Practical organic chemistry is assessed heavily in written papers, specifically relating to AQA Required Practical 10. You must understand the theoretical basis and the correct assembly of apparatus for synthesis, extraction, and purification.
🔑 Key Principle
Organic reactions are often slow, yield side-products, and rarely go to completion. Therefore, the experimental process is divided into three key phases: synthesis (reaction), isolation (separation), and purification followed by purity analysis.
Phase 1: Synthesis (Reflux and Distillation)
To prepare volatile organic compounds, we use two main heating techniques depending on whether we want to keep the reactants in the flask or isolate a product as it forms.
A technique involving the continuous boiling and condensation of a reaction mixture in a vertical condenser to prevent the loss of volatile components.
A separation technique based on differences in boiling points, used to isolate a volatile liquid product from a reaction mixture as it forms.
When drawing or identifying diagrams for reflux or distillation, check that the apparatus is not sealed. The top of the reflux condenser, or the receiver adapter vent in distillation, must remain open to the atmosphere. Heating a sealed system causes rapid pressure buildup from expanding gases, which leads to an explosion.
Phase 2: Isolation (Liquid Purifications)
Once a reaction is complete, the crude liquid product must be isolated from aqueous reagents, acids, and inorganic salts. This involves separation and drying steps:
- Separating Funnel: Pour the mixture into a separating funnel. Add water or aqueous wash solutions. Shake the funnel, releasing pressure periodically by opening the tap while inverted. Allow the layers to settle and separate based on density. Run off the aqueous and organic layers into separate beakers.
- Washing: To remove acidic impurities, shake the organic layer with sodium hydrogencarbonate (\(\text{NaHCO}_3\)) solution in the funnel. The acid reacts to release carbon dioxide gas, requiring frequent venting.
- Drying: Trace water must be removed from the organic layer. Transfer the organic liquid to a conical flask and add an anhydrous inorganic salt (such as anhydrous \(\text{CaCl}_2\) or \(\text{MgSO}_4\)) which acts as a drying agent. Swirl and leave until the liquid changes from cloudy to clear. Filter off the drying agent.
- Redistillation: Distil the dry organic liquid, collecting the fraction that boils at the precise boiling point of the pure target compound.
Phase 3: Purification of Solids (Recrystallisation)
Organic solids prepared in the lab (such as aspirin or benzoic acid) are purified using recrystallisation. The steps are highly structured:
A purification technique for organic solids, involving dissolving the impure solid in a minimum volume of hot solvent, cooling to form crystals, and filtering.
Recrystallisation Procedure:
- Dissolve: Dissolve the impure solid in the minimum volume of hot solvent. Using a minimum volume ensures the solution is saturated, maximizing the yield of crystals upon cooling.
- Hot Filtration: Filter the hot solution through fluted filter paper to remove insoluble impurities. (The fluted paper keeps the filtration fast, preventing the solid from crystallising in the funnel).
- Cool: Allow the filtrate to cool slowly to room temperature, and then in an ice bath. Pure crystals of the product will recrystallise out, while soluble impurities remain dissolved in the solvent.
- Cold Filtration: Filter the crystals under reduced pressure using a Buchner funnel and flask (connected to a water pump). This is much faster and dries the crystals more thoroughly than gravity filtration.
- Wash and Dry: Wash the crystals with a small volume of ice-cold solvent to remove remaining soluble impurities without dissolving the product. Dry the crystals in a desiccator or warm oven.
Phase 4: Assessing Purity (Melting Point Determination)
The purity of an organic solid is determined by measuring its melting point and comparing it to database values for the pure compound.
- Place a small amount of the dry solid into a capillary tube sealed at one end.
- Heat the capillary tube slowly in a melting point apparatus (or Thiele tube) near the expected melting point.
- Record the temperature at which melting begins and ends. This range is the melting point range.
⚠️ Purity Indicators
- A pure compound melts sharply at a precise temperature, matching the literature value within 1 to 2 °C.
- An impure compound melts over a wider temperature range and at a lower temperature than the literature value.
Worked Examples of Practical Scenarios
Solution:
- Distillation: Heat the reaction mixture in a distillation flask. Cyclohexene has a much lower boiling point than cyclohexanol, so it distils off first. Collect the distillate boiling below 100 °C.
- Separation: Transfer the distillate to a separating funnel. Add saturated sodium chloride solution (to help separate layers). Run off the lower aqueous layer, retaining the upper organic layer (cyclohexene).
- Washing: Wash the organic layer with sodium hydrogencarbonate solution to neutralise any traces of acid catalyst carried over. Vent the funnel regularly.
- Drying: Transfer the cyclohexene to a conical flask and add anhydrous calcium chloride (\(\text{CaCl}_2\)). Swirl and leave until the liquid is clear.
- Redistillation: Distil the dry cyclohexene, collecting the fraction that boils specifically between 81 and 85 °C.
a) Too much solvent is used in the first step.
b) The filtered crystals are washed with warm solvent.
Solution:
a) Too much solvent: The yield will decrease. When the solution is cooled, more of the product will remain dissolved in the excess solvent rather than crystallising out, meaning it is lost in the filtrate.
b) Washing with warm solvent: The yield will decrease. The warm solvent will dissolve some of the purified crystals on the filter paper, washing the product down into the flask.
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