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. Insoluble material is removed by filtration
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. Put flask on hot plate. To avoid crystallization in the funnel:
Use GLASS 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. Example A I1 I2 dissolves dissolves 100 mL water at 1000 insoluble
Solubility g/100 mL
Compound
25o
100o 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
Example

23. Example Cool to 25o A dissolves I1 dissolves I2 insoluble A I1
Solubility g/100 mL
Compound
25o
100o 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
Cool to 25o
A dissolves
I1 dissolves
I2 insoluble A I1
9.0 g crystallize
0 g crystallize
Example
Yield = 100 * 9.0/12.0 = 75%

24. Example 50 ml of water at 1000 A I1 I2 dissolves dissolves insoluble
Solubility g/100 mL
Compound
25o
100o 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
Example

25. Example NOT PURE! Cool to 25o 50 mL water at 1000 A dissolves
Solubility g/100 mL
Compound
25o
100o 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%
Example
NOT PURE!

26. What volume of solvent is needed to give the maximum yield of pure A?
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
Example

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