Recrystallization Impure benzoic acid Benzoic acid after recrystallization

Benzoic Acid

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

Insoluble material is removed by filtration 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.

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

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 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

Example 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 Example

Example 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 Example Yield = 100 * 9.0/12.0 = 75%

Example 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 Example

Example NOT PURE! Cool to 25o 50 mL water at 1000 A 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% Example 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

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 X g 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