To make Benzoic Acid in the laboratory, take 5 gram benzyl chloride, 4 gram sodium carbonate, 8 gram KMnO4 and about 200 ml water in a round flask. Apply a reflux condenser to the flask and heat it until the pink color of KMnO4 fades.
After cooling the flask mixture to about 80°C, the SO2 gases flow in it until the precipitate of MnO2 completely dissolves. Crystals of Benzoic Acid dissociate upon cooling the mixture. After filtering them, washing them and drying them, pure benzoic acid is obtained.