PAG 6 - Hydrolysis of an ester

What acid does the hydrolysis of methyl benzoate make?

Benzoic acid

Complete the following equation and give the symbols:

Methyl benzoate + sodium hydroxide

Methyl benzoate + sodium hydroxide → sodium benzoate + methanol

• C₆H₅COOCH₃ + NaOH → C₆H₅COO^-Na⁺ + CH₃OH

Give the method for reacting methyl benzoate with sodium hydroxide

1. Pour methyl benzoate into a round bottomed flask

2. Add equal amounts of sodium hydroxide solution and ethanol to flask

3. Add a few anti-bumping granules and set equipment up for refluxing

4. Reflux for 20 minutes

5. Allow the contents of the flask to cool with the condenser still in place. When cool, remove condenser from flask and decant into a beaker.

Give the reaction for the formation of benzoic acid

C₆H₅COO^- + H⁺ → C₆H₅COOH

Give the method for the production of benzoic acid from the mixture above.

1. Add 4-5 drops of methyl orange and stir into the solution

2. Acidify with small portions of hydrochloric acid, stirring after each addition

3. Filter the solid product under reduced pressure

4. Recrystallise the product (benzoic acid) using a minimum volume of boiling water

5. Filter the purified benzoic acid under reduced pressure and allow to dry in the air

After the reflux, what observations are made when adding the HCl portions?

Methyl orange changes to pink/red colour and a white solid forms, indicating an acid

Why is water used as a solvent for recrystallisation?

Water is polar and can dissolve the benzoic acid

Why is the first solution acidified with HCl?

Adding H⁺ ions allows the formation of benzoic acid

Suggest a significant procedural error and a modification which could be made

The use of an indicator as it means an extra impurity which we must get rid of

• We could instead do one experiment with indicator to work out amount of acid needed, then repeat without indicator

• Or we could use a pH probe