Testing for unsaturation with bromine on a microscale

In this practical, students use reverse disproportionation to produce a source of bromine from bromate ions, bromide ions and acid. They extract the bromine into hexane in a plastic pipette and decant the resulting solution into a well-plate.

Students can then test for unsaturated hydrocarbons, mixing the bromine solution with non-polar organic compounds such as cyclohexane, cyclohexene and limonene. Unsaturated compounds rapidly decolourise the bromine.

The experiment should take approximately 15 minutes.

Equipment

Apparatus

• Eye protection

Preparing a solution of bromine in hexane

• Plastic well-plate, 24 wells (eg Sigma ref: CLS3526)
• Plastic pipette, standard form (eg Aldrich ref: 213, 500-3)

Testing for unsaturation

• Plastic pipettes
• Plastic Petri dish

Chemicals

Preparing a solution of bromine in hexane

• Potassium bromate(V), 0.1 mol dm^–3
• Potassium bromide, 0.2 mol dm^–3
• Hydrochloric acid, 1 mol dm^–3
• Hexane

Testing for unsaturation

• Cyclohexane
• Cyclohexene
• Limonene
• Solution of bromine in hexane (see note 9 below for disposal)

Health, safety and technical notes

1. Read our standard health and safety guidance.
2. Wear eye protection throughout. Gloves would be sensible.
3. Care should be taken to ensure the solvents do not shoot out of the pipettes when inverted.
4. Cyclohexane – see CLEAPSS Hazcard HC045b. Cyclohexane is highly FLAMMABLE, a skin and respiratory IRRITANT and TOXIC to aquatic life.
5. Cyclohexene – see CLEAPSS Hazcard HC045c. Cyclohexene is a highly FLAMMABLE liquid and vapour, HARMFUL if ingested, TOXIC in contact with skin and for aquatic life.
6. Limonene – see CLEAPSS Hazcard HC045c. Limonene is FLAMMABLE as liquid and vapour, a skin IRRITANT and sensitiser and TOXIC to aquatic life.
7. Hexane – see CLEAPSS Hazcard HC045a. Hexane is highly FLAMMABLE liquid and vapour, a skin and respiratory IRRITANT, and TOXIC to the reproductive system. It is also a specific target organ toxin for the heart, CNS and respiratory system.
8. Ensure the laboratory is well-ventilated.
9. To dispose of the bromine solution, add 2 mol dm^–1 sodium carbonate solution (see CLEAPSS Hazcard HC095A and CLEAPSS Recipe Book RB080) until the colour is discharged – the solution can then be washed to waste with plenty of running water.

Procedure

Preparing a solution of bromine in hexane

1. Put ten drops of potassium bromate(V) solution into one well in a well-plate.
2. Add 20 drops of potassium bromide solution.
3. Add five drops of hydrochloric acid.
4. Leave for five minutes to allow the bromine to form fully.
5. Add hexane to the well until it is about half-full.
6. Using your plastic pipette, take up all the liquid in the well and invert the pipette. You should see two layers – the lower (aqueous) layer which should be coloured red-yellow by the bromine and the upper layer which should be colourless. Notice, too, the shape of the meniscus at the interface.
7. Gently flick the bulb of the pipette. This will mix the liquids and allow the bromine to be extracted into the upper hexane layer. Why?
8. When the upper layer is coloured red-yellow and the lower layer is colourless your extraction is complete.
9. Very carefully invert the pipette again and decant the lower aqueous layer into a well in your wellplate.
10. Into another well decant the upper layer of bromine dissolved in hexane. This is the solution you will use for carrying out tests for unsaturation.

Testing for unsaturation

1. Using a plastic pipette, add three drops of the bromine solution to each of the three wells in the well-plate.
2. Put three drops of each of the organic liquids under test in the wells and observe any changes.

Question

1. Which types of liquid decolourise bromide?

Teaching notes

One particular safety advantage of the microscale technique used to prepare a solution of bromine in hexane is that it eliminates the need for bottles of bromine and its associated hazards. In addition, the experiment is quite fun to do and illustrates several important chemical principles such as disproportionation, extraction techniques and hydrophilic/hydrophobic properties.

The reverse disproportionation reaction is:

BrO₃^–(aq) + 5Br^–(aq) + 6H⁺(aq) → 3Br₂(aq) + 3H₂O(l)