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Impure benzoic acid
Benzoic acid after recrystallization
Recrystallization
C-OH
O
Benzoic Acid
m.p. = 121 - 122o
Benzoic Acid
Solids: Recrystallization
Liquids: Distillation
Solids and Liquids:
Gas or liquid chromotography
Purification of Organic Compounds
Soluble
Insoluble
Partly Soluble
Types of impurities
Solubility of benzoic acid in water
Compound crystallizes upon cooling
Dissolve sample in hot solvent
Impure benzoic acid in hot water
Add water and heat until all soluble material dissolves
Insoluble material is removed by filtration
The hot solution is filtered to remove insoluble impurities
Use fluted filter paper to maximize surface area.
To avoid crystallization in the funnel:
Put flask on hot plate.
Use GLASS funnel.
Boil off excess solvent
Boiling Sticks
Cool Seed Scratch
Hot solution after filtration:
Next:
Cool, filter, wash, dry, weigh, mp
Crystals form in the warm solution
Cool in ice bath
Filter flask
Heavy-walledtubing
Vacuum
Buchner funnel..
Put filter paper in Buchner funnel..
Filter paper
Filter and wash with cold solvent
Filter, Wash
Insoluble impurities
Solubleimpurities
Purifiedbenzoic acid
Summary
Compound Solubility g/100 mL
25o 100o
A
I1
I2
1.0
1.5
0.0
80
50
0.0
Solubility Example
CompoundSolubility g/100 mL
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
CompoundSolubility g/100 mL
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 dissolves
I1 dissolves
I2 insoluble
Cool to 25o
A
I1
9.0 g crystallize
0 g crystallize
Yield = 100 * 9.0/12.0 = 75%Example
CompoundSolubility g/100 mL
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
CompoundSolubility g/100 mL
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 water at 1000
A dissolves
I1 dissolves
I2 insoluble
Cool to 25o
A
I1
9.5 g crystallize
0.25 g crystallize
Yield = 100 * 9.75/12.0 = 81%
NOT PURE!Example
What volume of solvent is needed to give the maximum yield of pure A?
Solubility of I1 = 1.5 g/100 mL
1.5 g100 mL
=1.0 gx mL
X = (100 mL * 1.0 g)/1.5 g = 67 mL
Example
What is the yield of pure A from 67 mL solvent?
Solubility of A = 1.0 g/100 mL
1.0 g100 mL
=X g67 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%
Example
Second Crop
Boil off some solvent
Cool, filter, wash, dry, weigh, mp
Fractional Crystallization
Recrystallization Solvents
• Does not react with compound
• Large temperature coefficient of solution
• Low boiling point
Mixed Solvents
• Ethanol – Water• Ether Pentane