Aspirin Synthesis and Stability

Aspirin Synthesis

• Aspirin is also known as 2-acetyl salicylic acid or 2-acetoxy benzoic acid.
• Its molecular formula is C₉H₈O₄, and its molecular weight is 180.15.
• Aspirin appears as colorless crystals or a white crystalline powder.
• It has a slightly acidic taste and no odor.
• Aspirin is slightly soluble in water (1:300 at 25°C, 1:100 at 37°C). It is more soluble in organic solvents like ether, ethanol, and chloroform.

Stability

• Aspirin (ASA) is stable in dry air when in crystal or powder form.
• In the presence of moisture, it undergoes hydrolysis to form salicylic acid and acetic acid.
• This hydrolysis occurs through base catalysis, where water molecules attack the carboxylate anion.

Mechanism

• The mechanism of aspirin synthesis involves a reaction between salicylic acid and acetic anhydride.
• The reaction is catalyzed by a weak acid (e.g., sulfuric acid). A detailed chemical reaction is shown.

Identification Test

• The presence of a phenolic group in a molecule can be detected using a ferric chloride test.
• During this test, applying ferric chloride, a violet-colored iron complex signifies the presence of a phenolic group.
• Aspirin doesn't have this group; hence it gives a negative result. Conversely, salicylic acid gives a positive test.

Methods of Synthesis

• Method 1: Uses acetic anhydride, which is readily available, easy to handle, and doesn't produce corrosive HCl gas. The process for this method is described in detail.
• Method 2: Uses acetyl chloride. This method is more vigorous, producing corrosive HCl gas, and pyridine is a teratogenic air pollutant, so this method generally is avoided.

Procedure for Method 1

• Steps for producing aspirin using acetic anhydride:

  • Place 1 gram of salicylic acid in a dry flask.
  • Add 2 ml of acetic acid.
  • Carefully add 1-2 drops of concentrated sulfuric acid (H₂SO₄).
  • Heat and stir until all the salicylic acid dissolves.
  • Allow the mixture to cool to obtain the aspirin crystals.
  • Add some water and filter the product.

• Important Notes:

  • If aspirin crystals haven't formed, add water to destroy excess acetic anhydride.
  • Acetic acid, or excess sulfuric acid, is not added, because those substances lead to hydrolysis of the aspirin.

Calculating Percentage Yield

• The steps to calculate the yield of aspirin are shown
• The theoretical yield is calculated using the mole ratios of the reactants and products.
• The actual yield is the mass of aspirin that was collected in the reaction.