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Identification of an Unknown ALdose

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Title: Identification of an Unknown ALdose


1
Identification of an Unknown ALdose
  • CHEM 203

2
In this experiment, you will determine the
identity of an unknown aldose by
  • Bial test to differentiate aldopentose and
    aldohexose.
  • Measurement of the specific rotation of the
    aldose.
  • Reducing the aldose to its alditol and
    determining its optical activity.
  • Converting the aldose to its phenylosazone and
    determining its melting point.

3
Possible Aldoses
4
Part 1 Bial test
  • This test is used to differentiate aldopentoses
    and aldohexoses.
  • Aldopentoses
  • react very rapidly
  • give a blue-green color
  • Aldohexoses
  • react slower
  • result in reddish-brown solutions

5
Part 1 Bial Test
  • Bial Reagent is FeCl3, HCl and orcinol
  • Experimental Dissolve a small amount (about 5
    mg) of your unknown aldose in 1 mL of water in a
    test tube. Add 1 mL of Bials reagent and a
    boiling stone. Heat the test tube on a steam
    bath until it just boils. Remove from the heat
    and record the color.

6
Part 2 Optical Activity
  • Remember that the Fisher Projection represents
    chiral centers
  • Each aldopentose and aldohexose will have its own
    unique combination of /- optically active
    centers resulting in a net rotation

7
Part 2 Optical Activity
  • Using a polarimeter you will determine the aobs
    for your aldose
  • Remember from our discussion of optical activity
    that the magnitude of rotation is cell length and
    concentration dependent

8
Part 2 Optical Activity
  • Once you have your aobs from our polarimeter you
    will need to convert that value to a specific
    rotation or aD
  • aD aobs / (l x c) where l path length (1
    dm) c concentration in g/mL
  • You are using a 1 decimeter cell that holds 10
    mL

9
Part 2 Optical Activity
10
Part 3 Reduction to Alditol
  • By reducing the aldehyde with NaBH4 you produce a
    new compound with unique properties optical
    activity / melting point

11
Part 3 Reduction to Alditol
  • For some aldoses, the introduction of a plane of
    symmetry will cause loss of optical activity!

12
Part 3Reduction to Alditol
  • Experimental Dissolve 1.0 grams of the unknown
    aldose in 5 mL of water in a small flask by
    stirring. Dissolve 0.1 grams sodium borohydride
    in 2.0 mL of 1M NaOH in a test tube. Cool both
    solutions in an ice/water bath for 5 minutes or
    more.
  • Remove both solutions from the ice bath. Using a
    Pasteur pipette, add the sodium borohydride
    solution to the aldose solution drop-by-drop with
    constant stirring over a 10-minute period. Do
    not let the temperature rise over 25C, if it
    does cool the flask in an ice/water bath for a
    minute or so, then continue your addition. When
    the addition is complete, let the solution stir
    at room temperature for 20 minutes. Add 6M HCl
    to the beaker dropwise until the foaming stops
    and the solution turns blue litmus paper red.
  • Cool the reaction mixture in an ice/water bath to
    crystallize the product it may be necessary to
    scratch the side of the beaker to induce
    crystallization. For best results, put Para film
    over the top of the beaker and place it in the
    refrigerator until next lab period
  • Once crystallization is complete, collect the
    crystals by vacuum filtration wash the crystals
    with two portions of cold 95 ethanol. Dry the
    crystals as well as you can by pressing them
    between two pieces of filter paper. Prepare an
    aqueous solution of the alditol (0.5 grams in 10
    mL of water) and determine its optical activity.

13
Part 4 Preparation of phenylosazone
  • Derivatives of compounds offer another unique set
    of physical properties to further ascertain the
    identity of a compound
  • Aldehydes and ketones react with hydrazines to
    form hydrazones

14
Part 4 Preparation of phenylosazone
  • Careful both D-glucose and D-mannose give the
    SAME phenylosazone. When the phenylosazones are
    formed both C-1 and C-2 form new C-N double
    bonds. Therefore, the difference in
    configuration at C-2 in glucose and mannose is
    lost. The remaining structural unit, C-3 through
    C-6, (shown in the boxes above) is the same for
    both glucose and mannose

15
Part 4 Preparation of phenylosazone
  • Experimental Dissolve 0.5 grams of the unknown
    aldose in 10 mL of water in a 50 mL Erlenmeyer
    flask, stir in 1.0 grams of phenylhydrazine
    hydrochloride, 1.5 grams sodium acetate
    trihydrate and 1 mL of saturated aqueous sodium
    bisulfite. Heat the flask on a boiling water
    bath for 30 minutes with occasional swirling.
    Add 15 mL of water to the flask and cool in and
    ice/water bath. Collect the crystals by vacuum
    filtration, wash the crystals with a small amount
    if ice-cold methanol. Allow the crystals to dry
    in your drawer until next lab period. Once they
    are dry, obtain a weight and melting point for
    your crystals

16
Part 4 Preparation of phenylosazone
  • Of the 12 D-aldohexoses, there are only 6
    possible phenylosazones whose melting points are
    in the handout

17
Part 5 Putting it together
  • With experimental error or possibly a failed
    experiment, not all of your data may match
    perfectly
  • Keep good observations here is where the
    notebook is key so that if you are forced into a
    3 of 4 or worse yet 2 of 4 situation, you know
    which of your experiments you can rely on most!

18
Part 5 Putting it together
  • Your report should have the following elements
  • Introduction
  • Data from each test
  • Discussion discuss your logic in applying the
    data into solving the identity of the unknown
  • Conclusion
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