Experiment 20

Objectives:  Diazotation and Coupling Reactions

Introduction:

●       Diazotation: When primary aryl amines react with nitrous acid (generated from NaNO₂ combined with H₂SO₄), a diazonium ion is formed. This reaction takes place under freezing conditions.

●       Coupling Reaction: The diazotized sulfanilic acid reacts with 2-naphthol and 1-naphthylamine, leading to the production of acidic and basic azo dyes. This represents an electrophilic aromatic substitution, where the diazonium cation (a weak electrophile) reacts with an aromatic ring, which must contain activating groups such as -OH or -NH₂. Electron-withdrawing groups on the aromatic ring of the diazonium ion facilitate this substitution. In 2-naphthol, substitution preferentially occurs at the 1-position, while the NH₂ group in 1-naphthylamine activates the 2- and 4-positions.

In azo compounds, the -N=N- group is part of an extended electron system involving aromatic rings, forming a chromophore. Groups like -OH and -NH₂ attached to chromophores can modify the dye's color.

Chemicals:

●       Sulfanilic acid

●       1-Naphthylamine

●       2-Naphthol

●       Sodium nitrite

●       2 N NaOH

●       2 N H₂SO₄

Pre-prepared Solutions:

●       Sulfanilic Acid Solution: 1.7 g of sulfanilic acid in 50 mL of 2 N H₂SO₄

●       Sodium Nitrite Solution: 0.6 g of NaNO₂ in 10 mL of distilled water

●       1-Naphthylamine Solution: 0.7 g of 1-naphthylamine in 50 mL of 2 N H₂SO₄

●       2-Naphthol Solution: 0.7 g of 2-naphthol in 50 mL of 2 N NaOH

Glassware:

●      Crystallizing dish (d = 190 mm, h = 90 mm)

●      3 beakers (250 mL)

●      Beaker (100 mL)

●      3 volumetric pipettes (50 mL)

●      Volumetric pipette (10 mL)

●      2 petri dishes (d = 9 cm)

●      3 pipette bulbs

●      3 glass stirring rods

Experimental Procedure:

1.  Half-fill a crystallizing dish with ice.

2.  Place the following beakers in the ice bath:

●       Beaker 1: 50 mL of acidic sulfanilic acid solution

●       Beaker 2: 10 mL of aqueous sodium nitrite solution

●       Beaker 3: 50 mL of acidic 1-naphthylamine solution

●       Beaker 4: 50 mL of alkaline 1-naphthol solution

3. Mix the sulfanilic acid solution with the sodium nitrite solution while stirring. Divide equal volumes of the mixture into beakers 3 and 4.

Observation:    The solutions in beakers 3 and 4 turn red.

Questions:

1.    What color is formed in the test? The solutions turn red, indicating the formation of azo dyes.

2.    What do you mean by diazotization? Diazotization is the process where primary aryl amines react with nitrous acid, forming a diazonium ion under cold conditions.

3.    Why is 2 N NaOH used? 2 N NaOH is used to create an alkaline environment, necessary for the coupling reaction with 2-naphthol.