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 3.3.1 Introduction to organic chemistry, 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 (\(0\text{ to }150\text{ }^\circ\text{C}\))
- Receiving adapter and conical flask
- Heating mantle or electric water bath
- Anti-bumping granules
- Separating funnel (50 cm³)
- Glacial ethanoic acid (\(10\text{ cm}^3\)) and Ethanol (\(10\text{ cm}^3\))
- Concentrated sulfuric acid (\(1\text{ cm}^3\))
- 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). |
Common Exam Questions
1. State why anti-bumping granules are added to the round-bottomed flask.
They provide nucleation sites that allow small bubbles of vapour to form, preventing sudden, uneven boiling (known as bumping) that could force liquid up into the condenser.
2. Explain why the water supply is connected to the bottom of the Liebig condenser rather than the top.
This ensures that the cooling jacket remains completely filled with cold water at all times, preventing air locks from forming and maximizing cooling efficiency.
3. Why is saturated sodium carbonate solution added to the separating funnel?
To react with and neutralise unreacted ethanoic acid and sulfuric acid catalyst. This converts them into soluble sodium salts that separate cleanly into the aqueous layer.
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.