Synthesis of Benzoic Acid from Benzyl Chloride

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BACKGROUND

Principle: If oxidation occurs to an aromatic compound having an aliphatic side chain then, fission of the side chain occurs between the first and second carbon atom from the benzene ring and the first carbon atom thus becoming part of a carboxyl (-COOH) group.

The oxidation process is carried out with a mixture of potassium permanganate and sodium carbonate in aqueous solution, or with dilute nitric acid. The reaction is quite slow if the side chain is a simple alkyl group. The side chain containing chlorinated alkyl group is more susceptible to oxidation. Hence in comparison to toluene, benzyl chloride more rapidly oxidizes in presence of an aqueous oxidizing agent. Here benzyl chloride is first hydrolyzed to benzyl alcohol, and then undergoes oxidation of a primary alcohol to the corresponding carboxylic acid.¹

Aim: To prepare benzoic acid from benzyl chloride.

Reaction:

 

 

Use: It is used as antibacterial, skin irritation, inflammation caused by burns, insect bites, fungal infections, or eczema etc.

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REQUIREMENTS

Chemicals:        Anhydrous sodium carbonate – 5 g.

Potassium permanganate – 10 g.

Benzyl chloride – 5 ml.

Sodium sulphite – 20g.

Conc. HCl – 50 ml

Apparatus:        Round-bottomed flask

    Reflux condenser set

    Beaker

    Measuring cylinder

    Buchner funnel etc

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PROCEDURE

200 ml of water is taken in a 500 ml flask 5 g of anhydrous sodium carbonate, 10 g of potassium permanganate, and 5 ml (5.5 g) of benzyl chloride are added one by one. The flask is fitted with a reflux water-condenser, and boiled gently for 1 to 1.5 hours to complete the reaction. During this boiling, the permanganate is slowly reduced, and manganese dioxide separates as a dark brown precipitate. Then the flask is cooled, and about 50 ml concentrated hydrochloric acid is added cautiously until the mixture is strongly acidic, and all the benzoic acid is precipitated. Then about 100 ml of 20% aqueous solution of sodium sulphite is added slowly with shaking until the manganese dioxide is completely dissolved and only the white precipitate of benzoic acid remains. The product is filtered at the pump when cold, and washed with water. It is recrystallised from boiling water. Benzoic acid is obtained as colourless needles, m.p. 121°.^1,2

Calculation of yield:

140.57 g of benzyl chloride yield Benzoic Acid = 122.12 g

5.5 g of benzyl chloride shall yield Benzoic Acid = 122.12 / 140.57 × 5.5 = 4.78 g

Hence, Theoretical yield = 04.78 g

If reported Practical yield = 4.5 g

Then, Percentage Practical yield = Practical yield / Theoretical yield × 100

= 4.5 / 4.78 × 100 = 94.1%

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CONCLUSION

The percentage yield of the benzoic acid is about 94.1% having mp 121°C.

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REFERENCES

1. Mann F. G., Saunders B. C., Practical Organic Chemistry, 4^th Edition, Dorling Kindersly India Pvt. Ltd., New Delhi, Page No. 239.
   
2. A class-book of organic chemistry, by J. B. Cohen, 295, 1918.