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Recrystallization / Filtration

Recrystallization / Filtration. Recrystallization – Purification of an organic compound by dissolving a solid in a hot solvent and recrystallizing the compound by slow cooling Vacuum Filtration - Separation of the solid solute from a liquid solvent References:

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Recrystallization / Filtration

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  1. Recrystallization / Filtration • Recrystallization – Purification of an organic compound by dissolving a solid in a hot solvent and recrystallizing the compound by slow cooling • Vacuum Filtration - Separation of the solid solute from a liquid solvent • References: • Slayden, et. al., – pp. 29 - 31 • Pavia Tech 11 – pp. 662 - 680 • Pavia Tech 6.2-6.3 – pp. 598 - 608 • Pavia Tech 8 – pp. 630 - 642 (Filtration) • Web Site: http//: classweb.gmu.edu/jschorni/

  2. Recrystallization / Filtration • Elements of the Experiment • Recrystallization • Solid compounds synthesized in the Organic laboratory usually need to be purified before final confirmation tests are performed • One of the most commonly used techniques is to purify a sample by dissolving it in a suitable solvent and recrystallizing the pure compound from the saturated solution by slow cooling • Vacuum Filtration • The purified crystals are separated from the solvent by Vacuum Filtration

  3. Recrystallization / Filtration • Background • Recrystallization • Solid organic compounds produced in the laboratory usually need to be purified • The most common technique involves Recrystallizing the sample from an appropriate Solvent • The Recrystallization process is a relatively slow and selective formation of crystals from a solvent • Precipitation is a rapid and nonselective process; thus not used to purify samples

  4. Recrystallization / Filtration • Recrystallization (Con’t) • The Recrystallization Process • Dissolve sample in a minimal amount of an appropriate solvent • Sample should be insoluble in solvent at room temperature, but soluble at elevated (near boiling point) temperature • If solution is colorized, it is sometimes necessary to add a decolorizing agent, such as activated charcoal (Norite) • Colorized solutions are first filtered through a fluted filter or a column containing alumina or silica gel Note: Unless otherwise instructed, decolorization will not be done in this experiment

  5. Recrystallization / Filtration • Recrystallization (Con’t) • The Recrystallization Process (Con’t) • The hot solution is cooled slowly to allow the purified crystals to form; leaving the more soluble impurities in solution • After solution and purified crystals have returned to room temperature, place the beaker in water/ice bath • Collect crystals by Vacuum Filtration • Rinse the crystals with small portions of cold solvent • Air-Dry the crystals for a week on a pre-weighed watchglass or weighing tray in your drawer or the instructor’s drawer • Determine the Melting Point Range of the purified sample

  6. Recrystallization / Filtration • Recrystallization (Con’t) • The Appropriate Solvent • The solute particles are generally insoluble in cold solvent, but soluble in hot solvent • The solvent (or mixed solvent) should have a steep solubility vs temperature curve for the solute B C Solubility A Temperature The “C” curve is a good solvent • Solute is sparingly soluble at room temperature • Solute is very soluble at elevated temperature

  7. Recrystallization / Filtration • Recrystallization (Con’t) • The Appropriate Solvent (Con’t) • Solubility of organic compounds is a function of the polarities of both the solvent and the solute: • “Like Dissolves Like” • Polar solvents dissolve polar solutes • Nonpolar solvents dissolve nonpolar solutes • The stability of the solute crystal lattice affects the solubility. The higher the melting point (higher stability), the less soluble the solute

  8. Recrystallization / Filtration • Recrystallization (Con’t) • The Appropriate Solvent (Con’t) • The Boiling Point of the solvent must be less than the Melting Point of the solute • If the Boiling Point of the solvent is higher than the melting point of the solute, the solute will “Melt” instead of “Dissolving” in the solvent at the elevated temperature • Upon cooling, the “Melted” solute will “Oil” out forming an insoluble mass that is not purified • The solvent should not react with the solute • Compounds with functional groups that can form hydrogen bonds (-OH, -NH-, -COOH, -CONH-) will be more soluble in hydroxylic (polar) solvents such as Methanol and Water

  9. Recrystallization / Filtration • Solvents in Decreasing Order of Polarity • H2O - Water • RCOOH - Organic Acids (Acetic Acid) • RCONH2 - Amides (N,M-dimethylformamide) • ROH - Alcohols (Methanol, Ethanol) • RNH2 - Amines (Triethylamine, Pyridine) • RCOR - Aldehydes, Ketones (Acetone) • RCOOR - Esters (Ethyl Acetate) • RX - Halides (CH2Cl2 > CHCl3 > CCl4) • ROR - Ethers (Diethylether) • ArH - Aromatics (Benzene, Toluene) • RH - Alkanes (Hexane, Petroleum Ether)

  10. Recrystallization / Filtration • Filtration • Two Purposes • Separate Purified Solid from the Soluble Impurities in the Solution from which it was Recrystallized • Remove Solid Impurities from a Liquid • Two Types • Gravity • Vacuum • Filter Paper • Retentivity – Opposite of Porosity; measure of the size of particles that can be retained on the filter paper • Porosity – Measure of the size of the particles than can pass through the paper

  11. Recrystallization / Filtration • Gravity Filtration Types • Filter Cones – Folded paper filter inserted into a class funnel with stem extending into a receiving flask Applicable Volume – > 10 mL. • Fluted Filters – Specially folded (many creases) filter paper inserted into a glass funnel with stem extending into a receiving flask. Applicable Volume – > 10 mL

  12. Recrystallization / Filtration • Gravity Filtration Types (Con’t) • Filtering Pipettes – Microscale technique used with Pasteur Pipets A piece of cotton is inserted into the top of the lower constriction Applicable Volume – < 10ml

  13. Recrystallization / Filtration • Vacuum Filtration – More rapid than gravity filtration • Buchner Funnels – Primarily used to filter large volumes of liquid from solids, such as crystals from the Recrystallization Process Applicable Volume – > 10 mL • Hirsch Funnels – Similar, but smaller than Buchner Funnel, with sloping sides Used in Microscale techniques Applicable Volume – < 10 mL

  14. Recrystallization / Filtration • The Experiment – Two Unknown Compounds • Purpose - Purify and isolate unknowns from impurities - Identify unknowns using melting points • List of unknown compounds is found on page 32 of the Slayden lab manual • Use literature resources to find Melting Points of compounds • Use an appropriate citation resource to find the melting points of all compounds in the list • Techniques - Recrystallization / Vacuum Filtration • Equipment - 2 100 mL (or 150 mL) Beakers, 250 mL Beaker - Hot Plate - Buchner Funnel w/filter paper - Filter Flask with Tubing - Mel-Temp Melting Point Apparatus • Reagents - Distilled Water for Water soluble compound - Methanol for Methanol soluble compound

  15. Recrystallization / Filtration • Elements of the Experiment • Mass of Reagents • Theoretical Volume of Solvent (Water & Methanol) Required to Dissolve Samples at Boiling Point • Mix solute & solvent in 100 mL or 150 mL beaker • Heat to “near” boiling point until sample dissolves • Recrystallize sample from solvent by slow cooling • Separate recrystallized particles from solution by Vacuum Filtration, washing with cold solvent • Air-Dry Samples on pre-weighed watchglass or weighing tray • Determine mass of dried sample • Compute Percent Recovery • Determine Melting Point Range

  16. Recrystallization / Filtration • Procedure • Obtain a vial for each of two unknown compounds: • Water soluble compound • Methanol soluble compound • Weigh each vial to the nearest 0.001 g • Empty the contents of the vial into a 100 or 150 mL beaker • Reweigh the empty vial to the nearest 0.001 g • Use the masses of the filled and empty vial to compute the mass of the compound

  17. Recrystallization / Filtration • Procedure (Con’t) • Using the mass of the compound you are going to recrystallize and the data in the table on page 32 of the Slayden manual, calculate the minimal volume of solvent required to dissolve your weighed sample at the boiling point of the solvent • Note: For each solvent use the compound with the lowest solubility per 100 mL of solvent at the boiling point. Ex. Let X be the solubility of the Water Soluble compound in 100 mL of Water at boiling point (100oC) Let Y be the mass of your weighed sample Let Z be the volume of solvent needed for your sample

  18. Recrystallization / Filtration • Procedure (Con’t) • Water Soluble Sample • Water boils at 100oC • Add the computed amount of solvent to the beaker containing your water-soluble sample • Heat the solution on a hot plate to just below the boiling point • Do not use a Thermometer or stirring rod, just swirl the solution • Heat solution only as long as it takes to dissolve the sample • If the sample does not completely dissolve after heating for several minutes, add additional distilled water in 2-3 mL increments and continue heating

  19. Recrystallization / Filtration • Procedure (Con’t) • Methanol Soluble Sample • Add the computed amount of solvent to the beaker containing your Methanol soluble sample • Methanol boils at 65oC • Place beaker containing sample and solvent in a 250 mL beaker containing about 1 inch of water (or just enough to allow the small beaker to sit on the bottom of the large beaker • Heat the water until the Methanol just begins to boil; then reduce the heat to keep water temperature around 65oC • Do not use a Thermometer or stirring rod, just swirl the solution

  20. Recrystallization / Filtration • Procedure (Con’t) • Heat solution only as long as it takes to dissolve the sample • If the sample does not completely dissolve after heating for several minutes, add additional Methanol in 2-3 mL increments and continue heating Note: Be patient; swirl the solution frequently and give the solid time to dissolve after each addition of solvent Note: If the flask appears to contain insoluble impurities, check with instructor, who will guide you on the following step Decant solution into new beaker leaving insoluble particles in original beaker

  21. Recrystallization / Filtration • Procedure (Con’t) • Remove the sample beaker from the hot plate or water bath beaker and place it on the bench top • Allow the solution to cool very slowly to room temperature • Crystals of the purified solute should start to come out of solution in a few minutes • When the mixture of crystals and liquid has reached room temperature, observe the relative amounts of solvent and solid particles • If there is insufficient crystal/solvent mixture to allow the mixture to be swirled, add an additional 2-3 mL of solvent and redissolve the solid particles; and cool again to room temperature slowly • Repeat previous step, if necessary

  22. Recrystallization / Filtration • Procedure (Con’t) • Place the beaker in a cool water bath for 1 minute • Set up Buchner Funnel and Filter Flask for Vacuum Filtration (Instructor will demonstrate) • Connect the rubber tubing to the vacuum flask and the vacuum apparatus on you lab bench • Turn on the water flow • Place a paper filter in the funnel top and moisten the filter paper with cold solvent to seal filter on bottom of funnel • Pour the crystal/solution slurry into the funnel • Rinse beaker with cold solvent to transfer all solid particles to funnel

  23. Recrystallization / Filtration • Procedure (Con’t) • Allow the filtration to continue until no more drops of solution pass into the filter flask • Wash crystals twice with 2-3 mL cold solvent • Transfer the crystals to the pre-weighed watch-glass or weighing tray • Place sample in your drawer or instructor’s drawer to dry for a week • Determine the net weight of the dried sample • Compute the percent (%) recovery of purified sample • Determine the Melting Point range of the sample • Identify the unknown compound by its melting point Note: Use literature resources to find the melting points of the compounds in the table on page 26 of the Slayden Manual

  24. Recrystallization / Filtration • The Report • From the “Elements of the Experiment” and the detailed steps, identify and setup the “Procedures” for this experiment. A procedure is a logical collection of steps leading to a new result • If a procedure involves a calculation, the equation must be setup as part of the procedure description and the computation of the results must be presented in the “Results” section including appropriate units and precision • Create neat, readable templates for presenting the results of each procedure. Use aligned columns and tables where applicable • The Summary section is a paragraph presentation of the results obtained in logical order

  25. Recrystallization / Filtration • The “Analysis/Conclusion” section should comment on the following using applicable arguments and selected results: • The amount of sample recovered relative to the impact of the procedure on potential loss of sample • Verification of the purity of the recovered sample in terms of appearance and results of the melting point determination

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