The oxidation of cyclohexanol by nitric acid

Read our standard health & safety guidance

Lesson organisation

This can be done either as a demonstration or a class practical, probably the former as a fume cupboard is needed.

Apparatus and chemicals

Goggles
Protective gloves (preferably nitrile)

Each group of students will need:

Beaker (250 cm³), for ice bath
Beaker (100 cm³), for hot water bath
Hot plate (or Bunsen, tripod and gauze)
Dropping pipettes, 3
Test-tube
Thermometer (0 - 100°C)
Vacuum filtration apparatus and filter paper

Access to:
Melting point apparatus
Thermometer (0 - 200 or 250°C)

Cyclohexanol (Harmful)
Nitric acid (1:1) (Corrosive, Oxidising) (see note 1)
Deionised or distilled water
Ice

Technical notes

Cyclohexanol (Harmful) Refer to CLEAPSS Hazcard 84C
Nitric acid (Corrosive, Oxidising) Refer to CLEAPSS Hazcard 67 and CLEAPSS Recipe Card 44.

1 1:1 nitric acid is about 5 mol dm^–3 and made by carefully mixing concentrated nitric acid (70%) with deionised water in equal volumes. Add the acid to water. It will get hot.

Procedure

HEALTH & SAFETY: Wear goggles throughout.

Work in a fume cupboard because the nitric acid is reduced to oxides of nitrogen which are Toxic - see CLEAPSS Hazard 68

a Half fill a 100 cm³ beaker with water and heat to 80 – 90°C.

b Add 1 cm³ of 1:1 nitric acid to a test-tube and place in the water-bath.

c Carefully add six drops of cyclohexanol to the test-tube. You will notice some bubbling and the nitric acid turns brown.

d Leave the tube for ten minutes in the hot water-bath

e Remove the tube and allow it to cool to room temperature.

f Place some crushed ice in a 250 cm³ beaker and add a little water. This makes an ice-bath.

g Cool the test-tube further in the ice bath. Crystals of hexanedioic acid should form.

h Filter off the crystals using a vacuum filter and wash the crystals with 2 cm³ of deionised water. Suck the crystals dry on the filter.

i Measure the melting point of your crystals, using the melting point apparatus.

j Compare your value for the melting point with the data book value (152°C).

Teaching notes

The reaction can usefully be represented:

Cyclohexanol is a secondary alcohol and thus will be oxidised to cyclohexanone by acidified dichromate(VI). See experiment The oxidation of cyclohexanol by potassium dichromate(VI).

Nitric acid is a strong enough oxidising agent to break open the ring as shown.

The value obtained for the melting point will probably be too low. This is because the hexanedioic acid is impure. It could be recrystallised (using water as solvent) to purify it.

Health & Safety checked, February 2008

Web Links

www.greener-industry.org/pages/nylon/7nylonPM2.htm is concerned with the manufacture of hexanedioic acid and its use in nylon manufacture.

http://en.wikipedia.org/wiki/Adipic_acid deals with wider uses of hexanedioic acid.

(Websites accessed June 2008)