Aspirin and Other Analgesics

Aspirin and Other Analgesics

Background

  • Aspirin: One of the most widely used nonprescription drugs.

    • Over 15,000 pounds sold annually in the U.S.
    • Medicinal Applications:
    • Analgesic (pain relief) for headaches, toothaches, neuralgia (nerve pain), muscle pain, joint pain (arthritis, rheumatism).
    • Antipyretic (fever reducer).
    • Anti-inflammatory (reduces swelling and redness).
    • Acts as an anticoagulant, preventing platelet aggregation to reduce stroke and heart attack risk.

  • Active Agent: Salicylic acid is responsible for analgesic properties; it is produced in the digestive tract after aspirin ingestion.

    • Historical Use: Ancient Greeks recommended chewing willow bark, a source of salicin, which converts to salicylic acid during digestion.

Aspirin Synthesis

Vacuum Filtration

  • Chemical Structure: Salicylic acid has phenolic and carboxylic acid groups, which cause stomach irritation (hemorrhaging).
  • Bayer Company's Contribution: Patented acetylsalicylic acid (aspirin) in 1899, found to be less irritating than salicylic acid because it is converted to salicylic acid in the small intestine.
  • Side Effects of Aspirin: Diffuses through the stomach lining and can lead to gastric bleeding; mitigated through coatings or buffering agents (e.g., magnesium hydroxide, magnesium carbonate, aluminum glycinate) to create carboxylate salt forms that are more water-soluble.

Experimental Method: Aspirin Preparation

  • Objective: Prepare aspirin via an esterification reaction using acetic anhydride and acid catalyst (sulfuric or phosphoric acid).
  • Vacuum Filtration: Fast method to remove a solid from a liquid mixture to recover products.
  • Cold Solvents: Ice-cold solvents should be used to minimize product loss due to decreased solubility at lower temperatures.

Limiting Reagents & Theoretical Yield

  • Balanced chemical equations are not always written for organic reactions, but the relative moles of reactants must be considered.
  • Limiting reagent: The reagent present in the smallest quantity that dictates the maximum theoretical yield.
    • Example of Limiting Reagent:
    • Hamburger Example: If 6 hamburger patties and 10 buns are available:
      • From patties: ext{Number of burgers} = 6 ext{ patties} imes rac{1 ext{ burger}}{1 ext{ patty}} = 6 ext{ burgers}
      • From buns: ext{Number of burgers} = 10 ext{ buns} imes rac{1 ext{ burger}}{2 ext{ buns}} = 5 ext{ burgers}
    • Limiting Reagent: Buns, maximum yield = 5 burgers.

Yield Calculation

  • Actual Yield < Theoretical Yield: Various factors cause lower actual yield: unreacted reagents, mechanical losses, side products.
    • Example: If a bun is lost to a dog, actual burgers produced = 4.
  • Percent Yield Calculation:
    • ext{Percent yield} = rac{ ext{actual yield}}{ ext{theoretical yield}} imes 100
    • If theoretical yield = 5 burgers and actual yield = 4 burgers:
    • ext{Percent yield} = rac{4 ext{ burgers}}{5 ext{ burgers}} imes 100 = 80 ext{5}

Determining Purity of Aspirin

  • Purity Requirement: Produced aspirin is not pure enough for use, unreacted materials often remain.
  • Unreacted Reagents: Water is added to hydrolyze acetic anhydride for removal of acetic acid which is water-soluble. Salicylic acid remains in product.
  • Detection Method: Use 1% iron (III) chloride solution; salicylic acid reacts to form a purple complex due to its phenol group:
    • Purple color intensity correlates to the quantity of salicylic acid present. Pure aspirin will not give a color reaction.

Analysis of Aspirin and Other Analgesics

  • Analgesics vs. Antipyretics: Analgesics relieve pain, while antipyretics reduce fever.
  • Examples: Aspirin, acetaminophen, ibuprofen, naproxen, with aspirin having additional anti-inflammatory properties.
  • Chromatography: Separation technique to analyze mixtures based on component properties.
    • Components: Stationary phase (e.g., silica plate) vs. mobile phase (various solvents).

Thin-Layer Chromatography (TLC)

  • TLC Setup:
    • Use a thin stationary phase and a TLC chamber.
    • Solvent moves up a plate via capillary action, with more soluble substances traveling farther.
  • Sample Handling: Use spotters to deposit sample solutions onto TLC plates, ensuring proper spacing to avoid overlap.
  • Visualizing Spots: Under UV light, fluorescing agents help visualize non-colored substances in the plate.

Rf Value Calculation

  • Retention Factor (Rf): Ratio of distances traveled by sample vs. solvent.
    • Rf = rac{ ext{distance traveled by component}}{ ext{distance traveled by solvent}}

Procedure: Aspirin Synthesis and Analysis

  1. Assemble materials: acetic anhydride, phosphoric acid, aspirin tablets, ethanol, iron(III) chloride, salicylic acid, glassware, etc.
  2. Safety note: Acetic anhydride and phosphoric acid are corrosive; proper PPE required.
  3. Follow detailed steps to synthesize aspirin, ensuring proper measurements and procedures for cooling, filtration, and drying.
  4. Data Analysis:
    • Calculate moles of salicylic acid and acetic anhydride.
    • Determine theoretical yield and percent yield.
    • Conduct melting point analysis to compare with known values.
    • Perform pH test and color tests with FeCl3 for aspirin samples.
    • Culminate findings through TLC to analyze analgesic samples.

Conclusion

  • Analyses include confirmation of synthesized aspirin's purity, yield, and comparison with commercial products. Record observations and improvements for future experiments.