Preparation and purification of organic compoundsEdexcel International A Level Chemistry: Revision notes

Making an organic compound has two parts: the reaction, and then separation and purification of the product from unreacted reagents, by-products and the solvent. The typical sequence is:

1. Carry out the reaction (often by reflux).
2. Separate the crude product (distillation, solvent extraction or filtration).
3. Wash to remove impurities.
4. Dry the product.
5. Purify (redistil a liquid or recrystallise a solid).
6. Check purity by measuring a boiling or melting temperature.

• Reflux: the mixture is heated with a condenser fitted vertically, so volatile liquids condense and return. This allows long heating without loss.
• Distillation: the apparatus is set up for the condensed liquid to be collected. It separates liquids with different boiling temperatures; collect the fraction at the boiling temperature of the wanted compound.
• Steam distillation: steam is passed through the mixture. It is used for compounds that are immiscible with water, have a high boiling temperature and may decompose if heated directly. The mixture boils below 100 °C because the vapour pressures of the two immiscible liquids add; the product distils with the steam and leaves non-volatile impurities behind.

Solvent extraction: shake the mixture with a solvent in a separating funnel; the organic compound dissolves in the organic layer and the layers are separated. Release pressure by opening the tap regularly. The layers are identified by density: run off the lower layer first.

Washing removes impurities:

• water removes water-soluble substances such as ethanol or salts
• aqueous sodium carbonate removes acid, giving CO₂ (vent the funnel): 2H⁺ + CO₃²⁻ → H₂O + CO₂

Drying: add an anhydrous salt such as magnesium sulfate, sodium sulfate or calcium chloride and swirl. When the liquid is clear and the solid no longer clumps, it is dry. Decant or filter off the drying agent.

To purify a solid:

1. Dissolve it in the minimum volume of hot solvent to make a saturated solution.
2. If there are insoluble impurities, filter while hot.
3. Cool slowly, then in ice: the pure solid crystallises because it is less soluble when cold; soluble impurities stay in solution.
4. Filter under reduced pressure (Buchner funnel and flask).
5. Wash with a small volume of ice-cold solvent to remove the remaining solution without dissolving much product.
6. Dry in a low-temperature oven, desiccator or between filter papers.

A good solvent dissolves the solid well when hot and poorly when cold. The yield is always less than the amount at the start because some product stays dissolved.

Pure substances have sharp, fixed melting and boiling temperatures.

• Melting temperature: heat a small sample in a capillary tube slowly in a melting temperature apparatus and record the range from first liquid to completely liquid. A pure solid melts over a narrow range (about 1 °C) at the literature value. Impurities lower the melting temperature and widen the range.
• Boiling temperature: record the temperature at which the liquid distils steadily. A pure liquid distils at a constant temperature; impurities give a range or a different value.

Compare the measured values with data book (literature) values to judge purity.

Aspirin is made by esterification of the OH group of salicylic acid with ethanoic anhydride, using a few drops of concentrated phosphoric(V) or sulfuric acid as catalyst:

C₇H₆O₃ + (CH₃CO)₂O → C₉H₈O₄ + CH₃COOH

1. Warm salicylic acid, excess ethanoic anhydride and the acid catalyst in a water bath.
2. Add cold water to hydrolyse the excess anhydride and precipitate the aspirin; cool in ice.
3. Filter under reduced pressure and wash with ice-cold water.
4. Recrystallise from the minimum volume of hot ethanol or ethanol and water.
5. Dry, weigh and measure the melting range.

Worked yield: 2.00 g of salicylic acid (M = 138.0) is 0.01449 mol. Theoretical aspirin (M = 180.0) is 0.01449 × 180.0 = 2.61 g. If 1.80 g is obtained, yield = 1.80 ÷ 2.61 × 100 = 69.0%.