Recrystallization
Impure benzoic
acid
Benzoic acid after
recrystallization
Benzoic Acid
O
C-OH
Benzoic Acid
m.p. = 121 - 122 o
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
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:
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 in ice bath
..
Buchner funnel
Heavy-walled
tubing
Filter flask
Vacuum
Filter paper
..
Put filter paper in Buchner funnel
Filter, Wash
Filter and wash with cold solvent
Summary
Insoluble
impurities
Soluble
impurities
Purified
benzoic acid
Solubility Example
Compound
Solubility g/100 mL
25o
100o
A
1.0
80
I1
1.5
50
I2
0.0
0.0
Solubility g/100 mL
Compound
A
25o
1.0
100o
80
I1
1.5
50
I2
0.0
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
Example
Solubility g/100 mL
Compound
A
25o
1.0
100o
80
I1
1.5
50
I2
0.0
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
A
9.0 g crystallize
I1 dissolves
I1
0 g crystallize
I2 insoluble
Example
Yield = 100 * 9.0/12.0 = 75%
Solubility g/100 mL
Compound
A
25o
1.0
100o
80
I1
1.5
50
I2
0.0
0.0
A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g
50 ml of water at 1000
A
dissolves
I1
dissolves
I2
insoluble
Example
Solubility g/100 mL
Compound
A
25o
1.0
100o
80
I1
1.5
50
I2
0.0
0.0
A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g
50 mL water at
Cool to 25o
1000
A dissolves
A
9.5 g crystallize
I1 dissolves
I1
0.25 g crystallize
I2 insoluble
Example
Yield = 100 * 9.75/12.0 = 81%
NOT PURE!
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
What is the yield of pure A from 67 mL
solvent?
Solubility of A = 1.0 g/100 mL
1.0 g
=
100 mL
Xg
67 mL
Example
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%
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