1. Recrystallization Impure benzoic acid
Benzoic acid after recrystallization

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

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

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

19. Filter and wash with cold solvent

20. 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. 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

23. 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
Yield = 100 * 9.0/12.0 = 75%

24. 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

25. NOT PURE! Cool to 25o 50 mL water at 1000 A dissolves I1 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%
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

27. 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 =
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%