Hydrolysis of an Ester - page 1
Keywords: hydrolysis ester practical methyl benzoate benzoic acid
By Jenny on 02/07/2009
Level: A Level (Year 13)
Page Number: 1 of 1
Hydrolysis of an Ester
For this practical I will hydrolyse the ester methyl benzoate to make benzoic acid, C6H5COOH.
The equation for the reaction is:
C6H5COOCH3 + OH- → C6H5COO- + H2O
C6H5COO- + H+ → C6H5COOH
Plan
• Measure 2.0 cm3 of methyl benzoate into a 100cm3 flask.
• Add 10 cm3 of dilute 2.0 mol dm-3 sodium hydroxide solution, 10 cm3 of ethanol and some anti-bumping granules.
• Attach water-cooled condenser.
• Using a Bunsen burner heat the flask gently for 5 minutes and then boil gently under reflux for another 15 minutes.
• Allow the flask to cool then remove the condenser and pour the solution into a beaker.
• Add 10 drops of methyl orange and then add 2.0 mol dm-3 hydrochloric acid until the solution is acidified.
• Filter the product under reduced pressure.
• Recrystallise the product using boiling water to purify it.
• Filter the purified product under reduced pressure again and leave it to dry in air.
• Dry a small sample of the product and determine its melting point.
• When the product is dry, weigh it and record the yield in a suitable table.
Measurements and Observations
When heating the solution over the Bunsen burner I started heating at 2.10, at 2.15 the mixture started to boil and at 2.30 I took it off the heat. The solution refluxed fairly smoothly due to the anti-bumping granules but there was some movement.
When I added the Methyl Orange the solution in the beaker turned green, then when I acidified it with the hydrochloric acid it slowly turned pink. I added a total of 18 cm3 of the acid to completely acidify it.
When my filtered crystals formed they were needle-like in shape and smelled sweet (slightly like the smell of Deep-heat).
The melting point of my crystals was 121°C and my yield was 1.42g.
Mechanism
Safety
I wore safety glasses at all times during this experiment and was careful not to allow any of the chemicals to get on my skin. The 2.0 mol dm-3 hydrochloric acid was particularly dangerous, contact with the eyes or skin can cause serious permanent damage. Concentrated solutions of this acid are extremely corrosive.
It is also toxic by inhalation (the concentrated solution releases dangerous quantities of hydrogen chloride) so it was important to make sure that the room was well ventilated.
I was also careful with Bunsen burner and I used the anti-bumping granules to the solution so that it wouldn’t react violently while it was being heated.
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