Core Practical 5: Synthesis of an Organic Liquid

Safety Notes

• Wear gloves and goggles when handling concentrated sulfuric acid.
• Work in a well-ventilated area or fume hood.
• Ethyl ethanoate is flammable – use electric heating.
• Vent the separating funnel regularly to avoid pressure buildup from CO₂.

Apparatus

• Round-bottom flask
• Condenser
• Heat source (water bath or electric heater)
• Separating funnel
• Dropping pipettes
• Boiling tubes
• Conical flask
• Measuring cylinders
• Thermometer
• Liebig condenser
• Distillation apparatus
• Drying agent (anhydrous calcium chloride or magnesium sulfate)
• Anti-bumping granules

Chemicals

• Ethanoic acid
• Ethanol
• Concentrated sulfuric acid (catalyst)
• Sodium carbonate or sodium hydrogencarbonate solution
• Saturated sodium chloride solution (brine)
• Anhydrous calcium chloride (drying agent)

Method – Synthesis and Purification

Preparation (Reflux)

1. Add 10 cm³ ethanoic acid, 10 cm³ ethanol, and a few drops of concentrated sulfuric acid to a round-bottom flask.
2. Add anti-bumping granules and attach a reflux condenser.
3. Gently heat under reflux for 20–30 minutes using a water bath.

Purpose: To heat a reaction mixture for an extended time without loss of volatile components, allowing the reaction to proceed completely.

How: Vapours condense and return to the flask. Anti-bumping granules prevent large bubbles from forming (‘bumping’).

Separation

1. After refluxing, transfer the reaction mixture to a separating funnel.
2. Add sodium hydrogencarbonate solution slowly to neutralise excess acid (CO₂ gas is produced – periodically invert and vent the funnel).
3. Shake and allow layers to separate. Remove the aqueous layer.

Purpose: To separate the organic product from the aqueous layer (e.g. unreacted acid, water-soluble impurities). Organic and aqueous layers form due to differences in density and solubility.

Washing

• Add sodium hydrogencarbonate to neutralise any remaining acid.
• Wash the organic layer with saturated sodium chloride (brine) to help remove water and separate layers more effectively.

Drying

• Add a few spatulas of drying agent (e.g. anhydrous calcium chloride or MgSO₄) to remove remaining water.
• Swirl until the liquid goes clear, then decant or filter to remove the solid.

Purpose: To remove water from the organic layer. The drying agent binds to water molecules but must not react with the organic product.

Distillation

• Distil the dry organic liquid and collect the fraction boiling at ~77 °C (for ethyl ethanoate).
• Record the boiling point and compare to data to confirm identity.

Purpose: To purify the organic product by separating it from other volatile substances based on boiling point. Ensure the thermometer bulb is level with the entrance to the condenser to measure vapour temperature accurately.

Example Calculation (Mole-Based)

A student obtained 10.1 g of ethyl ethanoate. Calculate percentage yield:

CH₃CH₂OH + CH₃COOH ⇌ CH₃COOCH₂CH₃ + H₂O

1. Moles of ethanol used = 0.1715 mol
   Based on 10.0 cm³ ethanol (density = 0.789 g cm⁻³, M_r = 46.0):
   Mass = 10.0 × 0.789 = 7.89 g
   Moles = 7.89 ÷ 46.0 = 0.1715 mol ethanol
2. Assume ethanol is limiting (ethanoic acid is in excess).
3. The reaction is 1:1 → theoretical ester moles = 0.1715 mol
4. M_r of ethyl ethanoate = 88.0
   Theoretical mass = 0.1715 × 88.0 = 15.09 g
5. Actual mass = 10.1 g
   % yield = (10.1 ÷ 15.09) × 100 ≈ 67.0%

Sources of Error and Improvements

• Loss of product during transfer: Minimise steps and rinse equipment.
• Incomplete separation or drying: Ensure proper shaking and add sufficient drying agent.
• Impure product: Confirm purity by recording boiling point and comparing with data.