Required practical 10: Preparation of a pure organic solid and a pure organic liquid

Introduction to Required Practical 10

In Chemistry, making a product is only half the battle. Once you have synthesized an organic compound, it is usually "dirty"—mixed with unreacted starting materials, catalysts, and side products. Required Practical 10 is all about the art of purification. We focus on two main skills: preparing a pure organic solid (like aspirin) and a pure organic liquid (like cyclohexene or a halogenoalkane).

Don’t worry if the long lists of steps seem overwhelming! Each step has a very logical reason behind it. Think of it like washing muddy clothes: first you scrub them, then you rinse them, and finally, you check if the stains are gone.

Part 1: Preparation of a Pure Organic Solid

The most common example for this practical is the preparation of aspirin. According to the syllabus, ethanoic anhydride is preferred over acyl chlorides for this reaction because it is safer, cheaper, and less reactive with water.

The Process: Recrystallisation

Recrystallisation is the "gold standard" technique for purifying solids. Here is the step-by-step breakdown:

1. Dissolve the impure compound: Dissolve your solid in a minimum volume of hot solvent.
Why? We use a minimum volume so the solution is saturated. We use a hot solvent because most solids are much more soluble when hot than when cold.

2. Hot Filtration: Filter the hot solution quickly through fluted filter paper.
Why? This removes any insoluble impurities (bits that won't dissolve). We use fluted paper and a warmed funnel to prevent the product from crystallising too early.

3. Cooling: Allow the solution to cool slowly at room temperature, then place it in an ice bath.
Why? As the solution cools, the solubility of our product drops, and it "crashes out" as pure crystals. Soluble impurities stay dissolved in the solvent.

4. Filtration under Reduced Pressure: Use a Büchner funnel and a side-arm flask connected to a vacuum pump.
Why? This is much faster than standard gravity filtration and leaves the crystals much drier.

5. Washing and Drying: Wash the crystals with a small amount of ice-cold solvent and let them dry in a warm oven or desiccator.
Why? Cold solvent removes any remaining soluble impurities from the surface of the crystals without dissolving the product itself.

Testing Purity: Melting Point Analysis

How do we know if our solid is actually pure? We use a melting point apparatus.

Pure substances: Have a sharp melting point that matches the value in a data book. For example, pure aspirin melts at exactly \(136 ^\circ C\).
Impure substances: Will melt over a wide range of temperatures and at a lower temperature than the pure compound.

Quick Review: If your crystals melt at \(125-132 ^\circ C\) instead of \(136 ^\circ C\), they are definitely still impure!

Part 2: Preparation of a Pure Organic Liquid

Purifying a liquid is slightly different. We often use a separating funnel to deal with different "layers" of liquids.

The Process: Using a Separating Funnel

1. Separation: Pour the mixture into a separating funnel. Two layers will form because the organic liquid and water are immiscible (they don't mix).
Tip: Usually, the aqueous (water) layer is on the bottom because it is denser, but you can check by adding a little water and seeing which layer grows!

2. Running off layers: Open the tap to let the bottom layer out into a beaker, then collect your organic layer in a separate flask.

3. Removing Acidic Impurities: If you used an acid catalyst, add sodium hydrogencarbonate (\(NaHCO_3\)) solution to the funnel and shake.
Important: This produces \(CO_2\) gas, so you must invert the funnel and open the tap frequently to release the pressure (venting). You'll know the acid is gone when it stops fizzing.

Drying the Liquid

Even after using a separating funnel, your organic liquid will have traces of water in it. It will look cloudy.
We add an anhydrous drying agent (like \(MgSO_4\) or \(CaCl_2\)).
Process: Add the drying agent and swirl. Keep adding until the liquid turns from cloudy to clear and the drying agent "clumps" less and moves freely like "snow" in a snow globe.

Final Step: Distillation

To get the highest purity, we perform a final distillation. We heat the liquid and collect the fraction that evaporates at the specific boiling point of our desired product. This separates it from any remaining organic impurities.

Safety and Hazards

In this practical, you are handling "real" chemicals, so safety is a priority (referenced as technique k in the syllabus):

• Flammability: Many organic liquids (like ethanol or cyclohexane) are highly flammable. Use an electric heating mantle or a water bath instead of a Bunsen burner.
• Corrosives/Irritants: Concentrated acids and ethanoic anhydride can cause burns. Wear gloves and safety goggles.
• Toxic Vapours: Conduct volatile reactions in a fume cupboard.

Summary Key Takeaways

For Solids:

• Recrystallisation: Dissolve (hot/min solvent) \(\rightarrow\) Filter (hot) \(\rightarrow\) Cool \(\rightarrow\) Filter (reduced pressure) \(\rightarrow\) Wash (cold).
• Purity Test: Sharp melting point = pure.

For Liquids:

• Separating Funnel: Removes water-soluble impurities; remember to vent gas!
• Drying Agents: Use anhydrous salts (like \(MgSO_4\)) until the liquid is clear.
• Distillation: Final purification based on boiling point.

Note: For more on basic distillation setups, you can cross-reference Required Practical 5. For identifying the final products using tests, see Required Practical 6.