings can undergo nitration,substitution of the nitrogroup XNO 2 , in a
mixture of concentrated nitric and sulfuric acids at moderate temperatures.

The explosive TNT (2,4,6-trinitrotoluene) is manufactured by the nitration of toluene
in three steps.



H 2 SO 4

3HNO 3 3H 2 O

toluene nitric acid 2,4,6-trinitrotoluene (TNT)

NO 2

NO 2

CH 3

O 2 N

CH 3



conc.

H 2 SO 4

50 

HNO 3 H 2 O

nitric acid nitrobenzene

NO 2



Cl

Cl 2 HCl

FeCl 3

benzene chlorobenzene

H

H

HH

H H



Cl

H

HH

H H

Cl 2 HCl

FeCl 3

Cl Cl

H

H

H

C

 HCl

heat or

UV

1,1-dichlorethane

bp  57 C

CCl

Cl

H

H

H

C

1,2-dichloroethane

bp  84 C

Cl CCl

H

H

H

H

H

C

H

CCl

H

The product mixture does not contain

equal numbers of moles of the

dichloroethanes so we do not show a

stoichiometrically balanced equation.

Because reactions of saturated

hydrocarbons with chlorine can

produce many products, the reactions

are not always as useful as might be

desired.

27-16 Substitution Reactions 1087

A model of a molecule of TNT.

When iron is added as a catalyst, it

reacts with chlorine to form iron(III)

chloride, which is the true catalyst.

The H 2 SO 4 is both a catalyst and a

dehydrating agent. The H in the

product H 2 O comes from the

hydrocarbon; the OH comes from

HNO 3.