Practical synthetic chemistry requires learning methods to prepare organic compounds safely, followed by purification. This guide explains how to prepare ethyl ethanoate by esterification and isolate it using distillation, liquid-liquid extraction, and drying agents.
Core Specification Link
This practical is key to topic 3.3.5 Alcohols, covering purification techniques and standard esterification reactions.
Esterification Reaction
\[ \text{CH}_3\text{COOH}(\text{l}) + \text{CH}_3\text{CH}_2\text{OH}(\text{l}) \overset{\text{H}_2\text{SO}_4}{\rightleftharpoons} \text{CH}_3\text{COOCH}_2\text{CH}_3(\text{l}) + \text{H}_2\text{O}(\text{l}) \]Ethanoic acid reacts with ethanol in a reversible esterification reaction. Concentrated sulfuric acid acts as both a catalyst and a dehydrating agent to shift the equilibrium position to the right.
Aim
To prepare a sample of ethyl ethanoate, isolate it using distillation, wash and separate unreacted impurities using a separating funnel, and dry the final ester.
Equipment List
- Round-bottomed flask (100 cm³)
- Still head and thermometer pocket
- Liebig condenser (with water hoses)
- Thermometer (0 to 150 °C)
- Receiving adapter and conical flask
- Heating mantle or electric water bath
- Anti-bumping granules
- Separating funnel (50 cm³)
- Glacial ethanoic acid (10 cm³) and Ethanol (10 cm³)
- Concentrated sulfuric acid (1 cm³)
- Saturated sodium carbonate solution and anhydrous calcium chloride granules
Experimental Method
- Add 10 cm³ of ethanol and 10 cm³ of glacial ethanoic acid to the 100 cm³ round-bottomed flask.
- Slowly add 1 cm³ of concentrated sulfuric acid down the side of the flask, swirling gently to mix.
- Add a few anti-bumping granules to prevent uneven, violent boiling.
- Assemble the distillation apparatus as shown below . Ensure that the water flows into the bottom of the condenser jacket and out from the top.
- Heat the mixture gently in a heating mantle or hot water bath . Do not use a Bunsen burner as organic reagents and products are highly flammable.
- Distil the reaction mixture, collecting the fraction that distils over between 74 °C and 79 °C into the receiving flask (ethyl ethanoate has a boiling point of 77 °C). Discard fractions outside this range.
- Pour the distillate into a separating funnel . Add 20 cm³ of saturated sodium carbonate solution.
- Stopper the funnel, invert it, and open the tap regularly to release excess carbon dioxide gas pressure.
- Allow the layers to separate . The aqueous layer is denser and forms at the bottom; discard it. Retain the upper organic layer of ethyl ethanoate in the funnel.
- Run the organic layer into a clean beaker and add a spatula of anhydrous calcium chloride (drying agent) . Swirl and leave for 10 minutes until the solution turns clear.
- Decant or filter the liquid into a clean, dry, weighed sample vial to record the yield.
Safety & Risk Assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Concentrated sulfuric acid | Highly corrosive, causing severe skin burns and permanent eye damage. | Wear safety goggles, gloves, and a lab coat. Add slowly with cooling. |
| Ethanol and Ethyl ethanoate | Highly flammable; vapours can ignite and cause fires. | Keep away from Bunsen burner flames. Use an electric heating mantle. |
| Glacial ethanoic acid | Corrosive with irritating, pungent acidic vapours. | Measure out and handle inside a fume cupboard. Wear gloves. |
Worked Yield Calculation
Step 1: Calculate the mass and moles of ethanol
\[ \text{Mass of ethanol} = 10.0\text{ cm}^3 \times 0.789\text{ g cm}^{-3} = 7.89\text{ g} \] \[ \text{Moles of ethanol} = \frac{7.89\text{ g}}{46.0\text{ g mol}^{-1}} = 0.172\text{ mol} \]Step 2: Calculate the mass and moles of ethanoic acid
\[ \text{Mass of ethanoic acid} = 10.0\text{ cm}^3 \times 1.05\text{ g cm}^{-3} = 10.5\text{ g} \] \[ \text{Moles of ethanoic acid} = \frac{10.5\text{ g}}{60.0\text{ g mol}^{-1}} = 0.175\text{ mol} \]Step 3: Determine the limiting reagent and theoretical yield
The reaction stoichiometry is 1:1. Ethanol has fewer moles (0.172 mol) than ethanoic acid (0.175 mol), so ethanol is the limiting reagent.
\[ \text{Theoretical moles of ethyl ethanoate} = 0.172\text{ mol} \] \[ \text{Theoretical mass of ethyl ethanoate} = 0.172\text{ mol} \times 88.0\text{ g mol}^{-1} = 15.14\text{ g} \]Step 4: Calculate the percentage yield
\[ \text{Percentage Yield} = \frac{\text{Actual Mass}}{\text{Theoretical Mass}} \times 100 = \frac{6.20\text{ g}}{15.14\text{ g}} \times 100 = 40.9\% \]The percentage yield is 40.9%.
Sources of Error & Improvements
| Error Source | Classification | Consequence & Mitigation |
|---|---|---|
| Reversible nature of esterification | Systematic | The reaction reaches dynamic equilibrium, preventing 100% conversion of reactants. Mitigation: Use an excess of one reactant or remove water continuously. |
| Loss of product during transfer | Systematic | Product remains on glassware walls during distillation and washing transfers. Mitigation: Minimise transfers; rinse glassware with small portions of solvent. |
| Incomplete separation in funnel | Systematic | Some ester remains dissolved in the aqueous layer. Mitigation: Extract the aqueous layer with a small volume of organic solvent (salting out). |
Exam questions, marked instantly
Write your answer and the AI examiner checks it against the mark scheme, showing exactly which marks you earned and what to add.
Exam-style question
2 marksExplain why cooling water enters a Liebig condenser at the bottom rather than the top.
Show Mark Scheme
- The outer jacket stays completely full of water, with no air pockets [1]
- So cooling is more efficient and all the vapour condenses [1]
Exam-style question
2 marksAn impure liquid ester is shaken with sodium carbonate solution in a separating funnel. Explain why.
Show Mark Scheme
- Sodium carbonate neutralises acidic impurities, such as unreacted carboxylic acid and the acid catalyst [1]
- The salts formed are soluble in water, so they move into the aqueous layer, which is then run off and discarded [1]
CPAC Skills Assessed
- CPAC 1: Follows multi-step written procedures to synthesise and isolate an organic product.
- CPAC 3: Safely uses organic synthesis apparatus, heating mantles, and separating funnels.
- CPAC 5: Correctly references boiling points and densities to calculate reaction yields.
When drawing a distillation setup in exams, make sure there are no closed systems. Leaving the collection flask sealed is a major safety hazard (pressure build-up) and will result in losing the diagram mark. Ensure a clear pathway from the flask through the condenser to the open air.