Recrystallization / Filtration

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 (Cont)
• 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 (Cont)
• The Recrystallization Process (Cont)
• 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
  instructors drawer
• Determine the Melting Point Range of the purified
  sample

6
Recrystallization / Filtration

• Recrystallization (Cont)
• 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

• 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 (Cont)
• The Appropriate Solvent (Cont)
• 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 (Cont)
• The Appropriate Solvent (Cont)
• 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 gt CHCl3 gt 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 gt 10 mL.
• Fluted Filters Specially folded (many creases)
  filter paper inserted into a glass funnel
  with stem extending into a receiving
  flask. Applicable Volume gt 10 mL

12
Recrystallization / Filtration

• Gravity Filtration Types (Cont)
• Filtering Pipettes Microscale technique used
  with Pasteur Pipets
• A piece of cotton is inserted into the top
  of the lower constriction Applicable
  Volume lt 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 gt 10 mL
• Hirsch Funnels Similar, but smaller than
  Buchner Funnel, with sloping sides
• Used in Microscale techniques
• Applicable Volume lt 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

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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 (Cont)
• 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 (Cont)
• 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 (Cont)
• 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 (Cont)
• 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 (Cont)
• 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 (Cont)
• 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 (Cont)
• 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 instructors
  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 25 of the Slayden Manual

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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

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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