Making esters from alcohols and acids

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 a variety of different smells.

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

Equipment

Apparatus

• Eye protection
• Glass specimen tubes x4 (note 3)
• Plastic dropping pipettes, access to adequate supply
• Beaker, 100 cm³ or 250 cm³ (note 3)
• Test tubes x4
• Test tube rack
• Bunsen burner
• Heat resistant mat
• Tripod
• Gauze
• Crucible tongs

Chemicals

• Glacial (concentrated) ethanoic acid (CORROSIVE), about 2 cm³
• Propanoic acid (CORROSIVE), about 2 cm³
• Benzoic acid (HARMFUL), about 0.2 g
• Concentrated sulfuric acid (CORROSIVE), 5–10 drops (note 7)
• Access to the following alcohols (about 10 drops of each required) (note 8):
  • Methanol (HIGHLY FLAMMABLE, TOXIC)
  • Ethanol (HIGHLY FLAMMABLE, HARMFUL if using Industrial Denatured Alcohol, IDA)
  • Propan-1-ol (HIGHLY FLAMMABLE, IRRITANT)
  • Butan-1-ol (HARMFUL)
• One or more other alcohols, as available, from the following (about 10 drops of each alcohol used) (note 8):
  • Butan-2-ol (IRRITANT)
  • Propan-2-ol (HIGHLY FLAMMABLE, IRRITANT)
  • 2-Methylpropan-1-ol (IRRITANT)
• Sodium carbonate solution, 0.5 M, about 10 cm³ per ester

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear goggles throughout.
• 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.
• Glacial (concentrated) ethanoic acid, CH₃COOH(l), (CORROSIVE) – see CLEAPSS Hazcard HC038a. 
• Propanoic acid, CH₃CH₂COOH(l) (CORROSIVE) – see CLEAPSS Hazcard HC038b.
• Benzoic acid, C₆H₅COOH(s) (HARMFUL) – see CLEAPSS Hazcard HC013A. 
• Concentrated sulfuric acid, H₂SO₄(l), (CORROSIVE) – see CLEAPSS Hazcard HC098a. 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.
• The alcohols (and organic acids) may be best provided as a central resource, away from flames, with a supply of plastic pipettes for each (or in small dropper bottles).
• Methanol, CH₃OH(l), (HIGHLY FLAMMABLE, TOXIC) – see CLEAPSS Hazcard HC040b.
• Ethanol, CH₃CH₂OH(l), (HIGHLY FLAMMABLE and HARMFUL if using IDA) – see CLEAPSS Hazcard HC040A. 
• Propan-1-ol, CH₃CH₂CH₂OH(l), (HIGHLY FLAMMABLE, IRRITANT) – see CLEAPSS Hazcard HC084A. 
• Butan-1-ol, CH₃CH₂CH₂CH₂OH(l) (HARMFUL) – see CLEAPSS Hazcard HC084B. 
• Propan-2-ol, CH₃CH(OH)CH₃(l), (HIGHLY FLAMMABLE, IRRITANT) – see CLEAPSS Hazcard HC084A. 
• Butan-2-ol, CH₃CH(OH)CH₂CH₃(l), (IRRITANT) – see CLEAPSS Hazcard HC084B. 
• 2-Methylpropan-1-ol, CH₃CH(CH₃)CH₂OH(l), (IRRITANT) – see CLEAPSS Hazcard HC084A. 
• Sodium carbonate solution, Na₂CO₃(aq) – see CLEAPSS Hazcard HC095A and CLEAPSS Recipe Book RB080.

Procedure

1. Add 10 drops of ethanoic acid (or propanoic acid) to the sulfuric acid in the specimen tube.
2. Add 10 drops of ethanol (or other alcohol) to the mixture.
3. Put about 10 cm³ of water into the 100 cm³ beaker. Carefully lower the tube into the beaker so that it stands upright.
4. Heat the beaker gently on a tripod and gauze until the water begins to boil, then stop heating.
5. 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.
6. After 1 minute, using tongs, carefully remove the tube and allow it to cool on the heat resistant mat.
7. 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.
8. 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!
9. Repeat this procedure for up to three more different esters.
10. 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.
11. For solid acids, the procedures in steps 1 and 2 need to be changed:
    1. Add 1 cm³ of methanol (or other alcohol) to the sulfuric acid in the specimen tube.
    2. Weigh out 0.2 g of benzoic acid (or another solid acid, such as salicylic (2-hydroxybenzoic) acid (HARMFUL – see CLEAPSS Hazcard HC052) 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).