A-level Chemistry exemplar for required practical No. 10 – part b Preparation of a pure organic liquid: The preparation of ethyl ethanoate Student sheet An ester is a chemical compound that is formed when an organic acid reacts with an alcohol. Esters frequently have distinctive odours and are naturally occurring flavour and fragrance chemicals in many fruits and plants. In this practical, the ester ethyl ethanoate is prepared and purified by distillation. Requirements Stage 1: Preparation of ethyl ethanoate 12 cm3 glacial ethanoic acid 10 cm3 ethanol 15 drops concentrated sulfuric acid anti-bumping granules disposable dropping pipette 10 cm3 measuring cylinder 25 cm3 measuring cylinder 250 cm3 beaker 50 cm3 pear-shaped flask condenser (with rubber tubing) clamp stand clamps and bosses heatproof mat Bunsen burner tripod and gauze. Stage 2: Isolation of ethyl ethanoate 4.5 g sodium carbonate 15 cm3 distilled or deionised water 100 cm3 beaker 50 cm3 beaker separating funnel and stopper anhydrous sodium sulfate boiling tube 50 cm3 pear-shaped flask. Stage 3: Purification of the ethyl ethanoate thermometer (0–100 oC) and adapter three-way adapter condenser (with rubber tubing) receiver adapter anti-bumping granules joint clips 250 cm3 beaker clamp stand clamps and bosses heatproof mat Bunsen burner tripod and gauze access to a balance. Suggested method Stage 1: Preparation of ethyl ethanoate a) Put a few anti-bumping granules in a 50 cm3 pear-shaped flask. b) In a fume-cupboard, add 10 cm3 ethanol, 12 cm3 glacial ethanoic acid and 15 drops of concentrated sulfuric acid to the flask. c) Place a 250 cm3 beaker containing some water on a tripod and gauze over a Bunsen burner. d) Clamp the pear-shaped flask in the beaker of water so that the reaction mixture is below the water line. e) Add a condenser so that it is set up for heating with reflux. Clamp the condenser. Do not insert a stopper. f) Light the Bunsen burner to heat the hot water bath. Raise the temperature of the hot water until the mixture in the flask is gently boiling. Continue the gentle boil of the reaction mixture for about 15 minutes. Turn off the Bunsen burner and cool the mixture by removing the hot water bath. Stage 2: Isolation of ethyl ethanoate g) Prepare a saturated solution of sodium carbonate by combining 4.5 g of sodium carbonate with 15 cm3 of distilled water in a 100 cm3 beaker. h) In a fume cupboard, transfer the reaction mixture from the pear-shaped flask to the beaker and stir. i) Transfer the mixture to a separating funnel. Stopper it and turn it upside down gently and then open the stopcock to vent the system. Invert at least 15–20 times, opening the stopcock each time. j) Allow the two layers to separate. Ethyl ethanoate is less dense than water, therefore the top layer is ethyl ethanoate. k) Remove the stopper, open the stopcock and slowly drain off the waste aqueous layer into a 50 cm3 waste beaker, then close the stopcock. l) Transfer the remaining ethyl ethanoate into a dry boiling tube containing about 1 g of anhydrous sodium sulfate. Agitate the tube so that any water is absorbed into the anhydrous solid. m) Decant the ethyl ethanoate into a clean, dry 50 cm3 pear-shaped flask.Stage 3: Purification of the ethyl ethanoate n) Add a few anti-bumping granules to the pear-shaped flask. o) Set up the apparatus for distillation. p) Place the flask in the 250 cm3 beaker and clamp it so the crude ethyl ethanoate is below the water line. q) Weigh a clean, dry 100 cm3 conical flask on an analytical balance. Record the mass in your Data Table. r) Place the conical flask under the receiver. s) Light the Bunsen burner and heat the flask in the hot water bath. Heat until the ethyl ethanoate is gently boiling. t) As the ethyl ethanoate vapours start to carry over and condense, record the temperature of the vapours in a suitable table. Record this temperature at the beginning and end of the distillation. u) Distil the ethyl ethanoate until no more distillate comes over. There should be some liquid remaining in the round-bottom flask. Never distil to dryness. v) Turn off the Bunsen burner. w) Reweigh the conical flask with the distilled ethyl ethanoate. Stage 4: Yield calculation Which is the limiting reagent – ethanoic acid or ethanol? What is the percentage yield from the limiting reagent?
188.8.131.52 Acylation (A-level only)
Required practical 10
• a pure organic liquid.