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.
🔑 Core Specification Link
This practical connects to 3.3.5 Alcohols (elimination/dehydration to alkenes) and 3.3.6 Organic Analysis (purity verification using boiling point).
Dehydration Chemical Equation
\[ \text{C}_6\text{H}_{11}\text{OH} \xrightarrow{\text{Conc. }\text{H}_3\text{PO}_4} \text{C}_6\text{H}_{10} + \text{H}_2\text{O} \]Cyclohexanol is heated under acid catalysis to undergo elimination, yielding cyclohexene and water. Phosphoric acid (\(\text{H}_3\text{PO}_4\)) is preferred over sulfuric acid as it yields fewer side products.
Experimental Method
Step 1: Synthesis & Initial Distillation
- Measure 10.0 cm³ (approx. 9.6 g) of cyclohexanol into a 100 cm³ round-bottomed flask.
- Carefully add 2.0 cm³ of concentrated phosphoric acid catalyst. Swirl the flask gently to mix.
- Add a few anti-bumping granules. These provide nucleation sites to prevent large bubbles forming and ensure smooth boiling.
- 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).
- 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 (\(\text{H}_3\text{PO}_4\) carried over). To purify it:
- Pour the distillate into a separating funnel.
- Add 10 cm³ of 10% sodium carbonate (\(\text{Na}_2\text{CO}_3\)) solution. This neutralises the acid impurities: \[ 2\text{H}_3\text{PO}_4(\text{aq}) + 3\text{Na}_2\text{CO}_3(\text{aq}) \rightarrow 2\text{Na}_3\text{PO}_4(\text{aq}) + 3\text{CO}_2(\text{g}) + 3\text{H}_2\text{O}(\text{l}) \]
- Stopper the funnel, invert it, and shake it gently. Important: Open the tap regularly while inverted to vent the built-up \(\text{CO}_2\) gas pressure.
- Allow the layers to separate. Cyclohexene has a density of \(0.81\text{ g cm}^{-3}\), while the aqueous sodium carbonate layer has a density of \(>1.00\text{ g cm}^{-3}\). Therefore, the organic layer sits on top.
- Remove the stopper and run off the lower aqueous layer into a beaker and discard it.
- Wash the organic layer by adding 10 cm³ of distilled water, shake, let separate, and discard the aqueous layer again.
Step 3: Drying
- Collect the upper organic layer from the separating funnel into a clean, dry conical flask.
- Add a spatula of anhydrous calcium chloride (\(\text{CaCl}_2\)), which acts as a drying agent.
- Stopper the flask, swirl it, and allow it to stand for 15 minutes.
- Observation: The liquid is dry when it turns from cloudy to completely clear. If it remains cloudy, add a little more drying agent.
- Decant or filter the liquid into a clean round-bottomed flask to remove the solid drying agent.
Step 4: Final Distillation (Fractional Distillation)
- Set up the distillation apparatus again using clean, dry glassware.
- Distil the product and collect the fraction that distils between 81 °C and 83 °C. This represents the pure cyclohexene.
- Weigh the final collection flask and determine the mass of cyclohexene obtained.
Yield & Atom Economy Calculations
Step 1: Calculate the mass of cyclohexanol reactant
\[ \text{Mass} = \text{Volume} \times \text{Density} = 10.0\text{ cm}^3 \times 0.96\text{ g cm}^{-3} = 9.60\text{ g} \]Step 2: Calculate moles of cyclohexanol
\[ \text{Moles of cyclohexanol} = \frac{9.60\text{ g}}{100.0\text{ g mol}^{-1}} = 0.096\text{ mol} \]Step 3: Calculate theoretical yield of cyclohexene
The reaction stoichiometry is 1:1, so \(0.096\text{ mol}\) of cyclohexanol theoretically produces \(0.096\text{ mol}\) of cyclohexene:
\[ \text{Theoretical mass of cyclohexene} = 0.096\text{ mol} \times 82.0\text{ g mol}^{-1} = 7.872\text{ g} \]Step 4: Calculate percentage yield
\[ \text{Percentage Yield} = \left(\frac{\text{Actual Mass}}{\text{Theoretical Mass}}\right) \times 100 = \left(\frac{4.50\text{ g}}{7.872\text{ g}}\right) \times 100 = 57.2\% \]The percentage yield is 57.2%.
Safety & Risk Assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Cyclohexene | Highly flammable; harmful if swallowed or inhaled. | Use a heating mantle instead of a Bunsen burner. Perform all transfers in a fume cupboard. Keep containers stoppered. |
| Concentrated Phosphoric Acid | Highly corrosive; causes severe skin burns and eye damage. | Wear safety goggles and nitrile gloves. Measure and add acid carefully inside a fume cupboard. |
| Carbon Dioxide pressure build-up | The separating funnel can burst, spraying corrosive contents. | Invert the funnel and open the tap frequently to vent \(\text{CO}_2\) gas during neutralisation. |
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.
Common Exam Questions
1. Explain why sodium carbonate solution is added to the impure cyclohexene, and why venting is necessary.
Sodium carbonate reacts with and neutralises any unreacted phosphoric acid catalyst carried over during the first distillation. Venting is required because the reaction produces carbon dioxide gas, which builds up pressure inside the closed separating funnel.
2. A student forgets to remove the stopper from the top of the separating funnel before opening the tap. State what will happen.
The liquid will not flow out of the tap easily, or it will flow very slowly and then glug and stop. A vacuum is created above the liquid as it attempts to drain, which prevents the liquid from running out smoothly.
3. Describe how the boiling point of the final distillate can verify its purity.
Measure the boiling point using a distillation setup. A pure sample will boil at a sharp, constant temperature matching the literature value (83 °C). An impure sample will boil over a wide range of temperatures, often starting lower or ending higher than the pure substance.
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.
When drawing a distillation apparatus in exams, ensure there are no closed seals at the receiver end. A closed system heated will build up gas pressure and explode, which is a major safety violation that loses marks.