Recrystallization

Impure benzoicacid

Benzoic acid afterrecrystallization

C-OH

O

Benzoic Acid

m.p. = 121 - 122o

Purification of Organic Compounds

Solids: Recrystallization

Liquids: Distillation

Solids and Liquids:

Gas or liquid chromotography

Types of impuritiesSoluble

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 allsoluble material dissolves

Insoluble material is removed by filtration

The hot solution is filtered toremove insoluble impurities

Use fluted filter paper tomaximize surface area.

To avoidcrystallizationin the funnel:

Use GLASS funnel.Put flask on hot plate.

Boil off excess solvent

Boiling Sticks

Hot solutionafter filtration:

Cool Seed Scratch

Crystals form in the warm solution

Next:

Cool, filter, wash, dry, weigh, mp

Buchner funnel..

Filter flask

Heavy-walledtubing

Vacuum

Put filter paper in Buchner funnel..

Filter and wash with cold solvent

Insoluble impurities

Solubleimpurities

Purifiedbenzoic acid

Solubility Example

Compound Solubility g/100 mL

25o 100o

A

I1

I2

1.0

1.5

0.0

80

50

0.0

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

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%

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

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!

What volume of solvent is needed togive the maximum yield of pure A?

Solubility of I1 = 1.5 g/100 mL

1.5 g100 mL = 1.0 g

x mL

X = (100 mL * 1.0 g)/1.5 g = 67 mL

What is the yield of pure A from 67 mLsolvent?

Solubility of A = 1.0 g/100 mL

1.0 g100 mL = X g

67 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%