1. 1/17/2015 1 Recrystallization / Filtration Purification and Isolation of an Impure Compound by Recrystallization and Vacuum Filtration  References: Slayden, et. al.,– pp. 31 – 34 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//:mason.gmu.edu/~jschorni/

2. 1/17/2015 2 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 Recrystallization / Filtration

3. 1/17/2015 3 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 Recrystallization / Filtration 11/22/2016

4. 1/17/2015 4 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 the 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 Recrystallization / Filtration

5. 1/17/2015 5 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 Recrystallization / Filtration The “C” curve is a good solvent  Solute is sparingly soluble at room temperature  Solute is very soluble at elevated temperature A B C Temperature Solubility

6. 1/17/2015 6 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 Recrystallization / Filtration

7. 1/17/2015 7 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 Recrystallization / Filtration

8. 1/17/2015 8 Solvents in Decreasing Order of Polarity  H 2 O- Water  RCOOH- Organic Acids (Acetic Acid)  RCONH 2 - Amides (N,M-dimethylformamide)  ROH- Alcohols (Methanol, Ethanol)  RNH 2 - Amines (Triethylamine, Pyridine)  RCOR- Aldehydes, Ketones (Acetone)  RCOOR- Esters (Ethyl Acetate)  RX- Halides (CH 2 Cl 2 > CHCl 3 > CCl 4 )  ROR- Ethers (Diethylether)  ArH- Aromatics (Benzene, Toluene)  RH- Alkanes (Hexane, Petroleum Ether) Recrystallization / Filtration

9. 1/17/2015 9 Filtration  Two Purposes  Separate purified solid from the soluble impurities in the solution from which it was recrystallized  Remove solid impurities remaining after recrystallizing a sample from a solvent  Two Types  Gravity  Vacuum  Filter Paper  Porosity–Measure of the size of the particles than can pass through the paper  Retentivity–Opposite of Porosity; measure of the size of particles that can be retained on the filter paper Recrystallization / Filtration

10. 1/17/2015 10 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 Recrystallization / Filtration

11. 1/17/2015 11 Gravity Filtration Types (Con’t)  Filtering Pipettes–Microscale technique used Pasteur Pipets A piece of cotton is inserted into the top of the lower constriction Applicable Volume – < 10ml Recrystallization / Filtration

12. 1/17/2015 12 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 Recrystallization / Filtration

13. 1/17/2015 13 Experimental Overview  This experiment will consist of 4 parts:  Part A Determination of an appropriate solvent for recrystallizing an impure sample of Fluorene  Part B Recrystallization of Fluorene from the applicable solvent  Part C Isolation of purified Fluorene by vacuum filtration  Part D Verification of compound identity by melting point Recrystallization / Filtration

14. 1/17/2015 14 Part A Determine the appropriate solvent for recrystallizing an impure sample of Fluorene  Notes:  Selecting a good solvent for recrystallization requires development of a practiced and consistent technique  There is no perfect procedure that is applicable to all compounds  Make sure the boiling point of the solvent is less than the melting point of the sample compound  The solvents to be evaluated include: Water, Methanol, and Petroleum Ether (a mixture of low- boiling alkanes, 30-60 o C) Recrystallization / Filtration

15. 1/17/2015 15 Recrystallization / Filtration Procedure – Part A (con’t)  Set up two hot water baths: 65 o C 95 o C  Place three (3) small test tubes in a test tube rack  Place a label with a solvent name on each test tube  To each test tube add about 50 mg (0.05 g) of Fluorene  To the test tube with the Petroleum Ether label, add 0.5 ml (10 drops) of Petroleum Ether  Stir the mixture with a microspatula  If all (or almost all) of the Fluorene solute dissolves at room temperature, then the solvent is probably not appropriate for recrystallizing Fluorene

16. 1/17/2015 16 Recrystallization / Filtration Procedure – Part A (con’t)  To the second test tube add 0.5 ml (10 drops) of Methanol  Repeat the above process observing the results at room temperature  If the crystals dissolve in the solvent at room temperature, Methanol is probably not a suitable solvent for recrystallizing Fluorene  To the third test tube add 0.5 ml (10 drops) of distilled water  Repeat the above process observing the results at room temperature

17. 1/17/2015 17 Recrystallization / Filtration Procedure – Part A (con’t)  If the solute in the test tube did not go into solution at room temperature, insert the test tube containing the solute and Methanol in the 65 o C water bath  Continuously insert and remove the test tube in a manner that gently heats the mixture  If the solution begins to boil; immediately remove the test tube from the water; it is essential to minimize the evaporation of the solvent Note:Be careful, Petroleum Ether (BP 42-65 o C) and Methanol (BP 65 o C) reach the boiling point quickly  If the solid does not dissolve, add more solvent dropwise, up to a maximum of 2.0 ml

18. 1/17/2015 18 Recrystallization / Filtration Procedure – Part A (con’t)  Reheat the solution to 65 o C and repeat the process above  If the solute does not go into solution after the addition of the additional solvent, then Methanol is probably not a suitable solvent for recrystallizing Fluorene  If the solute dissolves in the solvent at 65 o C, allow the mixture to cool in the test rack for a few minutes and then place the test tube in an ice/water bath for about a minute  If crystals do not come of solution, try evaporating some of the solvent by reheating the solution in the 65 o C water bath  If the crystals still do not come out of solution, then Methanol is probably not suitable for recrystallizing Fluorene

19. 1/17/2015 19 Recrystallization / Filtration Procedure – Part A (con’t)  If neither Petroleum nor Methanol is determined to be suitable for recrystallizing Fluorene, insert the 3 rd test containing the solute and water in the 100 o C water bath  Repeat the same process as used in testing the solubility of Fluorene in Methanol  Report your results to the instructor, who will then ask you to repeat parts of the experiment if you have selected the incorrect solvent.  If you have selected the correct solvent, proceed to the following steps

20. 1/17/2015 20 Procedure – Part B  Recrystallize a sample of Fluorene from the appropriate solvent Tare weigh a weighing tray Obtain about 1.0 g of impure Fluorene weighed to the nearest 0.001 g Empty the contents of the weighing tray into a clean, dry 100 ml Erlenmeyer Flask

21. 1/17/2015 21 Procedure – Part B (con’t)  Recrystallize Fluorene from Solvent (con’t)  Assemble the recrystallization apparatus as show in the figure below  Use the setup on the left when using water as the solvent  Use the setup on the right when using low-boiling point solvents. Use only enough water in the beaker to allow the flask to sit on the bottom of the beaker Recrystallization / Filtration

22. 1/17/2015 22 Recrystallization / Filtration Procedure – Part B (con’t)  Recrystallize Fluorene from Solvent (con’t) Add 25 mL of solvent to the flask Place the flask containing the sample solution on the hot plate if using water as the solvent or in the 65 o C water bath if using Methanol or Petroleum Ether as the solvent Stir gently (Do not use a Thermometer or stirring rod, just swirl the solution gently) Heat solution only as long as it takes to dissolve the sample

23. 1/17/2015 23 Procedure – Part B (con’t)  Recrystallize Fluorene from Solvent (con’t)  If the sample does not completely dissolve after heating for a minute or so, add additional solvent (0.5 – 1.0 ml) and continue heating  Repeat this process until sample has completely dissolved Note: Do not add too much or too little solvent ● Allow the mixture to cool slowly to room temperature Note: the purified crystals will come out of solution as the solubility decreases with decreasing temperature.  Place the cooled mixture in an ice/water bath for a minute or two Recrystallization / Filtration

24. 1/17/2015 24 Recrystallization / Filtration Procedure – Part B (con’t)  Recrystallize Fluorene from Solvent (con’t) If the cooled recrystallized mass can not be swirled, there is insufficient amount of liquid solvent to produce a filterable slurry Repeat the process using 30 ml of solvent Procedure to vacuum filtration If little or no crystals come out of solution, it will be necessary to evaporate some the solvent to obtain a saturated solution at elevated temperature Once a suitable mixture of purified crystals and solvent have been obtained, the crystals will be separated from the solvent using vacuum filtration

25. 1/17/2015 25 Procedure – Part C  Vacuum Filtration  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 the lab bench Recrystallization / Filtration

26. 1/17/2015 26 Procedure – Part C (con’t)  Vacuum Filtration (con’t)  Turn on the water flow  Place a paper filter in the Buckner Funnel top and moisten the filter paper with cold solvent to seat filter on bottom of funnel  Pour the crystal/solution slurry into the funnel  Rinse the flask with cold solvent to transfer all solid particles to funnel  Allow the filtration to continue until no more drops of solution pass into the filter flask  Wash crystals 3 times with 5 mL cold solvent Recrystallization / Filtration

27. 1/17/2015 27 Procedure – Part C (con’t)  Vacuum Filtration (con’t)  Dry the Sample Transfer the crystals to a pre-weighed labelled weighing tray or vial (if supplied) Place the sample in the instructor’s drawer and allow to dry for a week  Determine the Mass of the purified sample  Determine the % recovery  Determine the Melting Point range of the sample  In the conclusion section of report verify the compound by its melting point Recrystallization / Filtration

28. 1/17/2015 28 The Report  From the procedure description determine the number of logical procedures  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 Note: The computation for each computed result must be shown Recrystallization / Filtration

29. 1/17/2015 29 Report Notes (con’t)  The Summary section is a paragraph presentation of the results obtained in logical order  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 comparison of the melting point with the literature value  Discuss the reasons for any difference in the melting points between the pure and impure samples Recrystallization / Filtration