Aim:
To synthesize Benzoic acid from Benzyl chloride by an oxidation reaction
Reference:
F. G. Mann and B. C. Saunders, Practical Organic
Chemistry, 4th Ed, 1974, P. No. 239-240
Requirement:
Apparatus:
Beaker, Stirrer, Filtration assembly, RBF, Condenser, etc.
Chemicals:
Anhydrous
sodium carbonate, potassium permanganate, benzyl chloride, sodium sulfite
Theory/Principle:
When an aromatic compound having an aliphatic side
chain is subjected to oxidation, fission of the side chain occurs between the
first and second carbon atoms from the benzene ring, the first carbon atom thus
becoming part of a carboxyl (-COOH) group. Such oxidations are frequently
important for determining the position of a side chain relative to other
substituents in the benzene ring. The oxidation is usually carried out with a
mixture of potassium permanganate and sodium carbonate in aqueous solution, or
alternatively with dilute nitric acid. The reaction is quite slow if the side
chain is a simple alkyl group. The side chain containing a chlorinated alkyl
group is more susceptible to oxidation.
Hence in comparison to toluene, benzyl
chloride more rapidly oxidizes in the presence of an aqueous oxidizing agent. Here
benzyl chloride is first hydrolyzed to benzyl alcohol, and then undergoes
oxidation of primary alcohol to the corresponding carboxylic acid
 |
| Fig. 1 Synthesis of Benzoic Acid from Benzyl chloride |
Procedure:
- To 40 ml of water contained in a 500 ml bolt-head flask, add in turn 1 g of anhydrous sodium carbonate, 2 g of potassium permanganate, and finally 1 ml of benzyl chloride.
- To 40 ml of water contained in a 500 ml bolt-head flask, add in turn 1 g of anhydrous sodium carbonate, 2 g of potassium permanganate, and finally 1 ml of benzyl chloride.
- Fit the flask with a reflux water-condenser, and boil the mixture gently for 1-1.5 hours, i.e., until the reaction is complete and the liquid running down from the condenser contains no oily drops of unchanged benzyl chloride.
- During this boiling, the permanganate is slowly reduced, and manganese dioxide separates as a dark brown precipitate.
- Now cool the flask, and add concentrated hydrochloric acid (about 5 ml.) cautiously until the mixture is strongly acid, and all the benzoic acid has been precipitated.
- Then add a 20% aqueous solution of crystalline sodium sulfite (about 10 ml.) slowly with shaking until the manganese dioxide is completely dissolved and only the white precipitate of benzoic acid remains.
- When the mixture is quite cold, filter off the benzoic acid at the pump, and wash well with water.
- The benzoic acid is obtained as colorless needles, having m.p. 121°
