Overview
This lecture covers the laboratory synthesis of aspirin (acetylsalicylic acid) from salicylic acid, including reaction setup, purification, yield calculations, and product purity tests.
Aspirin Synthesis Overview
- Aspirin (acetylsalicylic acid) is made by acetylating the phenol group of salicylic acid.
- The reaction uses salicylic acid, acetic anhydride, and phosphoric acid as a catalyst.
Reaction Details and Calculations
- Phosphoric acid lowers activation energy, speeding up the reaction at lower temperatures (~50°C).
- The reaction is an equilibrium process (can reverse if water is present).
- Limiting reagent calculations require converting masses/volumes to moles using molar masses and densities.
- The reaction has a 1:1 molar ratio between salicylic acid and acetic anhydride.
- Theoretical yield is based on the limiting reagent and the molar mass of aspirin.
Experimental Procedure
- Use 0.208g salicylic acid and 0.480mL acetic anhydride in a 5mL conical vial.
- Add 1 drop of phosphoric acid and a spin vane for stirring.
- Heat mixture at 50°C for 10 minutes using a water bath and air condenser.
- After heating, cool to room temperature, then use an ice bath to induce crystallization.
- Use a Hirsch funnel and vacuum filtration to isolate the aspirin crystals.
- Wash crystals with several portions of cold water to remove impurities.
Product Purity Analysis
- Ferric chloride test: a purple color indicates unreacted salicylic acid (impurity).
- Melting point measurement: pure aspirin melts at a known value; impurities lower and broaden the melting range.
- Percent yield = (actual yield / theoretical yield) × 100%.
Common Sources of Error
- Product loss during washing and filtration.
- Impurities may come from incomplete reactions or contamination (e.g., water, acetic acid, phosphoric acid).
- Improper drying can cause melting point depression due to retained water.
Key Terms & Definitions
- Aspirin (Acetylsalicylic Acid) — Product of acetylation of salicylic acid's phenol group.
- Salicylic Acid — Starting material, contains a phenol group.
- Acetic Anhydride — Acetyl group donor in the reaction.
- Phosphoric Acid (H₃PO₄) — Acid catalyst that lowers activation energy.
- Fischer Esterification — Reaction forming an ester from an alcohol and an acid.
- Ferric Chloride Test — Qualitative test for phenols; purple color indicates presence.
- Limiting Reagent — Reactant that determines the maximum yield.
- Theoretical Yield — Maximum possible amount of product.
- Percent Yield — Ratio of actual to theoretical yield.
Action Items / Next Steps
- Calculate limiting reagent, theoretical yield, and percent yield.
- Record melting point and interpret results against literature values.
- Discuss results, errors, and ferric chloride test conclusions in your lab report.