Practical Skills: Elimination and Distillation Revision Notes | A Level AQA Chemistry

1. Why Distillation Matters in Organic Synthesis

You have just run a reaction and your flask contains a mixture of product, unreacted starting material, water, and acid. The question is: how do you get your pure product out? This is where distillation becomes the essential tool of the organic chemist.

Distillation works because different liquids have different boiling points. By carefully controlling temperature, you can vaporise your target compound, condense it back into a liquid, and collect it separately from everything else in the flask. For AQA Required Practical 5, you need to be able to carry out and describe this process for the distillation of a product from a reaction — specifically the elimination of an alcohol to form an alkene.

2. The Elimination Reaction: Turning an Alcohol into an Alkene

Before you can distil a product, you need to make it. In this practical context, the reaction is the elimination of water from an alcohol to produce an alkene. The alcohol is heated with concentrated phosphoric(V) acid ($\text{H}_3\text{PO}_4$) or concentrated sulphuric acid ($\text{H}_2\text{SO}_4$), which acts as a catalyst and dehydrating agent.

The most common A-level example is the dehydration of cyclohexanol to cyclohexene. Cyclohexanol (boiling point 161 °C) and cyclohexene (boiling point 83 °C) differ significantly in boiling point, which makes distillation an effective way to separate the product from unreacted starting material.

3. The Distillation Apparatus: Every Piece Has a Purpose

Once the reaction is complete, the mixture in the flask contains cyclohexene, unreacted cyclohexanol, water, and acid. Distillation separates the volatile cyclohexene from the higher-boiling components. Understanding why each piece of apparatus is present is essential — examiners frequently ask you to justify your setup.

• Round-bottomed flask: holds the reaction mixture; its curved base distributes heat evenly and withstands thermal stress better than a flat-bottomed flask.
• Anti-bumping granules: added before heating to provide nucleation sites for smooth, even boiling. Without them, the liquid can superheat and boil violently, causing liquid to shoot into the condenser.
• Still-head (distillation head) with thermometer: the thermometer bulb is positioned at the side-arm junction so it measures the temperature of the vapour entering the condenser, not the liquid in the flask.
• Liebig condenser: cold water flows through the outer jacket (entering at the bottom, leaving at the top — counter-current flow) to cool the vapour and condense it back to liquid.
• Receiver flask (conical flask): collects the distillate. The fraction is collected over a narrow temperature range corresponding to the boiling point of the target compound.
• Heat source (e.g. heating mantle or water bath): provides controlled, even heating. A naked flame is avoided when flammable organic solvents are present.

The thermometer position is a detail that examiners test regularly. The bulb must be level with the side-arm outlet — if it sits in the liquid, it reads the liquid temperature, not the vapour temperature, and you will collect the wrong fraction.

4. The Full Procedure: From Reaction to Pure Product

The practical has two distinct phases: the reaction phase (making the alkene) and the purification phase (isolating it). Both phases must be carried out carefully to maximise yield and purity.

5. Purification Steps: Washing, Drying, and Redistillation

The first distillate is not pure cyclohexene — it is a mixture of cyclohexene and water (they co-distil because cyclohexene is immiscible with water and the mixture has a lower boiling point than either pure component). Three purification steps follow distillation.

Worked example: Purification Sequence After Initial Distillation

The initial distillate contains cyclohexene, water, and traces of acid. Describe and explain the three purification steps used to obtain pure, dry cyclohexene.

1. Transfer distillate to separating funnel. Add saturated $\text{Na}_2\text{CO}_3(aq)$. Shake with venting. Run off and discard the lower aqueous layer. — Sodium carbonate is a base that neutralises any dissolved acid (e.g. $\text{H}_3\text{PO}_4$) carried over in the distillate. The reaction produces $\text{CO}_2$ gas — venting prevents pressure build-up. The aqueous layer (lower, denser) contains the sodium salt of the acid and is discarded. The organic cyclohexene layer (upper, less dense) is retained.
2. Transfer the organic layer to a conical flask. Add anhydrous $\text{CaCl}_2$ or anhydrous $\text{MgSO}_4$. Swirl and leave for several minutes. Filter off the drying agent. — Residual water dissolves into the drying agent, which absorbs it chemically. The drying agent is ready when it flows freely and no longer clumps together. Anhydrous $\text{CaCl}_2$ is commonly used for cyclohexene, though it cannot be used for alcohols (it forms complexes with them). Anhydrous $\text{MgSO}_4$ is a more general-purpose alternative.
3. Transfer the dried liquid to a clean round-bottomed flask with fresh anti-bumping granules. Redistil and collect the fraction boiling between 80 °C and 85 °C. — Redistillation removes any remaining impurities with different boiling points. Collecting over a narrow temperature range (80–85 °C, close to the literature boiling point of cyclohexene at 83 °C) ensures high purity. A wide collection range would include impurities.

Answer: Pure cyclohexene is obtained after: (1) washing with $\text{Na}_2\text{CO}_3(aq)$ to remove acid, (2) drying with anhydrous $\text{CaCl}_2$ to remove water, and (3) redistillation to collect the fraction at 80–85 °C.

Notice that the choice of drying agent matters. Anhydrous calcium chloride cannot be used to dry alcohols because it forms an addition compound with them (CaCl₂·4ROH), effectively removing your product along with the water. For cyclohexene (an alkene), it works well. Anhydrous magnesium sulphate is a safer general choice when you are unsure.

6. Simple Distillation vs Fractional Distillation

The distillation used in this practical is simple distillation. It is appropriate here because the boiling points of cyclohexene (83 °C) and cyclohexanol (161 °C) differ by approximately 78 °C — a large enough difference for effective separation in a single distillation.

In fractional distillation, the fractionating column provides a large surface area where vapour repeatedly condenses and re-vaporises as it rises. Each cycle enriches the vapour in the more volatile component. This is why fractional distillation is needed to separate liquids with similar boiling points, such as ethanol and water.

7. Yield, Purity, and Testing the Product

After redistillation, you can confirm the identity and purity of your product in two ways. First, compare the boiling point of your collected fraction with the literature value for cyclohexene (83 °C). A pure sample boils over a narrow range; an impure sample boils over a wider range at a temperature that may differ from the literature value.

Second, test with bromine water. Cyclohexene rapidly decolourises orange bromine water because the $\text{C=C}$ double bond undergoes electrophilic addition with $\text{Br}_2$. Unreacted cyclohexanol does not decolourise bromine water under these conditions, so a positive test confirms the presence of the alkene product.

Percentage yield is calculated from the actual mass of product collected compared with the theoretical maximum. Losses occur at every stage: some product remains in the flask, some is lost during washing and transfer, and some is lost as vapour. Careful technique — minimising transfers, using small volumes of washing solvent, and collecting over a precise temperature range — maximises yield.

8. Exam Technique and Common Mistakes

Questions on this practical appear in several formats: describe the apparatus, explain the purpose of a step, evaluate the procedure, or suggest improvements. Each command word demands a different type of response, and using the wrong approach loses marks even when your chemistry is correct.