The Amateur Chemist

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Synthesis of Benzoic Acid and Manganese Dioxide


30 grams Potassium Permanganate
70 ml toluene
350 ml water


Neither Benzoic acid nor manganese dioxide are very expensive, so this probably isn't the most economical way of getting these reagents. However, if you don't have access to the Internet or these chemicals for some reason, then this is a good way to get them. Both toluene and Potassium Permanganate can be found at your local hardware store. This does, however, require a fair bit of time and you need a reflux setup.

To start, add the permanganate to the water in a sufficiently sized boiling flask. I usually double or even triple this recipe to get more product and I use a 2000 ml flask. Put a stir bar in the flask and stir to dissolve as much of the permanganate as possible. Next, add the 70 ml of toluene and some boiling chips to the flask.

Reflux this for about 2 1/2 hours. I usually do it for about 3-4 hours just to be safe. Filter this mixture while it is hot. You should vacuum filter it, as it will go much faster. The black powder/mud is MnO2. It's purity depends on the purity of your KMnO4. By the way, do not try to wash the manganese dioxide. The filtrate should be clear. If it is not, then you still have some permanganate in it, and you need to reflux it more. Continue doing this until the filtrate is clear.

Then, put the cooled filtrate into a separatory funnel. Drain off the bottom layer. This is what you want. The top layer is extra toluene. You can drain this off and save it for another run. I wouldn't use it for anything else but this though. Anyway, boil down the aqueous layer (bottom) to 100 ml. Place this in a clean beaker and cool it. 

Add 37%, or something around that concentration, HCl to this slowly, drip by drip. This will produce a fluffy white precipitate to form. Keep adding until no more precipitate is formed. Then, vacuum filter this. Wash it with LOTS of cold water. Spread it out and let it dry, and there you go. Benzoic acid and Manganese Dioxide.