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Azo Coupling

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An azo coupling reaction a diazonium salt is reacted with an aryl group possessing a strong activating group. The product is an azo compound in which the two aryl groups are joined by an azo functionality. The linking of arene diazonium salts with aromatic amines or phenols to form diazoamino or azo compounds.

Arene diazonium salts are electrophilic reagents which react with primary and secondary aromatic amines in weakly acidic solution to form diazoamino compounds or 1,3-diaryl triazenes.

Azo Compounds

Synthesis

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Azo compounds have been synthesized by the reduction of azoxy derivatives with (EtO)3P or LiAlH4 and of ketazines with H2/Adkins catalyst. A cyclic nitroso dimer has been reduced to an azo compound with Si2Cl6. Some bicyclic substances have been alkylated to give diazenium salts.

Azo Synthesis

Reaction of a diaziridinone with an N,N-disubstituted hydrazine has afforded azo compounds.

Synthesis of Azo Compounds

Reaction

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Phase transfer catalysts have been used to aid azo coupling. Sodium dodecyl benzenesulphonate with H2O/CH2Cl2 accelerates C-coupling of 4-nitrobenzenediazonium chloride with amines which are relatively unreactive towards both N and C-coupling.

The kinetics of azo coupling between substituted bezenediazonium ions and some benzindoles have been studied. Micellar effects with the expected dependence on the charges on reactants and micelles have been found in azo coupling.

Azo Coupling Reaction

Mechanism

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The mechanism of azo coupling reaction is shown below.
  1. Naphthalene reacts with sodium hydroxide to form sodium salt.
  2. The sodium salt of naphthalene reacts with sulfanalic acid to form an azo compound.
  3. Further it rearranges to form an orange color azo dye.

Azo Coupling Mechanism
Aromatic azo compounds make excellent fade resistant dyes. By attaching different functional groups to the chromophore, the properties of the molecule are modified.

Additional functional groups attached Property modified Example
-SO3H The solubility of the dye in water is improved. Azo Coupling Reaction Example
-NH2 or NR2 The color of the dye is modified or enhanced. Example of Azo Coupling Reaction