EXPERIMENT 7 Part A
THE PREPARATION AND PURIFICATION OF BENZOIC ACID
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LEARNING AIMS
- To gain familiarity with the use of reflux and filtration apparatus
- To synthesise and purify a solid organic compound via a recrystallisation
- To record melting points
LEARNING OUTCOMES
- To understand reaction mechanisms and “curly arrows”
- To determine compound purity through mixed melting points
- To calculate chemical yields
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INTRODUCTION
Before attending the laboratory read the following sections on techniques:
Reflux
Weighing
Recrystallisation and filtration and, in the spaces in the instructions, estimate (in minutes) how long it will take you to reach each stage.
THEORY
Benzamide (containing a CONHB2B group) can be hydrolysed by heating with concentrated sodium hydroxide to yield the (water soluble) sodium salt of benzoic acid & liberating ammonia (NHB3B):-
Re-acidification, via the careful addition of concentrated acid, affords benzoic acid:-
METHOD
- Assemble an apparatus for refluxing (refer to Appendix booklet). Use a heating mantle for the source of heat. If in doubt, check with a demonstrator.
- Add benzamide (3.0g) and 10% sodium hydroxide (50 cmP3P), together with a few anti-bumping granules, to a 100 cmP3P round-bottom flask.
- Bring the mixture to boiling and then reflux for about 20 minutes.
- Allow to cool and then remove the condenser so that any residual ammonia disperses. Remove the anti bumping granules by decanting the cooled (ambient (room) temperature) solution into a beaker then slowly add concentrated hydrochloric acid until the solution is acidic to indicator paper (pH = 3-4). 5.Cool the solution in ice; benzoic acid should be precipitated. If not, scratching the inner wall of the glass vessel may lead to crystal formation. Filter off the solid product using vacuum filtration using a correctly fitting filter paper on a clamp-secured Buchner funnel (with rubber inset between filter funnel and Buchner flask) (see Appendix booklet) and wash the product with a little water. Note! Once the rubber tubing has been connected to the (expensive) Buchner funnel/flask, the whole apparatus has a tendency to tip over. Clamping the apparatus will prevent breakages and spillages.
6. Recrystallise the benzoic acid from hot water After collecting the crystals from the cold solution by vacuum filtration, dry them in an oven set at 80oC.
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EVALUATION OF RESULTS
Convert the mass of benzamide used into the number of moles of benzamide used using the R.M.M. of benzamide and the formula:
Using the mass of benzoic acid obtained, the R.M.M. of benzoic acid and the formula above, how many moles of benzoic acid was obtained?
Knowing the number of moles of benzamide you used will tell you how many moles of benzoic acid you would expect to have produced. By comparison of the number of moles of benzoic acid you expected to the number of moles of benzoic acid that you actually have produced, you can calculate the percentage yield using the formula:
% yield of benzoic acid = x 100%
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