Core Practical 16 Lab

Aims

To perform and explain the reactions of acid anhydrides

To synthesise aspirin from 2-hydroxybenzoic acid

Equipment

● 10 cm3 ethanoic anhydride ● 2 g 2-hydroxybenzoic acid ● 1 cm3 concentrated sulfuric acid ● distilled/deionised water ● two 10 cm3 measuring cylinders ● condenser ● pear-shaped flask ● stand, clamp and boss ● two 250 cm3 beakers ● dropping pipette ● ice ● Bunsen burner, tripod, gauze and safety mat ● mass balance (2 d.p.) and weighing boat ● Bϋchner funnel, Bϋchner flask, water/ suction pump and filter paper to fit funnel ● melting temperature apparatus and melting temperature tube

1. Assemble the apparatus provided for heating under reflux. Arrange the flask to fit in a water bath which is resting on a tripod and gauze above a Bunsen burner. Make sure that the inside of the flask and condenser are both dry. DO NOT LIGHT the Bunsen burner.

2. Weigh the stoppered container containing the 2-hydroxybenzoic acid, recording the mass.

3. Now carefully remove the flask from the apparatus.

4. Add the 2-hydroxybenzoic acid to the flask and then reweigh the stoppered container, recording the mass.

5. Using a measuring cylinder, transfer 4 cm3 of ethanoic anhydride to the flask. Add five drops of concentrated phosphoric acid to the flask, followed by a few anti-bumping beads. Carefully swirl the flask to mix the reagents.

6. Replace the flask in the reflux apparatus. Turn on the tap to allow a slow stream of water to pass up the Liebig condenser. Now ask your teacher to check your apparatus.

7. Heat the water bath until the solid in the flask dissolves. Continue to heat, with the water in the water bath just boiling, for a further five minutes.

8. Switch off the Bunsen and allow the reaction mixture to cool down for a few minutes.

9. Carefully add 3 cm3 of water down the condenser. The water hydrolyses the unreacted ethanoic anhydride in the flask and may cause the mixture in the flask to boil.

10. When the vigorous reaction in the flask has ended, pour the reaction mixture into about 7cm3 of water (or crushed ice) in a 100 cm3 beaker. Stir the mixture in the beaker. Solid aspirin should form, but if it does not, then further stirring and rubbing the inside of the beaker with the glass rod & placing on ice may help to cause crystallization.

11. Stand the flask in a beaker of iced water until precipitation appears to be complete.

12. Filter off the product using a Bϋchner funnel and suction apparatus.

13. Wash the crystals with the minimum volume of iced water and dry as much as possible.

14. Recrystallise the aspirin in the minimum volume of a mixture of hot distilled water.

15. Cool the flask in an ice-bath when crystals will form.

16. Filter and dry.

17. Measure the mass of the pure, dry crystals.

18. Measure the melting temperature of the product using melting temperature apparatus