RecrystallizationImpure benzoic acidBenzoic acid after recrystallization

Benzoic Acid

Purification of Organic CompoundsSolids: RecrystallizationLiquids: DistillationSolids and Liquids: Gas or liquid chromotography

Types of impurities

SolubleInsolublePartly Soluble

Solubility of benzoic acid in water

Dissolve sample in hot solventCompound crystallizes upon cooling

Impure benzoic acid in hot water

Insoluble material is removed by filtrationAdd water and heat until all soluble material dissolves

The hot solution is filtered to remove insoluble impurities

Use fluted filter paper to maximize surface area.

Use GLASS funnel.To avoid crystallization in the 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 funnelFilter flaskHeavy-walledtubingVacuum..

Filter paperPut filter paper in Buchner funnel..

Filter, WashFilter and wash with cold solvent

SummaryInsoluble impuritiesSolubleimpuritiesPurifiedbenzoic acid

Solubility ExampleCompoundSolubility g/100 mL25o100oAI1I21.01.50.080500.0

ExampleCompoundSolubility g/100 mL25o100oAI1I21.01.50.080500.0A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g100 mL water at 1000AI1I2dissolvesdissolvesinsoluble

ExampleCompoundSolubility g/100 mL25o100oAI1I21.01.50.080500.0A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g100 mL water at 1000A dissolvesI1 dissolvesI2 insolubleCool to 25oAI19.0 g crystallize0 g crystallizeYield = 100 * 9.0/12.0 = 75%

ExampleCompoundSolubility g/100 mL25o100oAI1I21.01.50.080500.0A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g50 ml of water at 1000AI1I2dissolvesdissolvesinsoluble

ExampleCompoundSolubility g/100 mL25o100oAI1I21.01.50.080500.0A = 10 g I1 = 1.0 g I2 = 1.0 g Total = 12.0 g50 mL water at 1000A dissolvesI1 dissolvesI2 insolubleCool to 25oAI19.5 g crystallize0.25 g crystallizeYield = 100 * 9.75/12.0 = 81%NOT PURE!

ExampleWhat volume of solvent is needed to give the maximum yield of pure A?Solubility of I1 = 1.5 g/100 mL1.5 g100 mL=1.0 gx mLX = (100 mL * 1.0 g)/1.5 g = 67 mL

ExampleWhat is the yield of pure A from 67 mL solvent?Solubility of A = 1.0 g/100 mL1.0 g100 mL=X g67 mLX = (67 mL x 1.0 g) / 100 mL = 0.67 gYield = 10.0 g - 0.67 g = 9.33 g% yield = 100 * 9.33 / 12 = 78%

Second CropBoil off some solventCool, filter, wash, dry, weigh, mp

Fractional Crystallization

Recrystallization SolventsDoes not react with compoundLarge temperature coefficient of solutionLow boiling point

Mixed SolventsEthanol WaterEther Pentane