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BACKGROUND
Bromobenzene reacts with magnesium metal to yield the Grignard Reagent (phenyl magnesium bromide), which reacts with carbon dioxide to yield the corresponding salt viz., benzoyloxy-magnesium bromide. The resulting salt on subsequent hydrolysis with HCl ultimately produces benzoic acid and magnesium bromochloride gets eliminated.1
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REQUIREMENTS
Chemicals: Dry Magnesium turnings – 2.4 g ;
Sodium Dry Ether – 30 ml ;
Dry Bromobenzene – 15.7 g (10 ml) ;
Iodine Crystals – 0.1 g ;
Dilute Hydrochloric Acid (6 N) – q.s.
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PROCEDURE
Transfer 2.4 g of dry magnesium turnings and 30 ml of sodium dry ether in a 250 ml round bottom flask fitted with an efficient reflux condenser. Add to the reaction flask slowly and carefully 15.7 g (10 ml) of absolutely dry bromobenzene along with a crystal of iodine. An immediate commencement of reaction shall take place thereby turning the etherial layer to milky white. In case, the reaction does not start off, warm the contents of the flask gently on an electric water-bath and remove it from the water-bath after the mixture starts refluxing. Once the reaction starts, the heat generated from it will promote the reaction itself. Boil the contents of the flask gently for a duration of 50-60 minutes. Place separately 20 g of crushed ice in a 250 ml beaker and pour into it slowly the Grignard reagent prepared earlier with constant vigorous stirring. A quick forceful reaction ensues and the contents in the flask happen to turn into a pasty mass. Continue stirring the mass till all CO2 cease to evolve. Add 50 ml of warm water and subsequently acidify the contents with dilute HCl carefully to litmus paper; thus benzoic acid shall start separating out and the magnesium salt will get dissolved. Cool the contents of the beaker in an ice-bath and filter the white residue in a Büchner funnel under suction, wash with water and air dry the product.
Calculation of Practical Yield: –
157.01 g of Bromobenzene after Grignardization yield Benzoic Acid = 122.12 g
15.7 g of Bromobenzene shall yield Benzoic Acid = 122.12 / 157.01 × 15.7 = 12.20 g
Hence, Theoretical yield of Benzoic Acid = 12.20 g
If reported Practical yield = 6.4 g
Then, Percentage Practical yield = Practical yield / Theoretical yield × 100
= 6. 40 / 12. 20 × 100 = 52.45 %
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CONCLUSION
The yield of the crude product is 6.4 g having mp 119-120.5°C.
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REFERENCES
- Kar A. Advanced Practical Medicinal Chemistry, New Age International (P) Limited Publication, New Delhi, Page No. – 169.
