The microscale synthesis of azo dyes

In this experiment, students prepare an azo dye and use it to dye a piece of cotton. The synthesis is unusual in that whereas most organic syntheses require ambient or elevated temperature, this synthesis requires low temperatures.

This practical should take 20 minutes.

Equipment

Apparatus 

• Beakers, 10 cm³, x3
• Thermometer
• Tweezers

Chemicals

• Ice
• Aminobenzene (aniline)
• Hydrochloric acid (concentrated)
• Sodium nitrite
• 2-Naphthol (also called β-Naphthol; naphthalene-2-ol)
• Sodium hydroxide solution, 2 mol dm^–3
• Ethanol
• Urea

Health, safety and technical notes

• Read our standard health and safety guidance.
• Students must wear suitable eye protection (splash resistant goggles to BS EN166 3).
• This experiment should be done in a fume cupboard.
• Aminobenzene (aniline) is TOXIC by all routes, a carcinogen and mutagen, a skin sensitiser, causes eye damage and is toxic to aquatic life .
• Hydrochloric acid, HCl(aq), is CORROSIVE and a respiratory irritant (see CLEAPSS HazCard HC047a).
• Sodium nitrite, NaNO₂, is an OXIDISER, toxic if swallowed and hazardous to aquatic life (see CLEAPSS HazCard HC093).
• 2-Naphthol is HARMFUL if swallowed or in contact with skin and toxic to aquatic life (see CLEAPSS HazCard HC070b).
• Sodium hydroxide solution, NaOH(aq), 2 mol dm^–3 is CORROSIVE (see CLEAPSS HazCard HC091a).
• Ethanol is highly FLAMMABLE (see CLEAPSS HazCard HC040a).
• Urea is of low hazard (see CLEAPSS HazCard HC035a).

Procedure

1. Put eight drops of aminobenzene in a 10 cm³ beaker and add 30 drops of deionised water followed by 15 drops of concentrated hydrochloric acid. Swirl the beaker and then put it in an ice bath.
2. Weigh 0.15 g of sodium nitrite into another beaker and add 1 cm³ of deionised water. Cool the beaker in the ice bath. Add one spatula of urea (this prevents side reactions occurring).
3. Mix the contents of the two beakers together and keep in the ice bath.
4. Weigh 0.45 g of 2-naphthol into another beaker and add 3 cm³ of sodium hydroxide solution. Swirl to dissolve.
5. Take a piece of cotton cloth 2 × 2 cm² and, using tweezers, dip it into the 2-naphthol solution. Allow the solution to completely soak the cotton.
6. Dip the cloth completely into the diazonium salt solution. A red dye forms in the fibres, dyeing the cloth.
7. Take the cloth out, wash it under the tap and leave to dry.

Observations

The orange-red azo dye forms in the fibres of the cotton, dyeing the cloth. The melting point of 1-phenylazo-2-naphthol is 133 °C.

The urea decomposes excess HNO₂ formed and prevents many side reactions from occurring. A better ‘red’ dye is usually produced.