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