The Synthesis and Analysis of Aspirin

Acetylsalicylic acid is the chemical name for aspirin, the ubiquitous pain reliever. One of the compounds used in the synthesis of aspirin is salicylic acid, which is itself a pain reliever that was known to many ancient cultures, including the Native Americans who extracted it from willow tree bark. Salicylic acid is extremely bitter tasting, and frequent use can cause severe stomach irritation. The search for a milder form of this pain reliever led to the successful synthesis of acetylsalicylic acid by the German chemist Felix Hoffmann in 1893.

Your two primary objectives in this experiment will be to synthesize and analyze aspirin. There is more than one way to synthesize aspirin; in this experiment, you will react acetic anhydride with salicylic acid in the presence of phosphoric acid (which acts as a catalyst). The reaction equation is shown below.

HO O HO O

O O O O O OH H PO (aq) 3 4 + + CH H C OH H3C O CH3 D 3 3

Salicylic acid Acetic anhydride Acetylsalicylic acid Acetic acid

You will conduct two tests of your synthesis to verify that you did indeed make aspirin, and to determine its relative purity. First, you will measure the melting temperature of a sample of your product, using a Melt Station. Second, you will use a Spectrophotometer to test the absorbance of salicylic acid impurity in your aspirin sample after it has been complexed with Fe3+ from iron (III) nitrate solution.

OBJECTIVES In this experiment, you will  Synthesize a sample of acetylsalicylic acid (aspirin).  Calculate the percent yield of your synthesis.  Measure the melting temperature of your aspirin sample.  Conduct a spectrophotometric analysis of your aspirin sample.

Organic Chemistry with Vernier © Vernier Software & Technology 1 MATERIALS

Part I Synthesis Materials 50 mL Erlenmeyer flask solid salicylic acid two 10 mL graduated cylinders 85% phosphoric acid solution, H3PO4 25 mL graduated cylinder liquid acetic anhydride 250 mL beaker distilled water Büchner funnel, filter, and filter paper cold distilled water spoon, spatula, or rubber policeman balance ice bath Temperature Probe or thermometer hot plate watch glass plastic Beral pipet or eyedropper

Part II Melting Temperature LabQuest or computer interface aspirin crystals (from Part I) LabQuest App or Logger Pro tissues (preferably lint-free) Vernier Melt Station mortar and pestle (optional) glass capillary tubes, one end closed

Part III Spectrophotometric Absorbance LabQuest or computer interface salicylic acid LabQuest App or Logger Pro aspirin crystals (from Part I) SpectroVis Plus spectrophotometer ethanol, denatured 50 mL graduated cylinder 0.025 M iron (III) nitrate solution, Fe(NO3)3 plastic cuvette with lid distilled water 250 mL beaker 100 mL volumetric flask 100 mL beaker 250 mL volumetric flask

PROCEDURE

Part I Synthesize Aspirin 1. Obtain and wear goggles. Protect your arms and hands by wearing a long-sleeve lab coat and gloves. Conduct this reaction in a fume hood. 2. Measure out 2.0 g of salicylic acid into a 50 mL Erlenmeyer flask.

3. Add 5.0 mL of acetic anhydride and 5 drops of 85% phosphoric acid solution. Swirl the mixture. If necessary, use a sparingly small amount of distilled water to rinse down any bits of solid that may be on the inner walls of the flask. CAUTION: Handle the phosphoric acid and acetic anhydride with care. Both substances can cause painful burns if they come in contact with the skin. 4. You are now ready to begin the synthesis of aspirin. a. Prepare a 70–80°C hot-water bath using a 250 mL beaker on a hot plate. Monitor the water temperature using a Temperature Probe or thermometer. b. Hold and partially submerge the 50 mL flask and contents in the water bath. c. Heat the mixture in the hot-water bath for 15 minutes, or until the mixture ceases releasing vapors. Stir the mixture occasionally during heating. Add 2 mL of distilled water about 10 minutes into the heating.

2 Organics Chemistry with Vernier The Synthesis and Analysis of Aspirin

5. Next you will crystallize the aspirin: a. When you are confident that the reaction has reached completion (no vapors appearing), carefully remove the flask from the hot plate and add 20 mL of distilled water. b. Allow the mixture to cool to near room temperature. Transfer the flask to an ice bath for about five minutes. As the mixture cools, crystals of aspirin should form in the flask. 6. Now you will wash the synthesized aspirin: a. Set up a vacuum filtration using a Büchner funnel. Be sure to weigh and record the mass of the filter paper to the nearest 0.01 g before filtering the solid. b. Transfer the contents of the flask to a Büchner funnel assembly. Filter the mixture with vacuum suction. c. When most of the liquid has been drawn through the funnel, turn off the suction and wash the crystals with 5 mL of cold, distilled water. d. After about 15 seconds, turn the suction back on. Wash the crystals with cold, distilled water twice more in this manner. e. Gently transfer the filter paper with your product onto the watch glass to air dry. As directed by your instructor, either direct a gentle stream of air (low flow) to help dry the solid, or let them air dry until the next lab period. f. Weigh the dried recrystallized product on the filter paper and record the mass to 0.01 g. 7. Determine the mass of your dry aspirin sample and record in the data table.

Part II Test the Melting Temperature of an Aspirin Sample 8. Prepare a sample for melting: a. Use a mortar and pestle to pulverize a small amount (about 0.2 g) of your synthesized aspirin and place it in a small pile in the mortar. b. Pack a capillary tube 3–4 mm (~1/8 inch) deep with your aspirin sample by inserting the open end into the small pile of aspirin. A small amount of the solid will be pushed up into the tube. c. Tap the closed end of the capillary tube on the table top to compress the sample into the closed end. d. Check the control knob on the Melt Station to confirm that it is in the Off position. e. Carefully insert the capillary tube of solid into one of the sample holders of the Melt Station. 9. Connect the Melt Station power supply to a powered electrical outlet. 10. Connect the Melt Station sensor cable to LabQuest or to a computer interface. 11. Start the data-collection program, then choose New from the File menu. You are now set up to take melting temperature data for up to 20 minutes. 12. In the first trial, you will want to observe the melting process and make a rough estimate of the melting temperature of your aspirin sample. Don’t worry if the heating rate is a bit too rapid and the sample melts too quickly. To do this: a. Start data collection. b. On the Melt Station, turn the control knob to a setting of 220ºC. The red light will turn on indicating active heating.

Organic Chemistry with Vernier 3 c. Carefully observe your sample. When the solid begins to melt, click Mark to mark the temperature on your graph (or press the D key on the computer or the OK button on LabQuest). When the entire solid has completely melted, click Mark again. The two values marked on your graph describe the estimated melting temperature range of your substance. d. When you have determined the approximate melting temperature range for the sample, stop data collection. Store the run by tapping the File Cabinet icon in LabQuest, or choosing Store Latest Run from the Experiment menu in Logger Pro. Discard the capillary tube and sample as directed by your instructor. e. On the Melt Station, turn the control knob to the Fan/Cooling setting to get ready for the next trial. The blue light will turn on indicating that the fan is cooling the Melt Station.

13. Now that you have a rough idea of the melting temperature, a more accurate determination of the melting temperature can be made. Use a previously prepared sample in a capillary tube, as described in Step 8, to determine the melting temperature of the sample: a. Start data collection. b. On the Melt Station, turn the control knob to the Rapid Heat setting. c. Carefully observe the temperature vs. time graph. When the temperature is within approximately 10ºC of the lowest possible melting temperature of your sample, turn the control knob to a temperature setting corresponding to your expected melting temperature. d. Carefully observe your sample. When the solid begins to melt, click Mark to mark the temperature on your graph. When the entire solid has completely melted, click Mark again. The two values marked on your graph describe the estimated melting temperature range of your substance. When you are finished with this step, stop data collection. e. Store the run. f. Discard the capillary tube and sample as directed by your instructor. g. On the Melt Station, turn the control knob to the Fan/Cooling setting to get ready for the next trial. 14. Repeat Step 13 until you have determined the melting temperature range of your aspirin. Record the melting temperature range in your data table. 15. At the end of your testing, turn the control knob on the Melt Station to Off.

Part III Test the Spectrophotometric Absorbance of an Aspirin Sample Your synthesis converted most, but not all, of the salicylic acid into acetylsalicylic acid. You will mix iron (III) nitrate with salicylic acid in your aspirin sample to complex the salicylic acid, which is a bluish-purple color. You will analyze several samples to determine the amount of salicylic acid impurity in your synthesized aspirin. You can use this information to calculate the purity of your aspirin sample. Follow Steps 16–21 to prepare a set of salicylic acid standard solutions and conduct testing to develop your own Beer’s law plot of the standards. Steps 22–25 will guide you through the set up and testing of your aspirin sample.

16. Quantitatively prepare the stock salicylic acid solution. a. Measure out about 0.20 g of salicylic acid. Record the mass, to the nearest 0.001 g. b. Transfer the salicylic acid to a 250 mL beaker and add 10 mL of ethanol. Swirl the beaker to dissolve the solid. c. Add 150 mL of distilled water to the beaker. Mix the solution.

d. Quantitatively transfer the solution from the beaker to a 250 mL volumetric flask. Thoroughly rinse the beaker with several portions of distilled water, and transfer the rinse water to the volumetric flask. Add distilled water, as needed, to fill the flask to the 250 mL mark. Mix the solution thoroughly. Calculate the precise molar concentration of your stock solution and record it in your data table. 17. Prepare five standard solutions of varying concentrations of salicylic acid. a. To prepare 100.0 mL of your standard solution (the solution you will use for Trial 1), quantitatively transfer 10.0 mL of the stock salicylic acid solution you prepared in Step 16 to a 100 mL volumetric flask.

b. Add 0.025 M Fe(NO3)3 solution to the flask to make precisely 100 mL. c. Prepare the remaining four salicylic acid standard solutions according to the table below, diluting the standard solution in the 100 mL flask with distilled water. Mix thoroughly. d. Calculate the precise molar concentrations of the five standard solutions in the table above and record them in your data table.

Standard salicylic Trial acid solution from Water Step 17 a–b (mL) (mL) 1 10.0 0 2 8.0 2.0 3 6.0 4.0 4 4.0 6.0 5 2.0 8.0

18. Disconnect the Melt Station and connect the Spectrophotometer to the USB port of LabQuest or a computer. Start the data-collection program, then choose New from the File menu. 19. Calibrate the Spectrophotometer.

a. To prepare a blank cuvette, fill a cuvette 3/4 full with 0.025 M Fe(NO3)3 solution. Place the blank cuvette in the Spectrophotometer. b. Choose Calibrate from the Sensors menu of LabQuest or the Experiment menu of Logger Pro. c. When the warmup period is complete, select Finish Calibration. Select OK. 20. Determine the optimal wavelength for creating the standard curve and set up the mode of data collection.

a. Empty the Fe(NO3)3 solution from the blank cuvette. Using the solution in the 100 mL volumetric flask of salicylic acid (Trial 1), rinse the cuvette twice with ~1 mL amounts, and then fill it 3/4 full. Wipe the outside with a tissue and place it in the Spectrophotometer. b. Start data collection. A full spectrum graph of the solution will be displayed. Stop data collection. The wavelength of maximum absorbance ( max) is automatically identified.

Organic Chemistry with Vernier 5 c. Change the mode to Events with Entry (absorbance vs. concentration) and select a wavelength for analysis:  In LabQuest App, the displayed wavelength of maximum absorbance ( max) is automatically identified on your graph with a point protector. Tap the Meter tab, then tap Mode. Change the mode to Events with Entry. Enter the Name (Concentration) and Units (mol/L). Select OK.  In Logger Pro, click the Configure Spectrometer button, . Click Abs vs. Concentration as the Collection Mode. The wavelength of maximum absorbance ( max) will be selected. 21. You are now ready to collect data for the five standard solutions. a. Leave the Trial 1 cuvette in the Spectrophotometer and start data collection. b. When the absorbance value displayed on the screen has stabilized, select Keep and enter the molar concentration. Select OK. The absorbance and concentration values have now been saved for the first solution. c. Discard the cuvette contents as directed by your instructor. Using the solution in the second 100 mL volumetric flask, rinse the cuvette twice with ~1 mL amounts, and then fill it 3/4 full. Wipe the outside, place it in the device, and close the lid. After closing the lid, wait for the absorbance to stabilize and select Keep. Enter the molar concentration, and select OK. d. Repeat the procedure for the remaining salicylic acid solutions that you prepared. e. Stop data collection to view a graph of absorbance vs. concentration. f. Record the absorbance and concentration values in your data table. You can do this either by examining your data points along the curve, or by viewing the data table. g. Choose Curve Fit from the Analyze menu. h. Choose Linear as the Fit Equation. Select OK. The graph should indicate a direct relationship between absorbance and concentration, a relationship known as Beer’s law. The regression line should closely fit the five data points and pass through (or near) the origin of the graph. i. After the preparation and testing of your aspirin sample in the following steps, you will be instructed to interpolate along this plot to determine the concentration of salicylic acid impurity in your aspirin sample. 22. Prepare the synthesized aspirin sample for testing. Complete this step quickly and be ready to proceed directly to Step 23. a. Measure out about 0.4 g of aspirin and transfer it to the 250 mL beaker. Record the mass of aspirin that you use to the nearest 0.01 g. b. Add 10 mL of ethanol to the beaker of aspirin sample. Swirl the mixture to dissolve the solid. c. Add 150 mL of distilled water to the beaker. Mix the solution. d. Quantitatively transfer the solution from the beaker to a 250 mL volumetric flask. Thoroughly rinse the beaker with several portions of distilled water, and transfer the rinse water to the volumetric flask. Add distilled water, as needed, to fill the flask to the 250 mL mark. Mix the solution thoroughly. e. Transfer 5 mL of the aspirin solution from the 250 mL volumetric flask to a clean and dry 100 mL volumetric flask. Add 0.025 M Fe(NO3)3 solution to the flask to make precisely 100.0 mL. Mix the solution thoroughly.

23. Measure and record the absorbance value of the treated aspirin sample. This must be done within 10 minutes of completing Step 22. To do this: a. Rinse and fill the cuvette 3/4 full with the sample. Cap the cuvette and place it in the Spectrophotometer. b. Monitor the absorbance value on the displayed Meter in Logger Pro (or tap the Meter tab in LabQuest). If the absorbance value falls within the range of the salicylic acid standard solutions, record it in your data table. Note: If the absorbance value does not fall within the range of the salicylic acid standard solutions in your data table, you can repeat Step 22e using a more dilute or more concentrated solution. 24. To determine the concentration of the salicylic acid impurity in the treated aspirin sample, interpolate along the regression line to convert the absorbance value of the unknown to concentration. a. Choose Interpolate from the Analyze menu. b. Click or tap any point along the regression curve (or use the ◄ or ► keys on LabQuest) to find the absorbance value that is closest to the absorbance reading you obtained in Step 23. The corresponding salicylic acid concentration, in mol/L, will be displayed to the right of the LabQuest graph, or on your Logger Pro graph. c. Record the concentration of salicylic acid in your data table. 25. Discard all solutions as directed.

DATA TABLE Part I Synthesis of Aspirin

Mass of salicylic acid used (g)

Volume of acetic anhydride used (mL)

Mass of acetic anhydride (1.08 g/mL) used (g)

Mass of aspirin and filter paper (g)

Mass of filter paper (g)

Mass of aspirin synthesized (g)

Part II Melting Temperature Data

Melting temperature range (°C)

Part III Salicylic Acid Standard Stock Solution

Initial mass of salicylic acid (g)

Moles of salicylic acid (mol)

Initial molarity of salicylic acid (mol/L)

Organic Chemistry with Vernier 7 Part III Beer’s Law Data for Salicylic Acid Standard Solutions

Trial Concentration (mol/L) Absorbance 1 2 3 4 5

Test of the Purity of the Synthesized Aspirin

Initial mass of aspirin sample (g)

Absorbance of aspirin sample

Concentration of salicylic acid (mol/L)

Moles of salicylic acid in aspirin sample (mol)

Mass of salicylic acid in aspirin sample (g)

Mass of aspirin in sample (g)

Percent aspirin in sample (%)

DATA ANALYSIS 1. What is the theoretical yield of aspirin in your synthesis? The mole ratio is 1:1 between salicylic acid and acetic anhydride in this reaction. 2. Using a literature source or the internet, find the accepted melting temperature value of pure acetylsalicylic acid. How does the melting temperature test of your aspirin compare to the accepted value? 3. Based on the results of the absorbance testing with the Spectrophotometer, what is the percent purity of your sample of aspirin? Does this percent purity compare well with the results of the melting temperature test? Explain. 4. Use your percent purity calculations to determine the percent yield of your synthesis of aspirin.

8 Organics Chemistry with Vernier The Synthesis and Analysis of Aspirin INSTRUCTOR INFORMATION 1. Each lab team will use about 2 g of salicylic acid.

2. The 0.025 M Fe(NO3)3 solution can be prepared by using 10.10 g of FeNO3•9H2O per 1 L. 3. The yield for this reaction will be lower than your students’ expectations. A low yield may not always be the result of sloppy work, but poor lab technique will certainly result in a disappointing yield. 4. You may choose to shorten Part III by providing your students with Beer’s law standards. 5. It is critical for your students to complete the spectrophotometric analysis of their samples (Part III) in one lab period, because of the instability of the prepped samples. 6. After students complete the Part 1 synthesis, they will need to dry the sample. You may choose to have them wait until the next lab period, to ensure that the sample is dry. Or, if you wish to have them complete Parts II and III in the same lab period, you will need to provide an air-flow supply (low flow) to fairly quickly air dry their aspirin sample.

HAZARD ALERTS Salicylic acid: Harmful if swallowed. Causes mild skin irritation. Causes serious eye damage. HMIS Classification: Health hazard2, Flammability0, Physical hazard0. Acetic anhydride: Strongly irritating and corrosive. Moderate fire risk (flash point 49°C). Causes severe eye damage. Wear eye protection. Have access to eyewash. Vapors are strongly irritating. Open and dispense in fume hood. Reacts (sometimes delayed) violently with water. Moderately toxic by ingestion and inhalation. HMIS Classification: Health hazard3, Flammability2, Physical hazard2. Phosphoric acid (o-phosphoric acid): Skin and eye irritant. Toxic by ingestion and inhalation. Burns tissue. HMIS Classification: Health hazard3, Flammability0, Physical hazard0. Ethanol: Fire risk (flash point 14.0°C). Flammable. Addition of denaturant makes the product poisonous. Store in dedicated flammables cabinet. Skin and eye irritant. Moderately toxic by ingestion and inhalation. HMIS Classification: Health hazard2, Flammability3, Physical hazard1. Iron (III) nitrate: May intensity fire; oxidizer. May be harmful if swallowed. Causes skin irritation. May cause respiratory irritation. HMIS Classification: Health hazard2, Flammability0, Physical hazard3. The hazard information reference is Sigma-Aldrich Co., 1-800-325-3010, www.sigmaaldrich.com/safety-center/msds-search.html

Organic Chemistry with Vernier 9 SAMPLE DATA Part I Synthesis of Aspirin

Mass of salicylic acid used (g) 2.01

Volume of acetic anhydride used (mL) 5.0

Mass of acetic anhydride (1.08 g/mL) used (g) 5.40

Mass of aspirin and filter paper (g) 2.52

Mass of filter paper (g) 0.56

Mass of aspirin synthesized (g) 1.96

Part II Melting Temperature Data

Melting temperature range (°C) 132.6–133.9

Part III Salicylic Acid Standard Stock Solution

Initial mass of salicylic acid (g) 0.201

Moles of salicylic acid (mol) 0.00145

Initial molarity of salicylic acid (mol/L) 0.00579

Part III Beer’s Law Data for Salicylic Acid Standard Solutions Trial Concentration (mol/L) Absorbance 1 5.79 × 10–4 1.005 2 4.63 × 10–4 0.832 3 3.47 × 10–4 0.616 4 2.32 × 10–4 0.423 5 1.16 × 10–4 0.211

Test of the Purity of the Synthesized Aspirin

Initial mass of aspirin sample (g) 0.40

Absorbance of aspirin sample 0.277

Concentration of salicylic acid (mol/L) 1.50 × 10–4

Moles of salicylic acid in aspirin sample (mol) 1.5 × 10–5

Mass of salicylic acid in aspirin sample (g) 0.10

Mass of aspirin in sample (g) 0.30

Percent aspirin in sample (%) 75

10 Organics Chemistry with Vernier The Synthesis and Analysis of Aspirin SAMPLE CALCULATIONS 1. For the sample data, the theoretical yield of aspirin is 2.63 g. The calculation is based on salicylic acid (the limiting reactant), mol of salicylic acid used = 2.01 g ÷ 138.13 g/mol = 0.0146 mol mol of salicylic acid = mol of aspirin produced (theoretical) theoretical mass of aspirin produced = 0.0146 mol × 180.16 g/mol = 2.63 g 2. The experimentally determined melting temperature of 132.6°C compares well with the accepted melting temperature of pure acetylsalicylic acid, 135°C. 3. The results of the spectrophotmetric test show that the percent purity of the synthesized aspirin is 75%. This compares reasonably well with the melting temperature test. For the sample data, the calculations are as follows. The absorbance of the aspirin sample is 0.081. The absorbance of 0.081 equals a salicylic acid concentration of 1.50 × 10–4 mol/L. Because the sample volume is 100 mL, the molar amount of salicylic acid impurity in the aspirin sample is 1.50 × 10–5 mol. There was 1.50 × 10–5 mol of salicylic acid in a 5 mL sample. This 5 mL sample was taken from 250 mL of solution. Thus, there is (1.50 × 10–5 mol × (250/5) = 7.50 × 10–4 mol of salicylic acid in the initial aspirin sample. The mass of salicylic acid is: 7.50 × 10–4 mol × 138.13 g/mol = 0.10 g The initial mass of the aspirin sample is 0.40 g. If 0.10 g of this sample is unreacted salicylic acid, then the percent of aspirin in the sample is: (30 g/40 g) × 100 = 75%. 4. Answers will vary. For the sample data, the mass of synthesized aspirin is 1.96 g. Corrected for purity, is: (1.96 × 0.75) = 1.47 g. The percent yield is: (1.47 g ÷ 2.63 g) × 100 = 55.9%.

SAMPLE DATA

Melting temperature determination for aspirin sample

Organic Chemistry with Vernier 11 Spectrum for salicylic acid with iron (III) nitrate

Absorbance vs. concentration for salicylic acid with interpolation

12 Organics Chemistry with Vernier