Required Practical 10b (AQA A-Level Chemistry): Revision Notes

8.3.2 Required Practical 10b

Aim

To synthesise a pure organic liquid by heating under reflux, purify it through distillation and washing, and assess its purity by measuring its boiling point. This experiment will utilise various laboratory techniques essential in organic synthesis, purification, and purity testing.

Apparatus and Equipment Needed

Apparatus:

• 100 cm³ round-bottomed flask

• Reflux condenser

• Anti-bumping granules

• Heating mantle or water bath

• Separating funnel

• Distillation apparatus (condenser, distillation flask, thermometer, and collection vessel)

• Drying agent (anhydrous calcium chloride or anhydrous sodium sulphate)

• Boiling point apparatus or simple setup with oil bath

• Thermometer (0–100°C) Chemicals:

• Organic reactants as specified for the target compound

• Sodium hydrogencarbonate solution (for washing acidic impurities)

• Saturated sodium chloride solution (for layer separation)

• Anhydrous calcium chloride or anhydrous sodium sulphate (for drying the organic layer)

Method

Step 1: Synthesis by Reflux

1. Preparation of Reaction Mixture:

• Place the organic reactants in a 100 cm³ round-bottomed flask along with any required solvents or catalysts.
• Add a few anti-bumping granules to ensure smooth boiling by creating small bubbles, which prevents sudden and vigorous boiling.

2. Reflux Setup:

• Attach a reflux condenser vertically to the flask.
• Ensure that water flows through the condenser from the bottom inlet to the top outlet for efficient cooling.
• Place the flask on a heating mantle or in a water bath (if the reagents are flammable) and heat the mixture to boiling. Refluxing allows continuous boiling and condensation, which keeps reactants and products from evaporating, allowing the reaction to proceed for the desired time without loss of volatile components. Note: The condenser should not be sealed at the top to prevent pressure buildup.

3. Completion of Reflux:

• After the specified reaction time, turn off the heat and allow the flask to cool slightly before proceeding.

Step 2: Initial Purification by Distillation

1. Distillation Setup:

• Transfer the reaction mixture to a distillation apparatus to separate the organic product from impurities, unreacted reactants, and by-products.
• Attach a thermometer in the distillation setup, ensuring it is positioned just above the boiling liquid to measure the temperature of the vapour accurately.

2. Distillation Process:

• Heat the flask gently. The pure product will distil over at its boiling point, which you should monitor and record.
• Collect the distillate in a clean container. This initial distillate is the impure product.

Step 3: Washing and Drying the Organic Product

1. Washing:

• Transfer the impure product into a separating funnel.
• Add approximately 25 cm³ of sodium hydrogencarbonate solution to neutralise any acidic impurities. Stopper the funnel and invert it several times, releasing the pressure periodically due to CO₂ gas formation.
• Allow the layers to separate, and then carefully run off and discard the aqueous (bottom) layer.

2. Further Washing:

• Add 25 cm³ of saturated sodium chloride solution to help further separate the organic and aqueous layers.
• Once again, invert the funnel and allow the layers to separate, then discard the aqueous layer.

3. Drying the Organic Layer:

• Transfer the organic layer to a clean, dry conical flask and add 3 spatulas of anhydrous calcium chloride or anhydrous sodium sulfate to absorb any residual water.
• Swirl the flask and allow it to sit until the liquid becomes clear, indicating that all water has been absorbed.

4. Final Distillation:

• Carefully decant the dried organic layer into a distillation flask and perform a second distillation to collect the pure product. Record the boiling point of the collected fraction to assess purity.

Data Collection

1. Observations during Reflux: Record any changes in colour, temperature, or consistency in the reaction mixture.
2. Boiling Point: Record the boiling point of the purified product. Compare it to the known value to assess purity.
3. Yield: Weigh the final pure product to determine the actual yield. Calculate the percentage yield if the theoretical yield is known.
4. Observations on Layer Separation: Note the volumes of organic and aqueous layers and any changes during washing and drying stages.

Purity Testing: Boiling Point Measurement

1. Boiling Point Setup:

• Set up a distillation apparatus or heat a small sample of the product in a boiling tube within an oil bath, measuring the boiling point.
• Ensure the thermometer is positioned above the liquid level to measure the vapour temperature accurately.

2. Boiling Point Determination:

• Note the boiling point range. A pure organic liquid will boil within a narrow range close to its standard boiling point.
• Record the atmospheric pressure, as changes in pressure can affect the boiling point.

Safety Considerations

1. Personal Protective Equipment (PPE):

• Wear safety goggles, lab coat, and gloves to protect against chemicals and splashes.

2. Handling of Chemicals:

• Sodium hydrogencarbonate and sodium chloride solutions are safe but can cause mild irritation; avoid contact with eyes.
• Anhydrous calcium chloride is an irritant. Handle it with gloves and avoid inhaling dust.

3. Heating Safety:

• Use a water bath if reagents are flammable, and never heat directly with an open flame if there's a risk of fire.
• When refluxing, ensure the top of the condenser is open to avoid pressure buildup, which could lead to explosion.

4. Use of Separating Funnel:

• When shaking the separating funnel with sodium hydrogencarbonate solution, release the stopper periodically to avoid pressure buildup from CO₂ gas.

5. Waste Disposal:

• Dispose of any waste liquids according to local regulations, particularly for organic solvents.

Conclusion and Analysis

• Purity Assessment: A pure product will have a boiling point close to the known value and a narrow range. Compare the boiling point obtained to a reliable reference to judge the product's purity.
• Yield Analysis: Calculate the percentage yield and discuss any possible reasons for product loss, such as incomplete reaction, evaporation, or losses during washing and drying.
• Reflection on Technique: This experiment covers key organic chemistry techniques: heating under reflux to prevent loss of reactants, using a separating funnel to wash away impurities, drying an organic product with anhydrous agents, and verifying purity through boiling point analysis.