Making esters from alcohols and acids

Read our standard health & safety guidance

Lesson organisation

As a class experiment this can be organised, if desired, as a class-cooperative investigation of the ability of a range of alcohols to react with a range of organic acids. Working groups could compare their results with others to build a general overview of this route to the formation of esters, with an interesting variety of smells.

Depending on the actual way the lesson is organised, this may be designed to take from 15 minutes to an hour.

Apparatus and chemicals

Each working group will require:

Eye protection

Glass specimen tubes, 4 (see note 1)
Plastic dropping pipettes, access to adequate supply
Beaker (100 cm³)
Test-tubes, 4
Test-tube rack
Bunsen burner
Heat resistant mat
Tripod and gauze
Crucible tongs

Access to the following alcohols – about 10 drops of each required (see note 2)

Methanol (Highly flammable, Toxic)
Ethanol (Highly flammable, Harmful)
Propan-1-ol (Highly flammable, Irritant)
Butan-1-ol (Harmful)
Butan-2-ol (Harmful)

One or more other alcohols, as available, from:

Propan-2-ol (Highly flammable, Irritant)
Butan-2-ol (Irritant)
2-Methylpropan-1-ol (Highly flammable, Harmful) (see note 2)

Ethanoic acid, pure (Corrosive), about 2 cm³
Benzoic acid (Harmful), about 0.2 g
Propanoic acid (Corrosive) (if available), about 2 cm³
Concentrated sulfuric acid (Corrosive), 5 - 10 drops (see note 3)
Sodium carbonate solution, 0.5 mol dm^-3 (Low Hazard at concentration used), about 10 cm³ per ester

Technical notes

Methanol (Highly flammable, Toxic) Refer to CLEAPSS Hazcard 40B
Ethanol (Highly flammable, Harmful) Refer to CLEAPSS Hazcard 40A
Propan-1-ol (Highly flammable, Irritant) Refer to CLEAPSS Hazcard 84A
Butan-1-ol (Harmful) Refer to CLEAPSS Hazcard 84B
Ethanoic acid, pure ('glacial') (Corrosive) Refer to CLEAPSS Hazcard 38A
Benzoic acid (Harmful) Refer to CLEAPSS Hazcard 13A
Propanoic acid (Corrosive) Refer to CLEAPSS Hazcard 38B
Propan-2-ol (Highly flammable, Irritant) Refer to CLEAPSS Hazcard 84A
Butan-2-ol (Irritant) Refer to CLEAPSS Hazcard 84B
2-Methylpropan-1-ol (Highly flammable, Harmful) Refer to CLEAPSS Hazcard 84B
Concentrated sulfuric acid (Corrosive) Refer to CLEAPSS Hazcard 98
Sodium carbonate solution (Low hazard at concentration used) Refer to CLEAPSS Hazcard 95A and Recipe Card 61

1 The essential requirements for these tubes are:

• neutral borosilicate glass
• a wide flat base, so that they are stable when stood in a beaker. If not available, small test-tubes could be used instead, standing in a larger (250 cm³) beaker.

2 The alcohols and acids may be best provided as a central resource, away from flames, with a supply of plastic pipettes for each.

3 For younger students, prepare the specimen tubes by adding one drop of concentrated sulfuric acid to each. This minimises the risks involved with such students handling this substance. Advanced students may be reliable enough to prepare their own tubes in this way.

Procedure

HEALTH & SAFETY: Wear goggles throughout

a Add 10 drops of ethanoic acid (or propanoic acid) to the sulfuric acid in the specimen tube.

b Add 10 drops of ethanol (or other alcohol) to the mixture.

c Put about 10 cm³ of water into the 100 cm³ beaker. Carefully lower the tube into the beaker so that it stands upright.

d Heat the beaker gently on a tripod and gauze until the water begins to boil, then stop heating.

e Stand for 1 minute in the hot water. If the mixture in the tube boils, use the tongs to lift it out of the water until boiling stops, then return it to the hot water.

f After 1 minute, using tongs, carefully remove the tube and allow it to cool on the heat resistant mat.

g When cool, pour the mixture into a test-tube half-full of 0.5 M sodium carbonate solution. There will be some effervescence. Mix well by pouring back into the specimen tube – repeat if necessary. A layer of ester will separate and float on top of the aqueous layer.

h Smell the product by gently wafting the odour towards your nose with your hand – do not put your nose near the top of the tube!

i Repeat this procedure for up to three more different esters.

Compare the odours of the different esters prepared by your group and by other groups. Write word equations for each reaction, and (for advanced students) chemical equations using structural formulae.

For solid acids, the procedures in steps 1 and 2 need to be changed:
j Add 1 cm³ of methanol (or other alcohol) to the sulfuric acid in the specimen tube.

k Weigh out 0.2 g of benzoic acid (or another solid acid, such as salicylic (2-hydroxybenzoic) acid (Harmful - refer to CLEAPSS Hazcard 52) and add it to the tube. Then proceed as above. Yields from solid acids are not as great, but odours are detectable and distinctive.

Teaching notes

This method is an updated version of the traditional test-tube scale approach to ester preparation, which minimises the risks involved in handling the reagents involved. For further information about this method of ester preparation, consult CLEAPSS Guidance Leaflet PS67-07 ‘Making esters’.

This method is only suitable for preparing small samples for characterisation by odour. Advanced students could scale up the quantities using larger test-tubes, but this would still not give sufficient product for isolation, characterisation by boiling point, or calculation of percentage yield.

Do NOT be tempted to use butanoic (butryric) acid, because of its very unpleasant odour (of rancid butter).

Health & Safety checked, July 2008

Web Links

For a broad review of esters, and interesting details of their odours and some uses, go to:
http://en.wikipedia.org/wiki/Esters

Another site which looks extensively at the odours and uses of esters in flavours and perfumes is:
http://www.hartnell.cc.ca.us/faculty/shovde/chem12b/esters.htm

For an article that looks specifically at the esters that give flavour to beer, go to:
http://www.evansale.com/esters_article.html

And for information from a manufacturer of an amazingly wide range of esters for uses from flavourings and perfumes to lubricants and paints, go to:
http://www.esterchem.co.uk/index.htm

(Last accessed July 2008)