Applied Science BTEC Nationals/Chemical Laboratory Techniques/Benzoic acid
Preparation of benzoic acid
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The experimental details given here are given in good faith and are believed to be safe and workable methods. However, the authors cannot take responsibility for the consequences of performing these experiments.
The experiments are written for experienced science teaching staff to use as instructions for a supervised class of students. The experiments are not designed for students or inexperienced members of the public to perform without supervision. If you wish to attempt the experiments, ensure that you have completed a legally adequate risk assessment beforehand and that you work within the constraints of the risk assessment.
Oxidations are a very important class of reactions in organic chemistry. Most oxidations require either a heteroatom in a reduced state (O in -OH, N in -NH2, etc.) or a carbon-carbon pi-bond.
The oxidation of an aromatic side chain does not require any of these factors. All that is needed is a benzylic carbon with at least one hydrogen attached to it. The oxidising agents that can be used are chromic acid (using Na2Cr2O7 or CrO3 in sulphuric acid and water) or potassium permanganate (KMnO4) . The product in each case is benzoic acid or a derivative of benzoic acid. The permanganate reaction requires work up with acid, because the permanganate reaction generates a basic solution as a by product):
- C6H5CH3 + 2 KMnO4 → C6H5COOK + 2 MnO2 + H2O + KOH
We will use potassium permanganate to oxidise methylbenzene (toluene) to benzoic acid. Permanganate solutions are less hazardous than those containing chromium compounds, which are often carcinogenic.
- Potassium manganate (VII) (permanganate) is a strong oxidiser, and is corrosive, wear gloves while handling it (it will also stain skin and clothing).
- Sodium hydrogen sulphite (sodium bisulphite) is an irritant—wear gloves while handling it.
- Concentrated hydrochloric acid is corrosive and toxic—wear gloves while handling it, and be sure to wash your gloves and your hands after handling it.
- Methylbenzene (toluene) is a flammable liquid, is toxic, and its vapours are narcotic—no flames will be allowed in lab, wear gloves while handling it, and avoid breathing its vapours.
 Day 1: Preparation of benzoic acid.
In a 100 cm3 round bottom flask (this will keep liquid to less than half of the capacity of the reaction vessel), mix the following:
- 3.0 g of KMnO4 (Use caution not to get on skin)
- About 0.7 g of methylbenzene (about 0.8 cm3 liquid)
- 35 g (cm3) of water.
Reflux (with a few added anti-bumping granules) the reaction mixture for up to 4 hours.
Refluxing for less time than this may result in left-over manganate (VII). Because of the volatility of methylbenzene, efficient cooling of the reflux condenser is necessary.
- C6H5CH3 + 2 KMnO4 → C6H5COOK + 2 MnO2 + H2O + KOH 
STOP, and leave your apparatus until the next lab period for purification.
Day 2: Separation of benzoic acid
Use suction filtration in a Büchner funnel (filter the hot solution, but do not wash with hot or cold water). Collect the filtrate (the liquid that passes the filter).
The insoluble MnO2 will be retained by the filter, while the filtrate is collected in the suction flask. It can be thrown away.
If the filtrate is still purple (indicating MnO4- ion is still present) transfer the filtrate to a new flask and cool on ice if it is still hot .
Slowly add small amounts of solid sodium hydrogen sulphite (NaHSO3 ) to reduce any remaining permanganate ion.
Be careful because the reaction between sodium hydrogen sulphite and potassium manganate (VII) is highly exothermic (heat released).
When no more purple colour is present (the mixture will be a brownish suspension), you have added enough sodium hydrogen sulphite.
When the purple colour is no longer present, centrifuge the suspension for 5 minutes at 3-4000 rpm. (The MnO2 formed at this stage is too fine to be filtered efficiently). Centrifugation will cause the precipitate to form a pellet and the rest of the solution can be decanted carefully.
If the solution is not colourless (it is still purple in colour) you will need to add more hydrogen sulphite and repeat the centrifugation in order to produce a clear solution (a slight brownish hue is acceptable, but it must not be purple).
Put the clear and colourless solution (a slight brownish colour is acceptable) in the ice bath. Acidify the mixture by adding about 2.5 cm3 of concentrated HCl. Add the acid drop by drop until a white precipitate of benzoic acid is formed. (It may redissolve if not enough acid has been added). As few as 4-5 drops may be enough to make the solution acidic, and initiate the formation of solid material. Adding more acid will not affect your results, as you cannot add too much acid.
- C6H5COOK + KOH + 2 HCl → C6H5COOH + H2O + 2 KCl 
Collect the precipitated organic product using vacuum filtration and a Büchner funnel. You can use small amounts of cold water to wash the solid material and to remove the solid from the crystallisation container, prior to drying on a watch glass.
The solid product will be stored in the drying oven until the next lab period for recrystallisation.
Stop and store your solid material until the next lab period for purification. If you were not able to produce solid material during this lab period, you can store your sample until the next lab period.
Day 3: Recrystallisation and analysis of the benzoic acid
The purification method involves recrystallisation. It is necessary to select an appropriate solvent in which
- the benzoic acid is very soluble at high temperatures but sparingly soluble at low temps, and...
- either; impurities are insoluble at all temps, or....
- impurities are soluble at all temps
A solvent which meets these criteria for this experiment is water.
Example recrystallisation method
This is an example of how the work might be recorded. Repeat this experiment – or improve on it if you feel able to. Note that not all data are recorded for you (the volume of hot water used, for example).
Benzoic acid was collected in a beaker (250 cm3).
A small quantity of water was poured into the beaker and the mixture brought to the boil over a Bunsen flame. The mixture was stirred to encourage dissolving.
Not all of the benzoic acid dissolved. A little more water was added, keeping the mixture stirred and simmering. The process was repeated until all of the benzoic acid appeared to dissolve.
The mixture was then filtered quickly through a pre-heated Buchner funnel. Any insoluble impurities remained on the paper as the residue.
The filtrate was cooled on ice and this caused most of the benzoic acid to crystallise. Soluble impurities were left in solution.
The mixture was again filtered at the pump, this time while cold. The residue on the filter is the purified benzoic acid.
This residue was washed by running a little deionised water over it in order to remove the salt solution. Running the pump for a short while after filtration encourages drying of the residue.
The residue was then carefully scraped off the filter paper onto a watch glass and allowed to dry in the oven at 60 oC. When dry, the final product was weighed.
Mass of benzoic acid at start 4.00 g
Mass of benzoic acid as product 3.50 g
Analyse the benzoic acid by determining the melt temperature. Compare your observed value to the literature value. Determine a percentage yield based on the limiting reagent (methylbenzene).
Show the oxidation state of the methyl carbon of the methylbenzene, the carbonyl carbon of the acid, and for the Mn in both the permanganate ion and the MnO2.
You may wish to record other details of the experiment which do not naturally fall in to any of the other categories e.g. mistakes which may have occurred; some theory which is new to you; some interesting application or detail which you may wish to record for future reference etc.