Required Practical 10b (AQA A Level Chemistry): Revision Note

Preparation of an Ester

• This required practical covers the following mandatory skills:

  • using a water bath, electric heater, or sand bath for heating

  • using laboratory apparatus for a variety of experimental techniques

  • purifying a liquid, including the use of a separating funnel

  • safely and carefully handling solids and liquids

• The preparation of ethyl ethanoate is a suitable experiment that provides opportunities to practise all of these skills

Key steps in the procedure

• Add 10 cm³ of ethanol, 12 cm³ of glacial ethanoic acid, and 15 drops of concentrated sulfuric acid to a pear-shaped flask

• Add a few anti-bumping granules to the flask

• Partially immerse the flask in a beaker of water

• Attach a condenser vertically and clamp it in place so the apparatus is set up for heating under reflux

• Heat the water bath and allow the reaction mixture to boil under reflux for about 15 minutes

• Stop the heating and allow the mixture to cool to room temperature

Separation of the product

• Pour the reaction mixture into a beaker containing saturated sodium carbonate solution

• This neutralises any unreacted acids and helps separate the ester as an oily layer

• Transfer the mixture to a separating funnel and insert the stopper

• Invert the funnel and open the stopcock to release pressure. Repeat this 15–20 times

• Allow the layers to separate. The ethyl ethanoate forms the upper layer

• Open the stopcock to drain off the aqueous layer

• Drain the ethyl ethanoate into a clean beaker

• Add a small amount of solid sodium sulfate to remove any remaining water

Purification of the product

• The ethyl ethanoate is purified by distillation

• Esters do not form hydrogen bonds like alcohols and carboxylic acids, so ethyl ethanoate has a lower boiling point than the reactants

• The distillate is collected, and the temperature at which it is obtained is recorded

Key hazards

• Concentrated sulfuric acid and ethanoic acid are highly corrosive; safety glasses and gloves must be worn

• Ethanol is flammable, so care must be taken to avoid naked flames

• Ethyl ethanoate is toxic, flammable, and has a strong odour; it should be handled in a fume cupboard

Identifying an Ester

Identification of an ester by hydrolysis

• The boiling point of an ester can be used as one piece of evidence to help establish its identity

• The ester can then be hydrolysed, and the resulting carboxylic acid purified by recrystallisation

• The melting point of the purified acid is measured and compared with known data values

• Acid hydrolysis of an ester is very slow, so the reaction must be heated under reflux with an acid catalyst to produce the carboxylic acid

• Methyl benzoate is a suitable ester for hydrolysis and identification

• The hydrolysis reaction is

C₆H₅COOCH₃     +   H₂O    →  C₆H₅COOH   + CH₃OH 

Key steps in the procedure

• Add 2.0 g of methyl benzoate to a pear-shaped flask

• Add 40 cm³ of 1.0 mol dm^-3 sulfuric acid

• Heat the reaction mixture under reflux for 30 minutes

• While still warm, pour the mixture into a beaker of cold water

• This causes benzoic acid to precipitate

• Collect the solid by Büchner filtration

• Recrystallise the solid using hot water

Identification data

• The melting point of the solid is measured and compared with data book values

• Expected melting point of benzoic acid = 122 °C

• Expected boiling point of methyl benzoate = 199 °C