Section 16.12 Mechanism for the Reaction of Amines with Nitrous Acid 651
PROBLEM 25
Give the structure of the activated benzene ring and the diazonium ion used in the synthesis
of the following compounds:
a. butter yellow b. methyl orange
(The structures of these compounds can be found in Section 8.12.)
16.12 Mechanism for the Reaction
of Amines with Nitrous Acid
We have seen that the reaction of a primary amine with nitrous acid produces a diazo-
nium salt. Both aryl amines and alkyl amines undergo this reaction, and both follow
the same mechanism.
Conversion of a primaryamino group to a diazonium group requires a nitrosonium
ion that is formed when water is eliminated from protonated nitrous acid.
The nitrosonium ion accepts a share of the amino nitrogen’s lone pair. Loss of a proton
from nitrogen forms a nitrosamine(also called an N-nitroso compoundbecause a
nitroso substituent is bonded to a nitrogen). Delocalization of nitrogen’s lone pair and
protonation of oxygen form a protonated N-hydroxyazo compound. This compound is
in equilibrium with its nonprotonated form, which can be reprotonated on nitrogen
(reverse reaction) or protonated on oxygen (forward reaction). Elimination of water
forms the diazonium ion.
N
NaNO 2 , HCl
aryl NH (^20) °C aryl Cl
- N −
N
NaNO 2 , HCl
alkyl NH (^20) °C alkyl Cl - N −
HO N O +N OH+ 2 O
H
HO N O
H Cl H Cl
Na+ N
Na+Cl−
−O O
nitrosonium
ion
sodium nitrite nitrous acid
Cl−
N
B
an N-hydroxyazo
compound
NOH
N
aniline a nitrosamine
a primary amine
HOHN
B
HN NO
H
H B
N NOH
H
N
a diazonium
ion
N
- H 2 O
−
B
−
B
−
HN HN
H
NO
- N O
H