Jce1098 p1261 synthesis of aspirin: a general chemistry experiment

In the Laboratory
Synthesis of Aspirin
John Olmsted III
Department of Chemistry and Biochemistry, California State University, Fullerton, Fullerton, CA 92834
In recent years, examples from organic chemistry have recognition, as the reactions modify the periphery of a struc- come to play increasingly prominent roles in the first-year tural core common to the starting material, intermediate university general chemistry course. The American Chemical product, and final product (Fig. 1).
Society has recommended more integration of biological Viewed from another perspective, this sequence exem- concepts into college-level introductory general chemistry plifies two of the most common types of chemical reaction, courses. Following this recommendation, at least two current general chemistry textbooks make liberal use of organic examples (1), and some universities are bringing organic chemistry much more into the forefront of the introductory course (2).
At California State University, Fullerton, the first-semester general chemistry laboratory has been redesigned over the past several years. The redesign has added an organic component As illustrated in Figure 2, the hydrolysis reaction pro- and provided students with explicit examples of several types of ceeds in several steps involving deprotonation and protona- operations in which chemists engage: observation, synthesis, tion as well as cleavage of a C–O bond (7 ). These encom- quantitative measurements, construction of apparatus, and pass examples of Brønsted acid–base proton transfer, another chemical analysis. Our experiment that accentuates accurate quantitative measurements was reported earlier (3).
These structural changes manifest themselves through Over the course of the first semester and the beginning readily observed macroscopic changes as the synthesis pro- of the second, our general chemistry students synthesize two ceeds. Addition of aqueous base to syrupy, fragrant oil of organic substances (aspirin and methyl orange) and two wintergreen yields a white odorless solid. Upon heating, this inorganic substances (alum and potassium ferrioxalate). Anearlier paper described our ferrioxalate synthesis experiment(4). This paper describes the first of our organic synthesis experiments, the synthesis, purification, and qualitative spec- troscopic characterization of aspirin.
Aspirin (acetylsalicylic acid) is a pain-relieving compound familiar to virtually all students. The synthesis of aspirin from oil of wintergreen is an example of one of the most prevalent, profitable, and honored activities of chemists: the conversionof a naturally occurring substance into one with therapeutic value. Simple enough to be accomplished and understood by beginning students, this synthesis nevertheless serves as a paradigm for the pharmaceutical industry, from tranquilizers and antibiotics to yet-undiscovered agents for treating cancer, Rationale
The two-step conversion of oil of wintergreen (methyl 2- hydroxybenzoate) into salicylic acid (5) and then into aspirin (6 ) serves as an introductory example of multistep sequen- tial synthesis. It also provides practice in molecular pattern Figure 1. The structural core common to oil of wintergreen, salicylic Figure 2. Reaction mechanism for the conversion of oil of winter- JChemEd.chem.wisc.edu • Vol. 75 No. 10 October 1998 • Journal of Chemical Education In the Laboratory
solid reacts further to yield a solution, from which a differentwhite solid precipitates upon acidification. On heating withacrid liquid acetic anhydride, salicylic acid reacts and dissolves.
Dilution with water and cooling results in precipitation ofaspirin, yet another white solid.
Synthesis must always be accompanied by isolation and purification of the product. Both salicylic acid and aspirinare sparingly soluble in water, making these procedures readilyaccessible to the general chemistry student. Isolation is easilyaccomplished by suction filtration using a Büchner funnelapparatus, and purification is equally easily accomplished bydissolving the crude product in hot water and chilling torecrystallize the pure product.
A newly synthesized chemical substance must be appro- priately characterized before the synthesis can be judged a Figure 3. FTIR spectra of oil of wintergreen, salicylic acid, and aspirin.
success. While characterization is multifaceted and typicallyinvolves techniques well beyond the scope of general chemistry,FTIR spectrophotometry is well suited to characterization ofaspirin and its precursors. By taking FTIR spectra of their deionized water. Cool the beaker in an ice bath until it is just products, our students not only “see” that their white solids warm to the touch. Without removing the beaker, slowly add, are distinctly different but also become familiar with one of with continuous stirring, 50 mL of 8 M H2SO4 (precipitate the most prevalent instruments of the contemporary laboratory.
forms). After chilling in the ice bath, isolate the product.
Figure 3 shows actual FTIR spectra taken under the same Rinse the beaker with iced deionized water, pour over the conditions that are used by our students. The three compounds precipitate, and continue suction for about 10 minutes.
share spectral features due to their common framework, for Transfer the crude solid to a 250-mL beaker containing example the aromatic C–H bending vibrations in the 600– 100 mL of deionized water. Heat to a gentle boil until the 800 cm᎑1 region. They differ substantially in the absorptions solid dissolves completely. Allow the beaker to cool (crystals form), then transfer the beaker to an ice bath and chill thor- the broad hydrogen-bonded OH absorption that is promi- oughly. Isolate the product. Rinse the beaker with 50 mL of nent in oil of wintergreen and salicylic acid between 3000 iced deionized water, pour over the precipitate, and continue and 3500 cm᎑1. On the other hand, whereas oil of winter- suction for 15 minutes. Spread the solid on a watch glass, green and salicylic acid have a single C=O absorption at about cover with filter paper, and store overnight until dry.
1700 cm᎑1, aspirin has two distinct peaks arising from its ester Conversion of Salicylic Acid into Aspirin Weigh 1.4 g of salicylic acid and transfer to a clean, dry Procedure
125-mL Erlenmeyer flask. Add 3.0 mL of acetic anhydride
(CAUTION: caustic vapors: use a hood) and 5 drops of con-
Outlined here is a compact version of the procedure centrated H3PO4. Stopper with a one-hole rubber stopper carried out by our students and the instructions for their fitted with 2 cm of plastic tubing. Float the Erlenmeyer flask laboratory report. The procedure given to the students includes in an 800-mL beaker containing 250 mL of water. Heat this more detailed specifications for standard techniques with beaker to 85 °C and maintain between 85 and 90 °C for five which they may not be sufficiently familiar. Pan balances minutes (CAUTION: do not boil; steam baths may be used if
provide sufficient accuracy for all weighings, and graduated available). Discontinue heating and immediately use a Pasteur cylinders provide sufficient accuracy for volume measurements.
pipet to deliver 2 mL of deionized water through the plastic Glassware can be cleaned by rinsing with deionized water and tubing (CAUTION: hot acid vapors). When the flask is suffi-
need not be dry except when explicitly noted. Isolation of ciently cool, remove it using a towel, remove the stopper, and solids is accomplished by cold suction filtration using a add 20 mL of deionized water. Allow to stand at room temperature until crystals begin to form. Then add 10 mL The filtrate from the first synthesis procedure is substan- of deionized water, swirl, and place the flask in an ice bath.
tially acidic and should be disposed of properly following After chilling, isolate the product. Rinse the flask with 15 procedures for acidic aqueous waste. All other liquid waste mL of iced deionized water, pour over the precipitate, and is relatively benign and can be rinsed down the sink.
continue suction for about 10 minutes.
Conversion of Oil of Wintergreen to Salicylic Acid Weigh the solid in a clean, dry 50-mL beaker and add to the beaker 10 mL of deionized water per gram of solid.
0.2 mL of oil of wintergreen to a previously Heat with continuous stirring until all solid dissolves. Transfer weighed clean, dry 250-mL beaker and reweigh. Add, with the beaker to an ice bath and chill (crystals form) until precipi- stirring, 40 mL of 6 M NaOH (precipitate forms). Heat with tation appears complete. Isolate the product and continue occasional stirring to a gentle boil, reduce the heating rate to suction for an additional 10 minutes. Then transfer the solid avoid “bumping”, and continue boiling gently for 15 min- to a clean, dry 50-mL beaker and oven-dry at 80 °C (CAUTION:
utes. Midway through heating, rinse any solids adhering to aspirin melts and decomposes at 100 °C) for one hour. Remove, the beaker walls into the solution with a small quantity of Journal of Chemical Education • Vol. 75 No. 10 October 1998 • JChemEd.chem.wisc.edu In the Laboratory
The use of FTIR in undergraduate laboratories is becom- Prepare a solution of each solid (salicylic acid and aspirin) ing increasingly prevalent as this technique becomes routine by placing about 1 mL of ethyl acetate in a clean, dry test and instruments are more affordable. Others have described tube, adding a spatula-tip amount of solid, and stirring to experiments using FTIR spectroscopy in general chemistry dissolve. Using a Pasteur pipet, transfer 4–5 drops of this for smog analysis (9) and for exploring Lewis structures (10).
solution to a salt plate and allow the solvent to evaporate.
The technique also finds applications in the organic laboratory Place the salt plate on the mount in the sample compartment (11) and throughout the undergraduate curriculum (12). At of an FTIR spectrophotometer. Scan the spectrum and print Fullerton we make use of FTIR not only in general chemistry a copy. Clean the salt plate three times by placing several laboratory but also in our undergraduate organic, analytical, drops of ethyl acetate on its surface and wiping thoroughly and physical chemistry laboratories; and our students often use the instruments in the course of their undergraduate Include in your report for this experiment a one-paragraph abstract, brief procedure, data and results, and discussion. In The use of FTIR spectroscopy to differentiate aspirin from the procedure, describe the details of what you did and other substances is not unique to this instructional exercise observations you made, but do not reproduce the procedure in chemical synthesis. Procedures for quantitative analysis of from the laboratory manual. Data and results should include aspirin using FTIR have been reported for pharmaceutical all masses used, the theoretical yields of products, and the preparations (13) and drug testing (14). Furthermore, con- percent yields of products. In the discussion, answer the temporary research utilizes FTIR to study aspirin–DNA (15) question “What explains why my yields are different from and aspirin–RNA interactions (16 ). Thus, the qualitative theoretical yields?” Also compare the IR spectra of oil of identification of aspirin by FTIR serves as a representation wintergreen and your two products and identify the peaks of how modern spectroscopic techniques facilitate analytical that are unique to each of the products.
Discussion
Acknowledgments
The FTIR instrument used for this work, a Perkin-Elmer The one-step synthesis of aspirin from salicylic acid is Model 1600 with 2 cm᎑1 resolution, was purchased using described in many general chemistry laboratory manuals (8) funding provided by NSF–ILI grant USE-9250902, FTIR and may be traced to a report by Brown and Friedman (6 ).
in Undergraduate Chemistry Laboratories.
The experiment described here differs from these typical syn-theses in three respects. First, it is a more elaborate two-step Literature Cited
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JChemEd.chem.wisc.edu • Vol. 75 No. 10 October 1998 • Journal of Chemical Education

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