Aim: Preparation of benzoic acid from ethyl benzoate
Introduction
Benzoic acid can be prepared by the alkaline hydrolysis of the ester, ethyl benzoate:
If sodium
hydroxide is used, then the residual solution will contain sodium benzoate.
Insoluble benzoic acid can be displaced from this solution by acidification. It
can then be filtered off and purified by recrystallisation from water. The
percentage yield of benzoic acid can be calculated and its melting point
determined.
Health and
Safety
Wear eye
protection and if any chemical splashes on your skin wash it off immediately.
Ethyl benzoate
is of no significant hazard.
2 mol l-1
sodium hydroxide is corrosive to the eyes and skin. Gloves and goggles should
be worn.
5 mol l-1
hydrochloric acid is irritating to the eyes, lungs and skin and if swallowed.
Wear gloves.
The product,
benzoic acid is a skin, eye and respiratory irritant.
Ethanol is volatile,
highly flammable, irritating to the eyes and intoxicating if inhaled or
ingested. But only a small amount of ethanol is produced in the reaction.
Requirements
|
ethyl benzoate |
2 mol l-1 sodium hydroxide |
|
5 mol l-1 hydrochloric
acid |
deionised water |
|
l00 cm3 round-bottomed flask |
cork ring |
|
condenser |
heating mantle |
|
anti-bumping granules |
dropper |
|
Buchner funnel and flask blue litmus
paper or pH paper |
100 cm3 measuring cylinder
|
|
glass filter funnel |
melting point apparatus |
|
capillary tube |
thermometer |
|
water pump |
balance (accurate to 0.01 g) |
Procedure
1. Weigh the 100 cm3 round-bottomed
flask supported on a cork ring. To the flask, add about 5 g of ethyl benzoate and
reweigh the flask and its contents.
2. To the ethyl benzoate, add
approximately 50 cm3 of 2 mol l-1 sodium hydroxide and a
few anti-bumping granules.
3. Set up the apparatus for heating under
reflux. Using a heating mantle, reflux the reaction mixture until all oily drops
of the ester have disappeared. This may take 45 - 60 minutes.
4. Allow the apparatus to cool and then
transfer the reaction mixture to a 250 cm3beaker.
5. Slowly and with stirring, add 5 mol l-1
hydrochloric acid to the reaction mixture to precipitate out the benzoic acid.
Continue adding the acid until no more precipitation takes place and the mixture
tums acidic - test with blue litmus paper or pH paper. (About 30 cm3 of
acid will be required)
6. Allow the mixture to cool to room
temperature and filter off the precipitate at the water pump and wash the crude
benzoic acid with a small volume of water.
7. Transfer the crude benzoic acid to a
250 cm3 beaker and recrystallise it from about 100 cm3 of
water.
8. Filter off the crystals of benzoic acid
at the water pump and wash them with a small volume of water. Allow air to be
drawn through the crystals for a few minutes in order to partially dry them.
9. Determine the melting point of the
benzoic acid.Calculate the percentage yield of benzoic acid.
Yield of Benzoic
acid = 2.3 gm
