Organic Chemistry

(Dana P.) #1

650 CHAPTER 16 Reactions of Substituted Benzenes


However, if the para position is blocked, substitution will occur at an ortho position.

The mechanism for electrophilic aromatic substitution with an arenediazonium ion
electrophile is the same as the mechanism for electrophilic aromatic substitution with any
other electrophile.

Azo compounds, like alkenes, can exist in cis and trans forms. Because of steric
strain, the trans isomer is considerably more stable than the cis isomer (Section 4.11).

We have seen that azobenzenes are colored compounds because of their extended con-
jugation and are used commercially as dyes (Section 8.12).

PROBLEM 24

In the mechanism for electrophilic aromatic substitution with a diazonium ion as the
electrophile, why does nucleophilic attack occur on the terminal nitrogen atom of the dia-
zonium ion rather than on the nitrogen atom bonded to the benzene ring?

N N

N N
trans-azobenzene cis-azobenzene

p-methylphenol benzenediazonium
chloride

2-hydroxy-
5-methylazobenzene

OH N

CH 3

+

CH 3

OH

NN

+
N Cl−

OH N

+

OH

N

N

+
N Cl−

Br

Br

an azo linkage

phenol meta-bromobenzenediazonium
chloride

3-bromo-4'-hydroxyazobenzene
an azo compound

3-D Molecule:
para-N,N-Dimethyl-
aminoazobenzene

N

N,N-dimethylaniline

mechanism for electrophilic aromatic substitution using an arenediazonium ion electrophile

N(CH 3 ) 2

+

p-N,N-dimethylaminoazobenzene

N
B HB
N

+

+


N

N(CH 3 ) 2 N(CH 3 ) 2

H N N
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