Synthesising an Ester in the School Lab
Students prepare ethyl ethanoate by refluxing ethanol, ethanoic acid and concentrated sulfuric acid. The mixture is then distilled, and the distillate is washed with sodium carbonate solution. A student proposes heating the mixture with a Bunsen burner without a reflux condenser 'to save time', and wants to use a naked flame throughout. The teacher notes that the product has a boiling point of about 77 °C and that ethanol boils at 78 °C.
1. Explain the role of each component and why reflux is used.
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Ethanoic acid + ethanol ⇌ ethyl ethanoate + water is an equilibrium (esterification). Concentrated H₂SO₄ is a catalyst and also removes water, shifting the equilibrium to the right. Reflux allows the mixture to be heated for a sustained period without losing volatile reactants, improving yield. Without reflux, ethanol and the ester would escape, lowering yield and creating a fire hazard. The judgement is that time and yield are linked: 'saving time' by removing reflux actually wastes material and increases risk.
2. Explain why a naked flame is dangerous here and what should be used.
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Ethanol (bp 78 °C) and ethyl ethanoate (bp 77 °C) are both highly flammable, and their vapours can form explosive mixtures with air. A naked flame near the apparatus or near open containers is a serious ignition risk. Heating should be by a water bath or electric heating mantle, with the apparatus set up away from flames. The boiling points are so close that simple distillation cannot cleanly separate ester from ethanol, so washing and further purification steps are also needed.
3. The sodium carbonate wash causes fizzing. Explain what is happening and why this step is necessary.
Read the worked answer
The wash neutralises unreacted ethanoic acid and any sulfuric acid: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O, and H₂SO₄ + Na₂CO₃ → Na₂SO₄ + CO₂ + H₂O. The CO₂ is the fizz. Removing the acid stops it contaminating the product and prevents it from catalysing reverse hydrolysis of the ester. The organic layer is then separated, dried (e.g. with anhydrous sodium sulfate) and distilled. The judgement is that purification is not optional: the fizz is evidence of acid still present, which must be removed before the product is considered pure.