Recrystallization Impure benzoic acid Benzoic acid after 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 Partly Soluble
Solubility of benzoic acid in water
Dissolve sample in hot solvent Compound crystallizes upon cooling
Impure benzoic acid in hot water
Dissolve Add water and heat until all soluble material dissolves Insoluble material is removed by filtration
Filter Hot The hot solution is filtered to remove insoluble impurities
Fluted Filter Paper Maximize surface area.
To avoid crystallization in the funnel: Use GLASS funnel Put flask on hot plate.
Boil off excess solvent
Boiling Sticks
Hot solution after filtration: Cool Seed Scratch
Crystals form in the warm solution Next: Cool, filter, wash, dry, weigh, mp
Cool
Buchner funnel Heavy-walled tubing Filter flask Vacuum
. . Put filter paper in Buchner funnel
Filter and wash with cold solvent
Insoluble impurities Soluble impurities Purified benzoic acid
Solubility Example Compound Solubility g/100 m. L 25 o 100 o A 1. 0 80 I 1 1. 5 50 I 2 0. 0
Compound Solubility g/100 m. L A 25 o 1. 0 100 o 80 I 1 1. 5 50 I 2 0. 0 A = 10 g I 1 = 1. 0 g I 2 = 1. 0 g Total = 12. 0 g 100 m. L water at 1000 A dissolves I 1 dissolves I 2 insoluble
Compound Solubility g/100 m. L A 25 o 1. 0 100 o 80 I 1 1. 5 50 I 2 0. 0 A = 10 g I 1 = 1. 0 g I 2 = 1. 0 g Total = 12. 0 g 100 m. L water at 1000 Cool to 25 o A dissolves A 9. 0 g crystallize I 1 dissolves I 1 0 g crystallize I 2 insoluble Yield = 100 * 9. 0/12. 0 = 75%
Compound Solubility g/100 m. L A 25 o 1. 0 100 o 80 I 1 1. 5 50 I 2 0. 0 A = 10 g I 1 = 1. 0 g I 2 = 1. 0 g Total = 12. 0 g 50 ml of water at 1000 A dissolves I 1 dissolves I 2 insoluble
Compound Solubility g/100 m. L A 25 o 1. 0 100 o 80 I 1 1. 5 50 I 2 0. 0 A = 10 g I 1 = 1. 0 g I 2 = 1. 0 g Total = 12. 0 g Cool to 25 o 50 m. L water at 1000 A dissolves A 9. 5 g crystallize I 1 dissolves I 1 0. 25 g crystallize I 2 insoluble Yield = 100 * 9. 75/12. 0 = 81% NOT PURE!
Maximize Yield What volume of solvent is needed to give the maximum yield of pure A? Solubility of I 1 = 1. 5 g/100 m. L 1. 5 g = 100 m. L 1. 0 g x m. L X = (100 m. L * 1. 0 g)/1. 5 g = 67 m. L
Maximize Yield What is the yield of pure A from 67 m. L solvent? Solubility of A = 1. 0 g/100 m. L 1. 0 g = 100 m. L Xg 67 m. L X = (67 m. L x 1. 0 g) / 100 m. L = 0. 67 g Yield = 10. 0 g - 0. 67 g = 9. 33 g % yield = 100 * 9. 33 / 12 = 78%
Second Crop Boil off some solvent Cool, filter, wash, dry, weigh, mp
Fractional Crystallization
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