Microscale preparation of ethyl benzoate

In this experiment, students mix benzoic acid and ethanol in a plastic pipette, before warming the mixture in a water bath. They then identify the ester by smell.

An organic preparation suitable as a class experiment for post-16 students, using a microscale technique that produces an identifiable product in about 20 minutes.

Equipment

Apparatus

• Eye protection
• Measuring cylinder, 5 cm³
• Beaker, 10 cm³
• Plastic dropping pipettes, 4
• Thermometer, 0–110 °C
• Beaker, 100 cm³
• Heat resistant mat
• Access to top-pan balance, weighing to 0.01 g
• Bunsen burner, tripod and gauze, or
  
• Access to an electric hotplate, with thermostatic control, or
  
• Access to thermostatically controlled water bath set at 70–80 °C

Chemicals

• Ethanol, (IDA, industrial denatured alcohol) (HIGHLY FLAMMABLE, HARMFUL), 1 cm³
• Benzoic acid (HARMFUL), about 0.5 g
• Concentrated sulfuric acid (CORROSIVE), access to small quantity

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Ethanol or IDA (industrial denatured alcohol), C₂H₅OH(l), (HIGHLY FLAMMABLE, HARMFUL) – see CLEAPSS Hazcard HC040A.
• Benzoic acid, C₆H₅COOH(s) (HARMFUL) – see CLEAPSS Hazcard HC013A. 
• Concentrated sulfuric acid, H₂SO₄(l), (CORROSIVE) – see CLEAPSS Hazcard HC098a.  
• Each group requires the stem of one of the three plastic dropping pipettes to be shortened, and a second pipette’s stem to be cut off completely, as illustrated below. This may be done in advance as part of technician preparation, but teachers may decide to leave this to the students. If so, they will require a sharp knife or scissors.

Procedure

1. Weigh 0.24 g of benzoic acid into the 10 cm³ beaker.
2. Add 1 cm³ of ethanol and swirl to dissolve.
3. Hold the shortened dropping pipette upside down. Carefully transfer, using a normal plastic dropping pipette, the contents of the beaker into the shortened dropping pipette.
4. Using a clean, normal dropping pipette, add one drop of concentrated sulfuric acid to the mixture.
5. Place the cut-off pipette bulb over the shortened pipette end.
6. Set up a waterbath at 70–80 °C using a small beaker about one-third full with water, and leave the assembled reaction vessel in the bath for ten minutes.
7. Remove from the water bath, take off the top and waft the vapour towards your nose. Describe the smell, and compare with the smell of other esters.

Teaching notes

After the practical, you may wish to build a class activity around writing an equation for the reaction that has occurred during the experiment.

The advantages of microscale techniques for organic preparation lie not only in the use of less reagents and safer conditions, but also in the speed with which a detectable product can be obtained.