Today's Experiment - Recrystallization

1

• Today's Experiment - Recrystallization
• References
• Slayden, et. al., p.19-20, p. 61 (Data Report)
• Pavia, et. al., p. 612 p. 634-664
• Due Today
• Pre-lab for Crystallization Experiment
• Problems on page 648 in Pavia
• Finish Melting Point Measurements
• Homework Assignment
• Obtain blank separation scheme diagram from Dr.
  Slaydens website. http//classweb.gmu.edu/chemlab
  /318/chem318.htm
• Prepare separation scheme for todays experiment

2

• Recrystallization
• Solid organic compounds produced in the
  laboratory usually need to be purified.
• The most common technique involves
  recrystallizing the sample.
• 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.

3

• Recrystallization (Cont)
• General Process
• Dissolve sample in a minimal amount of an
  appropriate solvent.
• Sample should be insoluble in solvent at room
  temperature, but soluble at elevated (boiling
  point) temperature.
• If solution is colorized, it is sometimes
  necessary to add a decolorizing agent (activated
  charcoal - Norite)
• Colorized solutions are first filtered through a
  fluted filter or a column containing alumina or
  silica gel.

4

• Recrystallization (Cont)
• General Process (Cont)
• The hot solution is cooled slowly to room
  temperature.As temperature changes the solute
  particles begin to come out of solution, leaving
  the more soluble impurities in solution.
• After crystallization, place beaker in ice-water
  bath.
• Collect crystals by vacuum filtration
• Rinse crystals with small portion of cold solvent
• Dry crystals in air or place in drying oven.

5

• Recrystallization (Cont)
• The Appropriate Solvent
• The solute particles are generally insoluble in
  cold solvent, but soluble in hot solvent.
• 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
• 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.

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• 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)

7

• Filtration
• Two Purposes
• Remove Impurities from Solid
• Separate Solids from Solution
• 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.

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• Gravity Filtration Types
• Filter Cones - The volume of liquid to be
  filtered is about 10 mL or greater.
• Solid collected on the filter is saved.
• Fluted Filters - The volume of liquid to be
  filtered is greater than about 10 mL.
• Commonly used in recrystallization
  procedures to remove impurities from the
  solvent.
• Fluted filters improve speed of filtration.
• Use short stem (stemless) funnels and keep
  solution near boiling point to avoid clogging.

9

• Gravity Filtration Types (Cont)
• Filtering Pipettes - Microscale technique used
  with volumes less than about 10 mL to remove
  solid impurities from a liquid.
• Place cotton into top of Pasteur pipet and
  push to constriction.
• Prewash with solvent.

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• Vacuum Filtration - More rapid than gravity.
• Buchner Funnels - Primarily used to collect a
  desired solid from liquid when the volume is
  greater than about 10 mL. Used frequently
  to collect the crystals from crystallization.
• Hirsch Funnels - Used in same way as Buchner
  funnels except for volumes less than 10 ml.

11

• The Experiment - Purification of Biphenyl
• Purpose - Isolate purify a sample of Biphenyl
• Technique - Hot Vacuum Filtration /
  Recrystallization
• Equipment - Beakers
• - Elenmeyer Flask (125 mL)
• - Water Bath Hotplate
• - Stemless Funnel Fluted Filter Paper
• - Buchner Funnel w/filter paper
• - Vacuum Plumbing / Tubing
• - Ice Bath
• Reagent - Methanol
• Impurities - Methyl Orange, Sand, Methanol,
  Charcoal

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• Procedure Purification of Biphenyl
• Obtain vial containing impure Biphenyl from
  instructor.
• Return clean, empty vial from drawer to
  instructor.
• Weigh vial containing approximately 2.5 g
  sampleon a balance to nearest 0.001 g.
• Transfer sample to 125 mL Erlenmeyer flask.
• Weigh empty vial.
• Record net weight of sample in notebook.
• Add boiling chip to Erlenmeyer flask.
• Add 2 - 3 mL methanol.

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• Procedure Purification of Biphenyl (Cont)
• Place Erlenmeyer flask in water bath on a
  hotplate and gently heat to boiling.
• Note Do not heat flask directly on hot
  plate Obtain water bath bowls from cabinet
• If sample does not completely dissolve at the
  boiling point, add 2 - 3 mL increments of
  Methanol until sample dissolves up to a maximum
  of 13 mL.
• Note Sand will not dissolve. Swirl sample and
  allow time for sample to dissolve after each
  addition of Methanol. Methanol boils at 65
  oC Do Not Boil Away.

14

• Procedure Purification of Biphenyl (Cont)
• When all sample dissolves, remove from water bath
  and add 10 excess (1.5 mL) of methanol.
• Allow to cool for 1 minute.
• Bring solution to boiling again.
• Heat some additional solvent (2 - 3 mL) in
  separate beaker.
• Add about 0.5 grams of Norite (activated
  charcoal) to solution.

15

• Procedure Purification of Biphenyl (Cont)
• Set up stemless funnel with fluted filter paper.
• Fold filter paper as per instructions or obtain
  fluted filter from front desk
• Obtain stemless funnel from hood drawer
• Use ring stand and iron ring to hold funnel
• Filter hot solution into beaker.
• Rinse filter with 1-2 mL of hot solvent.
• Set beaker aside and allow cool to room
  temperature.
• Place beaker in ice bath.
• Recrystallize to separate soluble impurities.

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• Procedure Purification of Biphenyl (Cont)
• Use a water bath on a hotplate to gently heat to
  boiling point swirling gently.
• Note Methanol boils at 65oC - Do not boil
  away heat only as long as it takes to
  dissolve the sample. Methanol is highly
  flammable Be Careful
• If all the solid does not dissolve, add
  approximately 2 mL portions of methanol until it
  does dissolve.
• When solute has completely dissolved, add an
  additional 10 of the total added.
• Remove the flask from the hot plate.
• Allow the flask to cool very slowly to room
  temperature.Crystals of the purified biphenyl
  should start to come out of solution in a few
  minutes

17

• Procedure Purification of Biphenyl (Cont)
• When solution has reached room temperature place
  the flask in an ice bath for 10 minutes.
• Set up Buchner funnel for vacuum filtration
  (Demo).
• Moisten filter paper with cold Methanol.
• Filter cold solution collecting crystals on
  filter paper.
• Wash crystals twice with 2-3 mL cold Methanol.
• Transfer dried crystals to a pre-weighed watch
  glass.
• Store the watch glass in your drawer until next
  week.
• Next week determine net weight of sample.
• Determine yield.