Organic Chemistry

646 CHAPTER 16 Reactions of Substituted Benzenes

Heating in dilute acid removes the sulfonic acid group (Section 15.12). Using a sulfonic

acid group to block the para position is a method commonly employed to synthesize high

yields of ortho-substituted compounds.

16.10 Synthesis of Substituted Benzenes

Using Arenediazonium Salts

So far, we have learned how to place a limited number of different substituents on a

benzene ring—those substituents listed in Section 16.2 and those that can be obtained

from these substituents by chemical conversion. However, the kinds of substituents

that can be placed on benzene rings can be greatly expanded by the use of

arenediazonium salts.

The drive to form a molecule of stable nitrogen gas causes the leaving group

of a diazonium ion to be easily displaced by a wide variety of nucleophiles. The mecha-

nism by which a nucleophile displaces the diazonium group depends on the nucleophile:

Some displacements involve phenyl cations, while others involve radicals.

A primary amine can be converted into a diazonium salt by treatment with nitrous

acid Because nitrous acid is unstable, it is formed in situ, using an aqueous

solution of sodium nitrite and HCl or HBr; indeed, is such a good leaving group

that the diazonium salt is synthesized at 0°C and used immediately without isolation.

(The mechanism for conversion of a primary amino group to a diazonium

group is shown in Section 16.12.)

Nucleophiles such as and will replace the diazonium group if

the appropriate cuprous salt is added to the solution containing the arenediazonium

salt. The reaction of an arenediazonium salt with a cuprous salt is known as a

Sandmeyer reaction.

• C‚N,Cl-, Br-

NH 2 N NCl−

+

NaNO 2 , HCl

0 °C

[+N‚N]

[NH 2 ]

N 2

(HNO 2 ).

NN

−Nu

Cl−

+

+

Nu

++N 2 Cl−

benzenediazonium

chloride

(N 2 )

NN Cl−

+

an arenediazonium salt

OH OH

SO 3 HSO 3 H

OH

Br Br

OH

H 2 SO 4

100 °C

Br 2 H 3 O+

100 °C

3-D Molecule:

Benzenediazonium chloride