Organic Chemistry

(Dana P.) #1
PROBLEM 22

Write the sequence of steps involved in the conversion of benzene into benzenediazonium
chloride.

PROBLEM 23 SOLVED

Show how the following compounds could be synthesized from benzene:
a.m-dibromobenzene d.m-nitrotoluene
b. m-bromophenol e.p-methylbenzonitrile
c.o-chlorophenol f. m-chlorobenzaldehyde

SOLUTION TO 23a A bromo substituent is an ortho–para director, so halogenation
cannot be used to introduce both bromo substituents of m-dibromobenzene. Knowing
that a bromo substituent can be placed on a benzene ring with a Sandmeyer reaction
and that the bromo substituent in a Sandmeyer reaction replaces what originally was
a meta-directing nitro substituent, we have a route to the synthesis of the target
compound.

16.11 The Arenediazonium Ion as an Electrophile


In addition to being used to synthesize substituted benzenes, arenediazonium ions
can be used as electrophiles in electrophilic aromatic substitution reactions. Be-
cause an arenediazonium ion is unstable at room temperature, it can be used as an
electrophile only in electrophilic aromatic substitution reactions that can be carried
out well below room temperature. In other words, only highly activated benzene
rings (phenols, anilines, and N-alkylanilines) can undergo electrophilic aromatic
substitution reactions with arenediazonium ion electrophiles. The product of the
reaction is an azo compound. The linkage is called an azo linkage. Because
the electrophile is so large, substitution takes place preferentially at the less steri-
cally hindered para position.


N“N

NO 2 NO 2

Br

NH 2

Br

Br

N

Br

Br


  1. Sn/HCl

  2. HO−


NaNO 2
0 °C
HBr

HNO 3
H 2 SO 4

Br 2

CuBr

FeBr 3

+
N Br−

Section 16.11 The Arenediazonium Ion as an Electrophile 649

+ 3 Br 2 + N 2

NH 2 NH 2
Br Br

Br

N
Br Br

Br

Br Br

Br

NaNO 2 , HBr H 3 PO 2
0 °C

+
NBr−

1,3,5-tribromobenzene
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