# Synthesis of Aspirin ## PAG 6: Synthesis of an organic solid - This practical covers a number of key laboratory skills - use appropriate apparatus to record a range of measurements - use water bath or electric heater or sand bath for heating - use laboratory apparatus for a variety of experimental techniques - purify a solid product by recrystallisation - use melting point apparatus - safely and carefully handle solids and liquids ## Method - Measure 6.0 g of salicylic acid into a conical flask - Add 10 cm³ of ethanoic anhydride and 5 drops of concentrated sulfuric acid - Swirl the mixture and place the flask in a warm water bath at around 60 °C for 20 minutes - Allow the flask to cool - Pour the contents into 75 cm³ of cold water in a beaker - Wait for the aspirin to crystallise out - Recover the aspirin using Buchner filtration and leave it to dry

The water bath provides controlled, even heating without risk of overheating the reaction mixture

## Recrystallisation

Recrystallisation is a purification technique where an impure solid is dissolved in hot solvent then cooled to form pure crystals

- Recrystallise the impure aspirin using ethanol - Once the solid dissolves, pour the solution into 40 cm³ of cold water - Recover the purified crystals using Buchner filtration, as before ### General recrystallisation principles - Hot solvent is used to dissolve both the organic solid and the impurities - Then as the solution cools the organic solid crystallises out - This leaves behind the impurities in the solution - The key is to use the minimum amount of solvent to dissolve the solid and avoid loss of the product

Using too much solvent reduces yield because more product remains dissolved in solution

- If any solid impurities remain in the solution, a hot filtration can be carried out - Once the solution has cooled down to room temperature and crystallised then the product crystals can be recovered by filtration - This is faster using Buchner apparatus in which filtration occurs under reduced pressure - After filtration the product is washed with fresh cold solvent and then allowed to dry on filter paper ### Tips for successful recrystallisation - Recrystallisation can be repeated more than once to ensure a very pure product, but each time the yield of product will decrease - Slow cooling results in bigger well-defined crystals which are easier to filter and dry ## Melting point analysis

The melting point of pure aspirin is 135 °C

- You can assess product purity by comparing to this value - The melting point of a solid provides insight into both its **identity** and **purity** - You can match a sample's melting point to a known value to help identify or confirm the product ### Purity assessment using melting point | Property | Pure substance | Impure substance | |----------|---------------|------------------| | Melting behaviour | Melts sharply at a well-defined temperature | Melts over a wide temperature range | | Comparison to data book | Closely matches the data book value | Lower than expected and less precise | - A pure solid melts sharply and closely matches the **data book value** - Impurities lower the melting point and broaden the temperature range over which the solid melts ### Melting point technique Although techniques can vary depending on the apparatus, key skills include: - Correctly preparing melting point tubes - Heating the sample slowly to prevent overshooting - Repeating measurements to improve accuracy (three runs is typical) **Sample preparation:** - Ensure the sample is completely dry and finely powdered - Crush it using the back of a spatula on filter paper or a tile to remove moisture **Taking measurements:** - Use the first trial to estimate the melting point range, then repeat using a slower heating rate for precision

Always quote a melting point as a range (±) and reference a data book value if you have one

### Common melting point apparatus Three main types of apparatus are used: 1. **Oil bath method** - Sample in melting point tube - Immersed in oil bath with thermometer - Heated with stirrer for even temperature distribution 2. **Thiele tube method** - Specialized triangular-shaped tube with mineral oil - Hot oil rises and circulates naturally - Melting point tube attached with rubber ring above oil level 3. **Melt station (digital)** - Modern electronic apparatus - Melting point tubes inserted into viewing window - Heating control dial and data logger connection - Most accurate and consistent results ## Practical skills summary This practical provides an excellent opportunity to develop several assessed skills, including: - **Accurately measuring** solids and liquids - **Using water baths** to heat mixtures safely - **Purifying solids** through recrystallisation - **Determining melting points** to assess product purity - **Handling, drying, and preparing** solid samples correctly