CHAPTER 10 ALKANOIC ACIDS AND ESTERS 231

10.1 PRACTICALACTIVITIES

PREPARATION OF AN ESTER

AimTo prepare an ester using the reflux technique

Materials• alkanoic acid (choose either glacial acetic acid or

butanoic acid)

• alkanol (choose one from ethanol, propan-1-ol, butan-1-ol or pentan1-1-ol)

• concentrated sulfuric acid (dropper bottle) (teacher use only)

• Quickfit round-bottom boiling flask (100 mL or 250 mL)

• reflux condenser with hoses

• water bath and Bunsen burner or electric heating mantle

• retort stand, boss heads and clamps

• boiling chips

• stirring rod

• beaker

• measuring cylinder

• Pasteur pipette

Safety• Wear safety glasses throughout this experiment.

• Sulfuric acid is corrosive. Clean up spills immediately. If the acid is spilled on the skin then wash the area with large quantities of water. Seek assistance.

• Organic chemicals are flammable. Do not allow liquids or vapours to come into contact with sparks or flames.

• Identify other safety precautions relevant to this experiment by reading the method.

MethodPreparation of the ester

Refer to Figure 10.11 in the text to help you set up the apparatus.

1. Select the alkanol and acid from the following tabulated information. Measure out the required volumes using clean measuring cylinders.

Experiment Alkanoic acid Alkanol

1 acetic acid (24 mL)

ethanol (8 mL)

2 acetic acid (24 mL)

propan-1-ol (8 mL)

3 acetic acid (24 mL)

butan-1-ol (18 mL)

4 acetic acid (24 mL)

pentan-1-ol (10 mL)

5 butanoic acid (24 mL)

ethanol (8 mL)

2. Pour the acid and the alkanol into the dry round-bottom flask. Your teacher will then add 2 mL (40 drops) of concentrated sulfuric acid to the flask. Now add 2 or 3 boiling chips.

3. Set up the flask and condenser for reflux as shown. Use a heating mantle, hotplate, Bunsen burner (with gauze) or hot-water bath to supply the heat once the apparatus is set up. A Bunsen burner can be used, but care should be taken that the reflux rate is controlled. A hot-water bath is not very suitable if the boiling points of the reactants and product are too high.

4. Before you begin to heat the flask, ensure that cold water is flowing from the tap to the base of the condenser jacket and then out the top and back into the sink.

5. Heat the flask and maintain reflux so that no vapours are lost, and that the condensate drips back into the reaction flask at about 2 drops per second.

6. Reflux the mixture for at least 30 minutes (or longer if time allows).

7. Turn off the heat and allow the apparatus to cool. (Remove the heating mantle, hotplate or water bath).

Isolation

You may wish to isolate the ester from the reaction mixture. If so, then omit this section and proceed with the optional extension.

8. Remove the boiling flask and carefully pour the reaction mixture into a beaker containing

PRACTICAL ACTIVITIES

232 THE ACIDIC ENVIRONMENT

PRACTICAL ACTIVITIES

70 mL of water. Stir with a glass rod. The mixture contains the ester plus unreacted alkanol and alkanoic acid. Compare the odour of this solution with the odours of the original reactants (see Results and analysis).

9. Clean up the equipment and workbench as directed by your teacher. Keep the aqueous mixture of the ester for further examination.

Results and analysis1. Depending on the solubility of the ester you

have made, the aqueous mixture will either remain as one phase, or separate into two layers. (e.g. 1-butyl acetate is insoluble in water). If separation occurs, the upper layer will be the crude ester. It can be removed carefully using a Pasteur pipette or by using a separation funnel.

2. Compare the odour of the ester (or the final aqueous solution) with that of the original alkanol and alkanoic acid. To do this you can waft vapours towards your nose with your hand. Describe what you smell in each case. Did the ester have a distinctly different odour from that of the reactants?

3. Explain how the ester could be extracted and separated from the unreacted alkanol and alkanoic acid.

4. Name the ester you have prepared and draw its structural formula.

ConclusionWrite an appropriate conclusion for your experiment.

Optional extension: Isolation of the ester

1. Add the reaction mixture to 30 mL of saturated salt water in a beaker.

2. Pour the mixture into a separating funnel and isolate the upper layer of crude ester.

3. Place the crude ester layer in a beaker and add saturated sodium hydrogen carbonate to remove any acid. Stop when fizzing ceases.

4. Isolate the ester once more using a separating funnel.

5. Dry the ester using crystals of anhydrous calcium chloride. Filter the mixture.

6. Set up a distillation apparatus and distil the dried ester. Collect the distillate in a clean beaker. The boiling points of the esters are:

ethyl acetate (77ºC)

1-propyl acetate (102ºC)

1-butyl acetate (127ºC)

1-pentyl acetate (147ºC)

ethyl butanoate (120ºC)

7. Compare the odours of the esters to those of the original alkanol and alkanoic acid. Name the ester you have prepared and draw its structural formula.