Exercise F2 Recrystallization Exercise F2 Recrystallization and Vacuum Filtrationand Vacuum Filtration

Organic Chemistry Lab IOrganic Chemistry Lab IFall 2009Fall 2009

Dr. MilkevitchDr. MilkevitchSeptember 21 & 23, 2009September 21 & 23, 2009

Steps in a Chemical SynthesisSteps in a Chemical Synthesis

Determine target compoundDetermine target compoundDesign synthesis (research reactions)Design synthesis (research reactions)Obtain starting materials & equipmentObtain starting materials & equipmentAssemble apparatus, begin synthesisAssemble apparatus, begin synthesisMonitor & complete synthesisMonitor & complete synthesisExtract and isolate productExtract and isolate productPurify productPurify productIdentify product (spectroscopic methods, physical Identify product (spectroscopic methods, physical properties)properties)Assess success of synthesisAssess success of synthesisModify synthesis, continue work if necessaryModify synthesis, continue work if necessary

Most Important Steps??Most Important Steps??

Determine target compoundDetermine target compoundDesign synthesis (research reactions)Design synthesis (research reactions)Obtain starting materials & equipmentObtain starting materials & equipmentAssemble apparatus, begin synthesisAssemble apparatus, begin synthesisMonitor & complete synthesisMonitor & complete synthesisExtract and isolate productExtract and isolate productPurify productPurify productIdentify product (spectroscopic methods, physical Identify product (spectroscopic methods, physical properties)properties)Assess success of synthesisAssess success of synthesisModify synthesis, continue work if necessaryModify synthesis, continue work if necessary

Introduction to RecrystallizationIntroduction to RecrystallizationExercise: RecrystallizationExercise: Recrystallization

Recrystallization: Recrystallization: “ “A purification A purification method in which a desired product is method in which a desired product is separated from impurities through separated from impurities through differential solubility in a hot solvent.”differential solubility in a hot solvent.”– Reaction Result: impurities may become trapped inReaction Result: impurities may become trapped in

crystal lattice or adsorbed onto crystal latticecrystal lattice or adsorbed onto crystal lattice

Can remove adsorbed impurities by washingCan remove adsorbed impurities by washing

How do you remove trapped impurities?How do you remove trapped impurities?Recrystallization!Recrystallization!

TheoryTheory

Organic solids usually more soluble in hot solvent Organic solids usually more soluble in hot solvent – Than a comparable volume of cold solventThan a comparable volume of cold solvent

Make a saturated solution by adding just enough hot Make a saturated solution by adding just enough hot solventsolvent– To a given amount of impure solid to To a given amount of impure solid to just just dissolve itdissolve it

When solution cools, solubility of solid decreasesWhen solution cools, solubility of solid decreases

Solid crystallizes (“crashes out”) of solutionSolid crystallizes (“crashes out”) of solution

Impurities are left behind in solventImpurities are left behind in solvent

Requirements for Recrystallization Requirements for Recrystallization SolventsSolvents

Dissolve all of the compound at the boiling point of the Dissolve all of the compound at the boiling point of the solventsolventDissolve very little or none of the compound when the Dissolve very little or none of the compound when the solvent is at room temperature (RT)solvent is at room temperature (RT)Have different solubilities for the compound and the Have different solubilities for the compound and the impuritiesimpuritiesHave a boiling point below the melting point of the Have a boiling point below the melting point of the compoundcompound

Compound dissolves, not melts into the solventCompound dissolves, not melts into the solvent

Relatively low boiling pointRelatively low boiling pointBe non-reactive with the compound, nontoxic, low odorBe non-reactive with the compound, nontoxic, low odorCheapCheap

Which Solvent to Choose?Which Solvent to Choose?

Compound should dissolve in a reasonable amount of hot Compound should dissolve in a reasonable amount of hot solvent, but not in cold solventsolvent, but not in cold solvent

““Like dissolves like”Like dissolves like”Polar in polar solventsPolar in polar solventsNon-polar in non-polar solventsNon-polar in non-polar solvents

Polarity determined by dipole moment Polarity determined by dipole moment

Polarity and solubility of an organic compoundPolarity and solubility of an organic compoundMolecular weight and proportion of hydrocarbon in moleculeMolecular weight and proportion of hydrocarbon in molecule

– Higher molecular weight less soluble than lower molecular weightHigher molecular weight less soluble than lower molecular weight– Higher proportion hydrocarbon, less polar Higher proportion hydrocarbon, less polar

Try a solvent and see how it doesTry a solvent and see how it does– Choose an alternate one if it doesn’t workChoose an alternate one if it doesn’t work

Tips on Performing a Tips on Performing a RecrystallizationRecrystallization

Dissolve compound in minimum amount of Dissolve compound in minimum amount of hot solventhot solventAfter compound dissolves, leave After compound dissolves, leave undisturbed, followed by placing in iceundisturbed, followed by placing in ice– Cool too quick, crystals too small and they will Cool too quick, crystals too small and they will

adsorb impuritiesadsorb impurities– Crystals too large, impurities get trapped insideCrystals too large, impurities get trapped inside

No crystals form?No crystals form?– Scratch inside of flask with glass rodScratch inside of flask with glass rod– Add a seed crystalAdd a seed crystal– Evaporate off some solvent in the hoodEvaporate off some solvent in the hood

Measuring Success of Measuring Success of RecrystallizationRecrystallization

Check purity of crystalsCheck purity of crystalsPerform a melting point determinationPerform a melting point determination

Determine % recoveryDetermine % recovery Mass of pure crystalsMass of pure crystals

Mass of impure crystals

X =100 % Recovery

Performing the RecrystallizationPerforming the Recrystallization

1. Weigh out 500 mg of impure compound, place in 50 ml Erlenmeyer• Add a stir bar

2. Set up hot plate. • Place 50 ml of ddH2O in a 100 ml erlenmeyer• Add a stir bar• Bring to boil

3. Add small amounts of boiling waterto the impure compound

4. Place erlenmeyer + compound back on hot plate to keep hot

5. Continue to add hot ddH2O until compound dissolves completely

6. Remove from heat, place on bench top to cool to RT.

7. After the erlenmeyer is at RT, place in ice bath for full recrystallization

ice

8. After about 10 min, remove from ice and vacuum filter

Vacuum FiltrationVacuum Filtration

1. Set up apparatus

Sidearm flask

Buchner funnelventuri

2. Turn on water Pour solution + crystals into Buchner funnel

Filter paper

venturi

venturi

3. Continue to filter until all your crystals are in the funnel•Continue to draw air through crystals until they are dry

4. Remove crystals, weigh them and record weight

Final NotesFinal Notes

Let solid dry in Buchner FunnelLet solid dry in Buchner Funnel– Place in crystallizing dish, place in hoodPlace in crystallizing dish, place in hood

to dryto dry

Weigh, calculate % recoveryWeigh, calculate % recovery– Next weekNext week

For your report, complete:For your report, complete:– Questions on the following pagesQuestions on the following pages– Place in your results sectionPlace in your results section

QuestionsQuestions

1) List the more important criteria for selecting a recrystallization solvent1) List the more important criteria for selecting a recrystallization solvent

2) Why should a recrystallization solvent have a fairly low boiling point?2) Why should a recrystallization solvent have a fairly low boiling point?

3) What problems may occur if a recrystallization occurs too rapidly?3) What problems may occur if a recrystallization occurs too rapidly?

4) Why should the boiling point of the solvent be lower than the melting pint 4) Why should the boiling point of the solvent be lower than the melting pint of the compound being recrystallized?of the compound being recrystallized?

5) Sometimes crystallization does not occur even after cooling the solution in 5) Sometimes crystallization does not occur even after cooling the solution in an ice bath. Describe the probably cause and explain how you could induce an ice bath. Describe the probably cause and explain how you could induce crystallization.crystallization.

6) During a suction filtration, what are the consequences of using a filter 6) During a suction filtration, what are the consequences of using a filter paper that is larger than the funnel diameter and one that is too small? What paper that is larger than the funnel diameter and one that is too small? What are the consequences for the use of each?are the consequences for the use of each?

7) A compound has a solubility in ethanol of 4.24 g / 100 ml at 78°C, and 7) A compound has a solubility in ethanol of 4.24 g / 100 ml at 78°C, and 0.86 g /100 ml at O°C. 0.86 g /100 ml at O°C.

– A) What volume of hot ethanol will be necessary to dissolve 50 mg of the compound (assume A) What volume of hot ethanol will be necessary to dissolve 50 mg of the compound (assume no impurities) ?no impurities) ?

– How much compound will remain dissolved in the solvent after recrystallization is complete?How much compound will remain dissolved in the solvent after recrystallization is complete?