EXPERIMENT 24

[aspirin.lab]

OBJECTIVES:

1. To learn the techniques used to prepare a common organic compound.

2. To learn the preparation and structure of ASPIRIN.

DISCUSSION:

Acetylsalicylic acid, commonly known as aspirin, is a drug that has many uses. Aspirin is both an analgesic and an antipyretic drug. Aspirin is a white solid that is almost insoluble in cold water. It is prepared by reacting salicylic acid with acetic anhydride. Concentrated H₂SO₄ is added to the mixture as a catalyst. Since aspirin is an ester (a "salt" of an organic acid), the reaction is known as esterification.

This is the equation for the reaction:

O OH O O OH O

\\ / \\ \\ / //

C OH C-CH₃ C O-C H-O

\ / / \ / \ \

C==C + O --> C==C CH₃ C-CH₃

/ \ \ / \ + //

C C C-CH₃ C C O

\\ // // \\ //

C--C O C--C

C₆H₄(COOH)(OH) (CH₃CO)₂O C₆H₄(COOH)(OOCCH₃) CH₃COOH

salicylic acetic acetylsalicylic acetic

acid anhydride acid acid

EXPERIMENTAL PROCEDURE: ( work in pairs )

1. Place an aluminum weighing dish on a top-loading balance. Tare it to zero. Add 9.00 g. of salicylic acid.

2. Pour 12 mL of acetic anhydride into a 125- or 250-mL Erlenmeyer flask. Do this in the hood. (CAUTION: Acetic anhydride is very irritating to the skin and eyes. Use CARE in pouring this chemical. If it is spilled, wash with plenty of water).

3. Add the salicylic acid to the acetic anhydride. Swirl the flask carefully to mix the two compounds.

4. From a dropper bottle, carefully add 12-14 drops of concentrated sulfuric acid (CON H₂SO₄) to the mixture in the Erlenmeyer flask. (Is the reaction mixture exothermic or endothermic after adding the acid?)

5. Heat 400 mL of tap water in a 600-mL beaker. Set it on a wire gauze on a ring and heat it to boiling.

6. Clamp the flask in the beaker of boiling water and boil for about 15 minutes. Shake or stir occasionally while heating. The compounds in the flask will react and form aspirin which should be soluble in the hot solution.

7. Remove the flask from the boiling water. Cool the flask in cold water to room temperature. Do not get any water in the flask.

8. Add 30 mL of ice water (a mixture of ice and distilled water) to the reaction mixture in the flask. Set the flask in a large beaker of ice and observe the white crystals of aspirin coming out of solution. This process is called crystallization.

9. Separate the aspirin crystals from the solution by using the suction apparatus as demonstrated by the Instructor. Put the correct sized filter paper in the Buchner funnel and attach a rubber tubing from the hose tip of the funnel to an aspirator over the trough in front of you. Pour the contents of the Erlenmeyer flask into the funnel. Turn on the aspirator. The white solid aspirin crystals will remain on the filter paper.

10. Wash the crystals twice with 30 mL of ice water each time. This will remove any left-over acetic anhydride and H₂SO₄.

11. After filtering, carefully lift up the filter paper with aspirin crystals on it and put the paper on a watch glass. Store it in your locker to dry until the next lab period. Discard the filtrate.

Next lab period:

12. Weigh a large filter paper. Record the mass.

13. Carefully transfer your dry aspirin to the weighed filter paper. Record the total mass.

14. Calculate your "percentage yield" by dividing the weight of your aspirin crystals by 11.73 and then multiplying by 100. You used 9 g. of salicylic acid in the preparation of aspirin. This 9 g. should give 11.73 g. of aspirin. This would produce a 100% yield, which is called the theoretical yield. Actually, many factors prevent us from obtaining a 100% yield. Some aspirin crystals may be lost or all of the salicylic acid or acetic anhydride may not react. Your actual yield will be less than 11.73 g, which means that your percentage yield will be less than 100%

15. Determine the melting point of the dry aspirin crystals you had prepared. Tap some dry aspirin crystals into a capillary tube. Fasten the tube to a thermometer with a small rubber band and immerse the tubes in a test tube of glycerine. Heat the glycerine slowly and watch the aspirin crystals. When they become liquid, record the temperature. This is the melting point of your aspirin.

Look up the melting point of aspirin in The Handbook of Chemistry and Physics. The purity of the aspirin you prepared can be determined by how close your melting point is as compared with the Handbook value.

DO NOT USE YOUR ASPIRIN AS A MEDICINE !! We are not sure of its purity and swallowing your preparation could be dangerous if it is impure. Commercial preparation is done using the same method, but the product is carefully washed and tested to insure purity.

EXP 24 - ASPIRIN LAB REPORT Name ____________________ Sec ____

1. Mass of a clean filter paper M1 = _______ g

2. Mass of aspirin crystals and filter paper M2 = _______ g

3. Mass of aspirin crystals M2 - M1 = M = _______ g

4. Percentage yield (show calculations) _______ %

M

% = -------- * 100 =

11.73

5. Melting point of your aspirin crystals mp = _______ ^o C

6. Melting point of aspirin from the Handbook mp = _______ ^oC

QUESTIONS:

1. What is the structural formula for aspirin?

2. What reactants are used to form aspirin?

3. What type of reaction do we call the synthesis of aspirin?

4. Was the reaction (a) exothermic or (b) endothermic?

5. Write the word equation for the reaction which produces aspirin.

6. Define the terms (a) ANALGESIC and (b) ANTIPYRETIC: