Acetylsalicylic Acid

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Acetylsalicylic Acid

• Experiment 8

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Read Before Class

• Experiment 8
• Essay Analgesics
• Techniques 8, 9, 25 (Part B) (4th ed.)
• Techniques 4, 6 (Part A), and Appendix 3 (3rd ed.)

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Overview of Aspirin

• Prevents synthesis of Prostaglandins
• Prostaglandins are responsible for evoking
  several immune responses in the body such as
  pain, fever, local inflammation
• Aspirin attaches an acetyl group (2 carbons) to
  the active site of cyclo-oxygenase, the enzyme
  which begins the synthesis of prostaglandins from
  precursor metabolites

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Reaction Objective

• Youll prepare acetylsalicylic acid via an
  esterfication reaction
• The reaction employs H3PO4 as the required acid
  catalyst to form an ester functional group _at_ the
  hydroxyl group on the benzene ring of salicylic
  acid.

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Tests for Impurities

• FeCl3 TestPhenols (benzene w/ a hydroxyl
  group) such as salicylic acid form a highly
  colored complex with FeCl3 in the range from red
  to violet. Use this test to check for any
  remaining salicylic acid in your final product.
• Iodine Test for StarchA deep blue-black
  color upon addition of iodine is a positive test
  for starch, which is commonly used as a binding
  agent in commercial aspirin tablets.

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Procedural Notes

• A.    Preparing the Acetylsalicylic Acid
• 1.      Prepare 50oC H2O bath.
• 2.      Add these to 5mL conical vial in the
  following order
• I)     0.210g Salicylic Acid
• II)     0.480mL Acetic Anhydride
• III) 1 drop H3PO4, concentrated
• B. Crystallization of Acetylsalicylic Acid
• 1. Cool to room temperature. Put in ice
  water bath briefly if needed.
• 2. Add 3.0mL of cold water to the vial.

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Procedural Notes

• C. Vacuum Filtration
• 1. Filter solid, rinsing 2x to 3x w/
  0.5mL cold water.
• 2. Air dry the crystals on the Hirsch
  funnel for 5-10 minutes.
• 3. Weigh a watch glass, collect the
  crystals on the watch glass, and
• then place in drying oven _at_ 100oC
  for 10 minutes.
• 4. Weigh the dried crystals on the
  watch glass.

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Analysis

• Peform the following analyses discussions of
  each
• Yield
• Melting point (crystals must be dry)
• FeCl3 Test
• Iodine Test for Starch
• What is the purpose of the concentrated H3PO4
  used in the 1st step?
• If the aspirin crystals were not completely dry
  before the melting point was determined, what
  effect would this have on the observed m.p.?