The conversion of alcohols to halogenalkanes
Class practical or Demonstration
is reacted with
in a phosphorus tribromide to give substitution reaction . bromoethane
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.
Protective gloves (preferably nitrile)
The teacher (or each group of students) will need:
Measuring cylinder (5 cm
3, or 10 cm 3)
Dropping pipettes, 2
Ethanol (HIGHLY FLAMMABLE) or Industrial denatured alcohol, IDA (HIGHLY FLAMMABLE, HARMFUL)
Phosphorus(III) bromide (CORROSIVE)
Deionised or distilled water
Refer to Health & Safety and Technical notes section below for additional information.
Health & Safety and Technical notes
Read our standard health & safety guidance
Wear goggles throughout. Work in a fume cupboard. Wear protective gloves when handling phosphorus tribromide.
Ethanol (HIGHLY FLAMMABLE) or Industrial Denatured Alcohol, IDA, C 2H 5OH(l), (HIGHLY FLAMMABLE, HARMFUL) - see CLEAPSS Hazcard.
Phosphorus(III) bromide (phosphorus tribromide), PBr 3(l) (CORROSIVE). Similar to phosphorus(III) chloride - see CLEAPSS Hazcard. The vapour will hydrolyse in the moisture in air to give HBr (CORROSIVE) - see CLEAPSS Hazcard.
a Put 20 drops of ethanol into a test-tube.
b By adding one drop at a time, put 15 drops of phosphorus tribromide in the test-tube.
c Leave the mixture to stand for 10 min.
d Put 2 cm 3 of deionised water in a measuring cylinder and, one drop at a time, add the water to the test-tube.
e 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:
3 + 3C 2H 5OH → 3C 2H 5Br + H 3PO 3
This is a nucleophilic substitution reaction.
The water removes excess PBr
3 by the hydrolysis reaction:
3 + 3H 2O → 3HBr + H 3PO 3
A few globules of bromobutane should be seen at the bottom of the tube.
Health & Safety checked, August 2016
This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.
© Nuffield Foundation and the Royal Society of Chemistry
Wikipedia - gives the mechanism for this reaction.
Page last updated October 2015