3' Reaction with nitrous acid, HNO2

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3. Reaction with nitrous acid, HNO2

• Nitrous acid test can be used to distinguish
• 1 aliphatic and 1 aromatic amines
• 1 and 2 aliphatic amines

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• ?Nitrous acid (HONO) is a weak, unstable acid.
• ?It is prepared by treating sodium nitrite
  (NaNO2) with an aqueous solution of a strong
  acid.

or
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• Nitrous acid react with all classes of amines.
• The products obtain from the reaction depend on
  whether the amine is primary, secondary or
  tertiary and whether the amine is aliphatic or
  aromatic.

video1
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(a) Primary aliphatic amines

• Primary aliphatic amines react with nitrous acid
  to yield highly unstable aliphatic diazonium
  salts.
• The salts decompose by losing nitrogen to form
  carbocations.
• The carbocations go on to produce alkenes by
  removal of a proton, alcohols by reaction with
  H2O, and alkyl halides by reaction with X-.

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ù
é
-

HCl
¾
¾

¾

Cl
N
CH
RCH
NaNO
NH
CH2
RCH
ú
ê
2
2
2
2
2
2
û
ë
- H
ù
é




N
H
C
RCH
CH
RCH
ú
ê
2
2
2
2
û
ë
H2O
X-
Observation Formation of gas bubbles (N2)
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(b) Secondary aliphatic amines

• Secondary amines react with nitrous
  acid to form secondary N-nitrosamines as
  yellow oil which is stable under the reaction
  conditions.

2o N-nitrosoamines
(yellow oil)
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Example
N-methyl-N-nitrosomethanamine
(yellow oil)
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1 aliphatic amines and 2 aliphatic amines
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(d) Primary aromatic amines

• Primary arylamines react with nitrous acid to
  give arenediazonium salts.

benzenediazonium salt
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4. Reaction with bromine water

• Like phenol, aniline reacts with aqueous bromine
  to yield white precipitate.
• NH2 group is an activating and ortho-para
  directors group.
• This reaction is used to test for aniline.

Identification test
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2,4,6-tribromoaniline (white precipitate)
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Formation of dye
1o arylamines react with nitrous acid to give
arenediazonium salts which are stable at 0?C
Arenediazonium salts also undergo coupling
reaction with aromatic compounds with strong
electron donating group, such as OH and NR2 at
the para position to yield azo compounds
Azo compounds are usually intensely coloured and
relatively inexpensive compounds, they are used
as dyes.
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