SCH4C                         Lab #15 - Preparation and Testing of Aspirin

Introduction:  One of the most useful of the benzene derivatives is aspirin, which is acetylsalicyclic acid.  Aspirin is synthesized by the reaction of salicyclic acid and acetic anhydride:

In this experiment you will prepare and purify aspirin and test the product.

Saftey:  Safety glasses must be worn throughout this experiment.  Acetic anhydride deomposition vapours are very irritating.  Do the warm acetic anhydride decomposition in the fume hood.

Procedure: Part I:  Preparation of the aspirin

1.      Obtain a sample of about 4 g of salicyclic acid and determine its mass to the nearest 0.01 g   Transfer Transfer the dry salicyclic acid powder to a small Erlenmeyer flask adn add exactly 8 mL og acetic anhydride.  Acetic anhydride is a powerful lachymator ( a substance that irritates the eyes) so this process must be done in the fume hood.)

2.       Stir the salicyclic acid-anhydride mixture with a swirling motion until all the solid has dissolved.  Add, with continued stirrng, 4 drops of 18 M H2SO4.  Place the Erlenmeyer contining the mixture in a beaker of boiling water, and allow the reaction to proceed for 15 minutes.

3.    To separate the aspirin from the reaction mixture, pour the into a 150 mL beaker containing 20 g of crushed ice and 20 mL of cold water.  Rinse the Erlenmeyer with 10 mL of ice water, of ice water, and add this rinse water to the rest of the mixture.

4.    Place the beaker contining the reaction mixture into an ice bath  and stir until crystallization appears to be complete, which is about 5 to 10 minutes.

5.    Filter the crystals.  Wash the crystals with three 5 mL portions of ice water, and allow to dry.  Determine the mass of dry aspirin obtained.  (Warning: The aspirin you have prepared contains contaminates and is NOT to be taken internally.)

Part 2:  Purification of the Aspirin.
Aspirin does not like to dissolve in water.  It sometimes dissolves slightly in hot alcohols.  We will use this fact to help purifiy the aspirin.

1.    To purify by recrystallization, place your crude aspirin in a 50 mL Erlenmeyer flask and dissolve it in a minimum amount of boiling propanol (Do this in a water bath).
2.    Remove the flask and add warm water dropwise until the solution becomes slightly cloudy.
3.    Allow the solution to cool until crystallization starts then finish crystallizing in the ice-bath.
4.    Collect your product by filtration, wash with small quantities of cold water. Place in the oven and complete its drying overnight.

Part 3:      Testing the Purity of the Aspirin

1.      Using capillary  tubes and a melting point apparatus as demonstrated by the teacher, determine the the mass of dry melting point of your aspirin and that of commercial aspirin as follows:

        Obtain two capillary melting-point tube.  Make one shorter than the other by cutting off a centimter or so.  Seal one end of each tube by placing it in a burner flame.  We will be placing crushed commerical aspirin in the shorter tube to identify it.

        Push the open end of the shorter tube into some dry, powdered commerical aspirin.  Tap the closed end gently on the table  so the solid packs down into the tube.  Continue to do this until the depth of solid is about 1 cm.  Similarily, fill the longer tube with some of the aspirin you have prepared.

    Attach the tubes to a thermometer (labeled -10oC to +50oC) with a small rubber circle cit from the end of a piece of rubber tubing.   Insert into a Thiele tube and warm slowly, after securing the Thiele tube assembly to a retort stand.

Heat slowly and watch the capillary tubes closely as the temperature approaches 120oC.  At the first sign of melting of each solid, record the temperature.  DO NOT exceed the 150oC maximum temperature of the thermometer.

2.    Stir a small sample of your aspirin into 2 mL of cold 0.5 M sodium bicarbonate (NaHCO3) solution.  Observe the solubility.  Add 0.5 M Hydrochloric acid, drop by drop, until a change is observed.  Record your observation.

3.    Place a few crystals of your aspirin and of commerical aspirin on apiece of moist blue litmus paper.  Record your observations.

4.    Place a few crystals of salicyclic acid in a clean, dry test tube.  Add 1 mL of ethanol and 1 drop of 0.1 M ferric chloride, FeCl3 solution.  Observe the colour.  Repeat,  using the aspirin you prepared and commercial aspirin.  What differences are noted?

Calculations and Results.
1.    Calculate the percentage yield of aspirin obtained?

2.   a)  Use the results of your tests on salicyclic acid, commerical aspirin, and your preparation of aspirin to suggest a reason why your aspirin and commerical aspirin may have differnece melting points.

b)    Commerical aspirin contains 5 grains of aspirin (0.324 g) in a tablet that weighs approximately 0.35 g.  Suggest another reason for any melting-point differences that you noted.

3.    Aspirin passes through the stomach unchanged, but is absorbed in the intestines.  Suggest a reason for this.