Try this practical or demonstration to produce bromoethane in a substitution reaction between ethanol and phosphorus tribromide
In this experiment, students observe what happens when they add drops of phosphorus tribromide to a test tube containing ethanol. A vigorous substitution reaction occurs, producing bromoethane and phosphorus acid. After leaving the mixture to stand, students add water to the test tube to remove excess phosphorus tribromide by hydrolysis, leaving a small amount of bromobutane at the bottom of the tube.
This experiment could be done by advanced pupils, if sufficient fume cupboards were available. Alternatively it could be a teacher demonstration. It takes about 20 minutes.
- Eye protection (goggles)
- Protective gloves (preferably nitrile)
- Test tube
- Test tube rack
- Measuring cylinder, 5 cm3 or 10 cm3
- Dropping pipettes, x2
- Ethanol (HIGHLY FLAMMABLE) or industrial denatured alcohol, IDA (HIGHLY FLAMMABLE, HARMFUL)
- Phosphorus(III) bromide (CORROSIVE)
- Deionised or distilled water
Health, safety and technical notes
- Read our standard health and safety guidance.
- Wear eye protection (goggles) throughout. Work in a fume cupboard. Wear protective gloves when handling phosphorus tribromide.
- Ethanol (HIGHLY FLAMMABLE) or industrial denatured alcohol, IDA, C2H5OH(l), (HIGHLY FLAMMABLE, HARMFUL) – see CLEAPSS Hazcard HC040A.
- Phosphorus(III) bromide (phosphorus tribromide), PBr3(l) (CORROSIVE). Similar to phosphorus(III) chloride – see CLEAPSS Hazcard HC074. The vapour will hydrolyse in the moisture in air to give hydrobromic acid, HBr (CORROSIVE) – see CLEAPSS Hazcard HC047C.
- Put 20 drops of ethanol into a test tube.
- By adding one drop at a time, put 15 drops of phosphorus tribromide in the test tube.
- Leave the mixture to stand for 10 min.
- Put 2 cm3 of deionised water in a measuring cylinder and, one drop at a time, add the water to the test tube.
- Observe the test tube carefully. What do you notice?
This is an alternative method to heating the alcohol under reflux with sodium bromide and concentrated sulfuric acid, generating HBr in situ. Yields by the HBr method are seldom impressive.
The phosphorus tribromide reacts vigorously with the ethanol and it is important that it is added only one drop at a time. The reaction is:
PBr3 + 3C2H5OH → 3C2H5Br + H3PO3
This is a nucleophilic substitution reaction.
The water removes excess PBr3 by the hydrolysis reaction:
PBr3 + 3H2O → 3HBr + H3PO3
A few globules of bromobutane should be seen at the bottom of the tube.
The Wikipedia article on phosphorus tribromide includes the mechanism for this reaction.