Purifying ethyl ethanoate from a crude esterification mixture
A student prepares ethyl ethanoate by refluxing ethanol with ethanoic acid and a few drops of concentrated sulfuric acid. The crude distillate is cloudy, has a strong fruity smell, and turns blue litmus red. It is believed to contain ethyl ethanoate, unreacted ethanol, unreacted ethanoic acid, water and a little sulfuric acid. The student has a separating funnel, sodium carbonate solution, anhydrous sodium sulfate, pH paper and simple distillation apparatus. A previous attempt to distil the crude liquid directly gave a distillate that still smelled strongly of acid and did not give a clean boiling point. The teacher asks for a purification sequence that will give a neutral ester sample, and for a judgement on whether the first distillation was useful or should be replaced by washing.
1. Explain why simple distillation of the crude liquid did not give a clean, neutral ester, and identify the main species likely to be present in the distillate.
Read the worked answer
Simple distillation separates volatile components from non-volatile sulfuric acid, but it does not cleanly separate the organic components from each other. Ethyl ethanoate (bp about 77°C), ethanol (bp about 78°C) and water (bp 100°C) can all distil, and ethanoic acid can be carried over as vapour or as an aerosol. Ethanol and the ester have very close boiling points, and the ester can form a low-boiling azeotrope with water, so a single distillation cannot give a sharp boiling point. The distillate therefore still contains ethanoic acid, which is why it turns litmus red and smells acidic, and water, which makes it cloudy because the ester is not fully miscible with water. The first distillation was not useless because it removed the bulk of the non-volatile sulfuric acid and some high-boiling material, but it must be followed by a chemical wash and a drying step before a final distillation.
2. Give a safe purification sequence using the available apparatus and explain the purpose of each step.
Read the worked answer
- Cool the crude distillate, then transfer it to a separating funnel. Add cold dilute sodium carbonate solution in small portions, shaking gently and venting frequently, until no more carbon dioxide is produced. This neutralises ethanoic acid to sodium ethanoate, which is ionic and stays in the aqueous layer; it also removes some ethanol. Keep the contact time short and avoid warming, because sodium carbonate solution can slowly hydrolyse the ester.
- Allow the layers to separate. Ethyl ethanoate is less dense than water (about 0.90 g cm−3), so the ester is the upper organic layer. Run off the lower aqueous layer.
- Wash the organic layer with a small volume of cold water to remove residual sodium ethanoate and carbonate, then separate again. Test a drop of the organic layer with pH paper; it should no longer be strongly acidic.
- Add anhydrous sodium sulfate, swirl and leave until the solid no longer clumps. This removes dissolved water. Decant or filter off the drying agent.
- Distil the dried ester and collect the fraction around 74–79°C. Discard the first few drops, which may contain residual water or ethanol, and do not distil to dryness. The final sample should be neutral to pH paper and have a sharper boiling point.
3. The student obtains 3.2 g of purified ester from 0.100 mol of ethanol, with ethanoic acid in excess. Calculate the percentage yield and judge whether the loss during purification is acceptable.
Read the worked answer
- Mr(ethyl ethanoate, CH3COOCH2CH3) = 88.0 g mol−1.
- Maximum amount from 0.100 mol ethanol, if ethanol is limiting and the reaction goes to completion, is 0.100 mol ester.
- Theoretical mass = 0.100 × 88.0 = 8.80 g.
- Percentage yield = (3.2/8.80) × 100 = 36.4%. A yield of about 36% is low but not surprising for a school esterification: the reaction is an equilibrium, so it does not go to completion, and the purification removes ethanol, acid, water and some ester in the aqueous washes. The loss is acceptable as a teaching preparation if the sample is pure, but it is not an efficient synthesis. The yield could be improved by using a larger excess of ethanol or by removing water during the reaction, and by keeping wash volumes small and avoiding prolonged contact with sodium carbonate solution, which can hydrolyse the ester.