Organic Chemistry

648 CHAPTER 16 Reactions of Substituted Benzenes

PROBLEM 21

Explain why HBr should be used to generate the benzenediazonium salt if bromobenzene is

the desired product of the Sandmeyer reaction and HCl should be used if chlorobenzene

is the desired product.

An iodo substituent will replace the diazonium group if potassium iodide is added

to the solution containing the diazonium ion.

Fluoro substitution occurs if the arenediazonium salt is heated with fluoroboric acid

This reaction is known as the Schiemann reaction.

If the aqueous solution in which the diazonium salt has been synthesized is acidified

and heated, an OH group will replace the diazonium group. ( is the nucleophile.)

This is a convenient way to synthesize a phenol.

Phenols can also be prepared at room temperature using cuprous oxide and aqueous

cupric nitrate.

A hydrogen will replace a diazonium group if the diazonium salt is treated with

hypophosphorous acid This is a useful reaction if an amino group or a

nitro group is needed for directing purposes and subsequently must be removed. It

would be difficult to envision how 1,3,5-tribromobenzene could be synthesized

without such a reaction.

(H 3 PO 2 ).

p-cresol

NNCl

CH 3

Cu 2 O

Cu(NO 3 ) 2 , H 2 O

+

+

− OH

CH 3

N 2

N

+

NCl− OH

+ H 3 O+ + N 2 + HCl

phenol

∆

H 2 O

N

+

NCl− N

+

NBF 4 − F

HBF 4 ∆

+ BF 3 + N 2 + HCl

fluorobenzene

Schiemann reaction

(HBF 4 ).

N

+

NCl− I

CH 3

+ KI + N 2

CH 3

+ KCl

p-toluenediazonium p-iodotoluene

chloride

Günther Schiemann (1899–1969)
was born in Germany. He was a
professor of chemistry at the
Technologische Hochschule in
Hanover, Germany.