Recrystallization Impure benzoic acid Benzoic acid after recrystallization

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.

Use GLASS funnel. Put flask on hot plate. 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

Cool, filter, wash, dry, weigh, mp Crystals form in the warm solution Next: Cool, filter, wash, dry, weigh, mp

.. Buchner funnel Heavy-walled tubing Filter flask Vacuum

Put filter paper in Buchner funnel .. Put filter paper in Buchner funnel

Filter and wash with cold solvent

Purified benzoic acid Insoluble impurities Soluble impurities

Solubility Example Solubility g/100 mL Compound 25o 100o A I1 I2 1.0 1.5 0.0 80 50 0.0

A I1 I2 dissolves dissolves 100 mL water at 1000 insoluble Solubility g/100 mL Compound 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

Cool to 25o A dissolves I1 dissolves I2 insoluble A I1 Solubility g/100 mL Compound 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 Cool to 25o A dissolves I1 dissolves I2 insoluble A I1 9.0 g crystallize 0 g crystallize Yield = 100 * 9.0/12.0 = 75%

50 ml of water at 1000 A I1 I2 dissolves dissolves insoluble Solubility g/100 mL Compound 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

NOT PURE! Cool to 25o 50 mL water at 1000 A dissolves I1 dissolves Solubility g/100 mL Compound 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 Cool to 25o 50 mL water at 1000 A dissolves I1 dissolves I2 insoluble 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 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

What is the yield of pure A from 67 mL solvent? Solubility of A = 1.0 g/100 mL 1.0 g 100 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%