In this experiment I’ll be making Benzoic Acid from the oxidation of Toluene. Before starting, let me remind you that it is much easier and cheaper to simply buy Benzoic Acid or obtain it using other methods. This procedure was done to simply explore the chemistry behind it. However, it might be a good source of Benzoic Acid if you can’t find it anywhere.
For this experiment, the following reagents are needed:
- Potassium Permanganate;
- Hydrochloric Acid (at least 20%);
- Distilled Water (regular tap water can also be used without any visual loss of yield);
This experiment isn’t particularly dangerous. It is recommended to work in a well ventilated area throughout the entire procedure due to Toluene vapors and HCl vapors.
- Toluene is highly flammable, irritant to the skin and mildly toxic when inhaled. Avoid inhaling too much of it.
- Potassium Permanganate stains everything. Literally. Gloves are recommended to avoid staining your hands. Stains in your workplace may be cleaned using a solution of Sodium Metabisulfite.
- Hydrochloric Acid is corrosive and releases HCl gas that is also corrosive.
- Manganese Dioxide doesn’t pose any special danger.
- Benzoic Acid doesn’t pose any special danger.
First, start by preparing the mixture.
- Weight into a flask (ground glass joints recommended) 16g of Potassium Permanganate.
- Add 180mL of water.
- Add 20mL of Toluene.
After preparing the mixture, set up an apparatus for simple reflux.
The mixture was allowed to reflux for about 2,5 hours.
- Beginning the reflux:
- Ending the reflux:
Disassemble the apparatus and allow the mixture to cool. Meanwhile, set up an apparatus for Vacuum Filtration.
Note: This is not your typical Vacuum Filtration apparatus. My Kitasato (also known as Buchner Flask, Vacuum Flask or Filter Flask) was KIA (killed in action). For the time being I must improvise my set up for vacuum filtration.
Filter the mixture. If the filtrate is still pink (Potassium Permanganate color) you must remount your reflux apparatus and reflux the solution again. If the filtrate is colorless you’re good to go.
- Manganese Dioxide in the filter:
- Mixture after filtration:
Note: I dumped my Manganese Dioxide. If you wish to keep yours, add it to a flask with water. Shake it and decant the water. Repeat this process at least three times. Vacuum filter and set aside to dry.
Now, the leftover Toluene is removed by means of physical separation. A separatory funnel is recommended. The Toluene layer is the top one and can be discarded.
Now measure about 100mL of 33% Hydrochloric Acid. This quantity doesn’t need to be exact but I recommend using concentrated Hydrochloric Acid.
On the following picture you can see that a precipitate has formed, after the addition of the Hydrochloric Acid. This is Benzoic Acid.
Now set up a Vacuum Filtration apparatus and vacuum filter the mixture.
- Here is a picture of the Benzoic Acid after filtration:
Now wash your product with 20mL of cold water. Wash again with the same amount of cold water and set aside to dry.
Here is the final product:
The final product weights about 2 grams.
Potassium Permanganate is a powerful oxidizer and can oxidize Toluene to Benzoic Acid. The mechanism for the reaction is quite complex. However, it can be simplified by the overall equation:
C7H8(l) +2 KMnO4(aq) –> KC7H5O2(aq) + 2 MnO2(s) + KOH(aq) + H2O(l)
It can also be represented as (image from http://commons.wikimedia.org/wiki/File:Benzoic_acid_Synthesis.JPG):
Potassium Permanganate: 158,034 g/mol
Toluene: 92,14 g/mol
Potassium Benzoate: 160,21 g/mol
Manganese Dioxide: 86,937 g/mol
Potassium Hydroxide: 56,106 g/mol
Water: 18,015 g/mol
Hydrochloric Acid: 36,460 g/mol
Benzoic Acid: 122,12 g/mol
I used 16 grams of Potassium Permanganate. Thus, the theoretical amount of Toluene should have been 4,66g (about 5,4mL). However, 20mL of Toluene (large excess) was used. I chose to use such a large excess for three reasons:
- Toluene and water are immiscible. Having excess Toluene increases the contact area during reflux, decreasing reflux time.
- Potassium Permanganate can oxidize the Benzoate ion all the way to Carbon Dioxide. This would affect yield. Having excess Toluene prevents this because Toluene is oxidized much more easily.
- Having excess Toluene also guarantees that all Potassium Permanganate gets reduced, avoiding additional separation processes to remove it.
After refluxing, filtering and removing the Toluene, HCl is added. It reacts as follow:
- First, it neutralizes all Potassium Hydroxide formed in the main reaction.
HCl(aq) + KOH(aq) –> KCl(aq) + H2O(l)
- After all KOH is neutralized, the HCl reacts with the Potassium Benzoate in solution to form Benzoic Acid.
KC7H5O2(aq) + HCl(aq) –> C7H6O2(s) + KCl(aq)
Potassium Benzoate is quite soluble in water. However, Benzoic Acid is much less soluble. Because there is a lot of HCl dissolved, the solubility of Benzoic Acid decreases even more (Common-Ion Effect).
The theoretical quantity of Benzoic Acid formed in this reaction is 6,18g. The amount obtained was 1,9g. This represents a yield of 30,7%. This is the “common” yield for this reaction.
The reaction is very interesting and is a very good way to practice lab skills. Please leave some feedback and feel free to comment. If you have any doubts, feel free to ask.