RP10: Preparation of a Pure Organic Liquid

Preparing a pure organic liquid requires an initial synthesis followed by a series of purification steps: separating layers, washing away acidic residues, drying out trace moisture, and a final distillation to isolate the desired fraction. This guide uses the dehydration of cyclohexanol to cyclohexene as the primary example.

Aim

To prepare a pure sample of an organic liquid (specifically cyclohexene from cyclohexanol) using distillation, liquid-liquid extraction in a separating funnel, drying, and fractional distillation.

Equipment List

• Round-bottomed flask (100 cm³)
• Pear-shaped flask (100 cm³)
• Condenser with tubing
• Still head and receiver adapter
• Thermometer (0-150 °C) with holder
• Heating mantle or water bath (for flammable liquids)
• Separating funnel (100 cm³) and stand
• Beakers (100 cm³ and 250 cm³)
• Conical flasks (100 cm³)
• Measuring cylinders (10 cm³ and 25 cm³)
• Dropping pipettes
• Solid anhydrous calcium chloride (CaCl₂)
• Sodium carbonate solution (NaHCO₃, 1.0 mol dm⁻³)
• Concentrated phosphoric acid (H₃PO₄) or concentrated sulfuric acid (H₂SO₄)
• Cyclohexanol
• Anti-bumping granules

Dehydration Chemical Equation

Cyclohexanol is heated under acid catalysis to undergo elimination, yielding cyclohexene and water. Phosphoric acid (H₃PO₄) is preferred over sulfuric acid as it yields fewer side products.

Experimental Method

Step 1: Synthesis & Initial Distillation

1. Measure 10.0 cm³ (approx . 9.6 g) of cyclohexanol into a 100 cm³ round-bottomed flask.
2. Carefully add 2.0 cm³ of concentrated phosphoric acid catalyst . Swirl the flask gently to mix.
3. Add a few anti-bumping granules . These provide nucleation sites to prevent large bubbles forming and ensure smooth boiling.
4. Set up the distillation apparatus (as shown below) . Heat the mixture gently using a heating mantle (never a Bunsen burner, as cyclohexene is highly flammable).
5. Collect the fraction boiling between 80 °C and 85 °C in a receiving flask.

Step 2: Washing and Separating

The distillate is a mixture of cyclohexene, water, and acidic impurities (H₃PO₄ carried over). To purify it:

1. Pour the distillate into a separating funnel.
2. Add 10 cm³ of 10% sodium carbonate (Na₂CO₃) solution . This neutralises the acid impurities:
\[ 2\text{H}_3\text{PO}_4(\text{aq}) + 3\text{Na}_2\text{CO}_3(\text{aq}) \rightarrow 2\text{Na}_3\text{PO}_4(\text{aq}) + 3\text{CO}_2(\text{g}) + 3\text{H}_2\text{O}(\text{l}) \]
3. Stopper the funnel, invert it, and shake it gently. Important: Open the tap regularly while inverted to vent the built-up CO₂ gas pressure.
4. Allow the layers to separate . Cyclohexene has a density of 0.81 g cm⁻³, while the aqueous sodium carbonate layer has a density of >1.00 g cm⁻³. Therefore, the organic layer sits on top.
5. Remove the stopper and run off the lower aqueous layer into a beaker and discard it.
6. Wash the organic layer by adding 10 cm³ of distilled water, shake, let separate, and discard the aqueous layer again.

Step 3: Drying

1. Collect the upper organic layer from the separating funnel into a clean, dry conical flask.
2. Add a spatula of anhydrous calcium chloride (CaCl₂), which acts as a drying agent.
3. Stopper the flask, swirl it, and allow it to stand for 15 minutes.
4. Observation: The liquid is dry when it turns from cloudy to completely clear. If it remains cloudy, add a little more drying agent.
5. Decant or filter the liquid into a clean round-bottomed flask to remove the solid drying agent.

Step 4: Final Distillation (Fractional Distillation)

1. Set up the distillation apparatus again using clean, dry glassware.
2. Distil the product and collect the fraction that distils between 81 °C and 83 °C . This represents the pure cyclohexene.
3. Weigh the final collection flask and determine the mass of cyclohexene obtained.

Yield & Atom Economy Calculations

Safety & Risk Assessment

Sources of Error & Improvements

• Reversible reaction limit: The dehydration of cyclohexanol is an equilibrium reaction. Improvement: Distil off the cyclohexene product as soon as it forms. Removing a product shifts the equilibrium to the right (Le Chatelier's principle), improving conversion.
• Transfer losses: Moving liquids between flasks, separating funnels, and cylinders leaves film residues behind. Improvement: Use small volumes of solvent to rinse vessels and combine rinses with the main organic layer.

CPAC Skills Assessed

• CPAC 1: Follows detailed written instructions for multi-stage synthesis and purification.
• CPAC 2: Correctly sets up standard laboratory distillation glassware.
• CPAC 3: Manages flammable organic risks by utilizing heating mantles instead of open flames.