The conversion of ethanol to bromoethane

Read our standard health & safety guidance

Lesson organisation

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.

Apparatus and chemicals

Goggles
Protective gloves (preferably nitrile)

The teacher (or each group of students) will need:

Test tube
Test-tube rack
Measuring cylinder (5 cm³, or 10 cm³)
Dropping pipettes, 2

Access to:

A fume cupboard

Ethanol (Highly flammable) or Industrial denatured alcohol, IDA (Highly flammable, Harmful)
Phosphorus tribromide (Corrosive)
Deionised or distilled water

Technical notes

Ethanol (Highly flammable) and IDA (Highly flammable, Harmful) Refer to CLEAPSS Hazcard 40A
Phosphorus tribromide (Corrosive) Similar to phosphorus trichloride - refer to CLEAPSS Hazcard 74. The vapour will hydrolyse in the moisture in air to give HBr (Corrosive) Refer to CLEAPSS Hazcard 49

Procedure

HEALTH & SAFETY: Wear goggles throughout. Work in a fume cupboard. Wear protective gloves when handling phosphorus tribromide.

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 cm³ 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?

Teaching notes

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.

Health & Safety checked, February 2008

Web Links

www.docbrown.info/page06/OrgMechs3.htm#ALCOHOLS shows the mechanism for the bromide ion substitution of alcohols.

http://en.wikipedia.org/wiki/Phosphorus_tribromide gives the mechanism for this reaction

(Websites accessed June 2008)