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Recrystallization

Recrystallization. Impure benzoic acid. Benzoic acid after recrystallization. Benzoic Acid. Purification of Organic Compounds. Solids: Recrystallization. Liquids: Distillation. Solids and Liquids: Gas or liquid chromotography. Types of impurities. Soluble Insoluble

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Recrystallization

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  1. Recrystallization Impure benzoic acid Benzoic acid after recrystallization

  2. Benzoic Acid

  3. Purification of Organic Compounds Solids:Recrystallization Liquids: Distillation Solids and Liquids: Gas or liquid chromotography

  4. Types of impurities Soluble Insoluble Partly Soluble

  5. Solubility of benzoic acid in water

  6. Dissolve sample in hot solvent Compound crystallizes upon cooling

  7. Impure benzoic acid in hot water

  8. Add water and heat until all soluble material dissolves Insoluble material is removed by filtration

  9. The hot solution is filtered to remove insoluble impurities

  10. Use fluted filter paper to maximize surface area.

  11. Use GLASS funnel. To avoid crystallization in the funnel: Put flask on hot plate.

  12. Boil off excess solvent

  13. Boiling Sticks

  14. Hot solution after filtration: Cool Seed Scratch

  15. Crystals form in the warm solution Next: Cool, filter, wash, dry, weigh, mp

  16. Cool in ice bath

  17. .. Buchner funnel Heavy-walled tubing Filter flask Vacuum

  18. Filter paper .. Put filter paper in Buchner funnel

  19. Filter, Wash Filter and wash with cold solvent

  20. Summary Purified benzoic acid Insoluble impurities Soluble impurities

  21. Solubility Example Solubility g/100 mL Compound 25o 100o A I1 I2 1.0 1.5 0.0 80 50 0.0

  22. Solubility g/100 mL Compound 25o 100o Example A I1 I2 1.0 1.5 0.0 80 50 0.0 A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g 100 mL water at 1000 A I1 I2 dissolves dissolves insoluble

  23. Solubility g/100 mL Compound 25o 100o Example A I1 I2 1.0 1.5 0.0 80 50 0.0 A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g Cool to 25o 100 mL water at 1000 A dissolves I1 dissolves I2 insoluble A I1 9.0 g crystallize 0 g crystallize Yield = 100 * 9.0/12.0 = 75%

  24. Solubility g/100 mL Compound 25o 100o Example A I1 I2 1.0 1.5 0.0 80 50 0.0 A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g 50 ml of water at 1000 A I1 I2 dissolves dissolves insoluble

  25. Solubility g/100 mL Compound 25o 100o Example A I1 I2 1.0 1.5 0.0 80 50 0.0 A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g Cool to 25o 50 mL water at 1000 A dissolves I1 dissolves I2 insoluble A I1 9.5 g crystallize 0.25 g crystallize Yield = 100 * 9.75/12.0 = 81% NOT PURE!

  26. What volume of solvent is needed to give the maximum yield of pure A? Example Solubility of I1 = 1.5 g/100 mL 1.5 g 100 mL 1.0 g x mL = X = (100 mL * 1.0 g)/1.5 g = 67 mL

  27. What is the yield of pure A from 67 mL solvent? Example Solubility of A = 1.0 g/100 mL 1.0 g 100 mL X g 67 mL = X = (67 mL x 1.0 g) / 100 mL = 0.67 g Yield = 10.0 g - 0.67 g = 9.33 g % yield = 100 * 9.33 / 12 = 78%

  28. Second Crop Boil off some solvent Cool, filter, wash, dry, weigh, mp

  29. Fractional Crystallization

  30. Recrystallization Solvents • Does not react with compound • Large temperature coefficient of solution • Low boiling point

  31. Mixed Solvents • Ethanol – Water • Ether Pentane

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