Exercise F2 Recrystallization and Vacuum Filtration Organic Chemistry Lab I Fall 2009 Dr. Milkevitch September 21 & 23, 2009
Steps in a Chemical Synthesis • Determine target compound • Design synthesis (research reactions) • Obtain starting materials & equipment • Assemble apparatus, begin synthesis • Monitor & complete synthesis • Extract and isolate product • Purify product • Identify product (spectroscopic methods, physical properties) • Assess success of synthesis • Modify synthesis, continue work if necessary
Most Important Steps?? • Determine target compound • Design synthesis (research reactions) • Obtain starting materials & equipment • Assemble apparatus, begin synthesis • Monitor & complete synthesis • Extract and isolate product • Purify product • Identify product (spectroscopic methods, physical properties) • Assess success of synthesis • Modify synthesis, continue work if necessary
Introduction to RecrystallizationExercise: Recrystallization • Recrystallization: “A purification method in which a desired product is separated from impurities through differential solubility in a hot solvent.” • Reaction Result: impurities may become trapped in crystal lattice or adsorbed onto crystal lattice • Can remove adsorbed impurities by washing • How do you remove trapped impurities? • Recrystallization!
Theory • Organic solids usually more soluble in hot solvent • Than a comparable volume of cold solvent • Make a saturated solution by adding just enough hot solvent • To a given amount of impure solid to just dissolve it • When solution cools, solubility of solid decreases • Solid crystallizes (“crashes out”) of solution • Impurities are left behind in solvent
Requirements for Recrystallization Solvents • Dissolve all of the compound at the boiling point of the solvent • Dissolve very little or none of the compound when the solvent is at room temperature (RT) • Have different solubilities for the compound and the impurities • Have a boiling point below the melting point of the compound • Compound dissolves, not melts into the solvent • Relatively low boiling point • Be non-reactive with the compound, nontoxic, low odor • Cheap
Which Solvent to Choose? • Compound should dissolve in a reasonable amount of hot solvent, but not in cold solvent • “Like dissolves like” • Polar in polar solvents • Non-polar in non-polar solvents • Polarity determined by dipole moment • Polarity and solubility of an organic compound • Molecular weight and proportion of hydrocarbon in molecule • Higher molecular weight less soluble than lower molecular weight • Higher proportion hydrocarbon, less polar • Try a solvent and see how it does • Choose an alternate one if it doesn’t work
Tips on Performing a Recrystallization • Dissolve compound in minimum amount of hot solvent • After compound dissolves, leave undisturbed, followed by placing in ice • Cool too quick, crystals too small and they will adsorb impurities • Crystals too large, impurities get trapped inside • No crystals form? • Scratch inside of flask with glass rod • Add a seed crystal • Evaporate off some solvent in the hood
Measuring Success of Recrystallization • Check purity of crystals • Perform a melting point determination • Determine % recovery Mass of pure crystals X 100 % Recovery = Mass of impure crystals
Performing the Recrystallization • Weigh out 500 mg of impure • compound, place in 50 ml Erlenmeyer • Add a stir bar • 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 water • to 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
Filter paper Buchner funnel venturi venturi Sidearm flask Vacuum Filtration 1. Set up apparatus 2. Turn on water Pour solution + crystals into Buchner funnel
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 Notes • Let solid dry in Buchner Funnel • Place in crystallizing dish, place in hood to dry • Weigh, calculate % recovery • Next week • For your report, complete: • Questions on the following pages • Place in your results section
Questions • 1) List the more important criteria for selecting a recrystallization solvent • 2) Why should a recrystallization solvent have a fairly low boiling point? • 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 of the compound being recrystallized? • 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 crystallization. • 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 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 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 no impurities) ? • How much compound will remain dissolved in the solvent after recrystallization is complete?