Esters

Goal of the practical

• Make a small, fragrant sample of ethyl ethanoate by reacting ethanol with ethanoic acid under acid catalysis:

CH₃COOH(l) + C₂H₅OH(l) → CH₃COOC₂H₅(l) + H₂O(l)

Apparatus

• Pear-shaped or round-bottom flask, fitted with a still-head and a Liebig (water-cooled) condenser
• Collection flask (small conical or round-bottom) for the distillate
• Thermometer
• Heating mantle, electric heater, or water bath — not a Bunsen burner, since ethanol vapour is highly flammable
• Reagents: ethanol, ethanoic acid (glacial), concentrated sulfuric acid (the catalyst), sodium carbonate solution (for the work-up wash)

Method

• Step-by-step:
  • Add about 5 cm³ of ethanol and 5 cm³ of glacial ethanoic acid to the reaction flask
  • Carefully add about 2 cm³ of concentrated sulfuric acid (this is corrosive — wear gloves and safety glasses)
  • Drop in a few anti-bumping granules (they keep the boiling smooth)
  • Fit the still-head, condenser and collection flask
  • Heat the mixture gently with the electric heater or in a hot water bath
  • As the mixture warms, the ester forms and distils off through the condenser, where it cools back to a liquid and collects in the receiving flask
  • The ester has the lowest boiling point in the flask (about 77 °C, against ethanol 78 °C, water 100 °C and ethanoic acid 118 °C), so it is the first substance to come over the condenser
  • Because ethyl ethanoate and ethanol boil within about 1 °C of each other, the distillate is a crude mixture; the work-up step below cleans it up
  • The distillate has a noticeably sweet, fruity, glue-like smell — this is the diagnostic sign that the ester has been made

Working up the crude distillate

• The crude distillate also contains a small amount of unreacted ethanol, ethanoic acid and a trace of sulfuric acid carried over
• These can be removed before bottling the pure ester:
  • Add a small volume of sodium carbonate solution to the distillate; it neutralises any acid carried over, releasing CO₂ (visible effervescence) until no more bubbles form
  • Anhydrous calcium chloride can then be added to absorb any residual ethanol
  • Decant the now-clear ester layer into a clean stoppered bottle

Why the ester is distilled off as it forms

• The esterification reaction is reversible: leaving the ester in the hot flask with water and the acid catalyst would let the reverse reaction (hydrolysis of the ester back to acid + alcohol) take place
• Distilling the ester out of the reaction mixture as soon as it forms removes one of the products and pulls the equilibrium toward more ester — a direct application of Le Chatelier's principle from topic 20