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