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Transcript
Exp't 88 The Acid-Catalyzed Reaction of Acetic Anhydride and Isopropyl Alcohol by Robert Minard and Kurt Rublein, Organic Instructional Laboratories, Revision 3/21/02 Ester syntheses are one of the more enjoyable experiments in organic chemistry, as one can't help but notice the special odors associated with esters - some fruity, some more like nail polish. In contrast, the acids which are used in their syntheses usually have a rotten odor. The reverse reaction, hydrolysis of the ester, gives the alcohol and the acid from which it was synthesized. Because the hydrolysis can occur in the presence of sweat, esters are not used as perfumes. OH + CH H3C CH 3 H3C O O C C O H+ ? (two organic products) CH 3 In this experiment, you should predict the product(s) of the reaction above and interpret your analysis data to confirm or negate your prediction. You may need to refer to your Organic textbook to review the reactions of acid derivatives (i.e., acetic anhydride, and acid anhydride) with alcohols. PreLab Be sure to include all sections that accompany the PreLab for Synthetic Experiments. Be sure to include an entry in the Chemical Data Table for the predicted product. Properly named, the product can be found in Aldrich. Use the formula index if you are not sure about the name. Include answers to the following questions in your PreLab: 1. The same experiment could be performed using acetic acid, isopropyl alcohol, and sulfuric acid; in this particular experiment, what advantage could acetic anhydride have over acetic acid? (Hint: consider what by-product is formed if acetic acid were used, and how this could affect the equilibrium between reagent and products.) 2. In developing this experiment, precautions were taken to ensure that acetic anhydride and sulfuric acid were mixed together before the isopropyl alcohol was added. What reaction is likely to take place if concentrated sulfuric acid was added directly to the isopropyl alcohol? (Hint: Look up acid catalyzed reactions of alcohols.) Precautions Concentrated sulfuric acid is very reactive and corrosive - it will eat holes in your clothes. Acetic anhydride is also very reactive and will react vigorously with water to give acetic acid and heat. Wear gloves when handling these chemicals or when handling solutions that contain these. Remember that the sulfuric acid is a catalyst, so when the reaction is complete, it is still present until after the work up and neutralization with sodium bicarbonate solution. Also, when water is added to the reaction, this hydrolyzes the acetic anhydride to acetic acid, which is also neutralized by sodium bicarbonate. 1 Exp't 88 Procedure Wear gloves while mixing the reactants. Measure 1.6 mL of acetic anhydride into the short-neck round-bottomed flask from your microscale kit. Clamp the flask upright using the three-finger microclamp. Using a glass Pasteur pipet, cautiously add 3 drops of concentrated sulfuric acid, one drop at a time, letting the sulfuric acid run down the inside wall of the flask neck. Add 4 boiling chips and gently swirl the liquids so that they are thoroughly mixed. Now slowly add 1.3 mL of isopropyl alcohol. Caution--the flask will get very warm at this point--which is why the flask should already be clamped upright. Connect the air-cooled condenser column to the flask. Heat the mixture gently, just to the point that it begins to reflux. Be careful to observe that condensed vapors are returning to the reaction flask and that the reaction product is not distilling out of the top of the condenser. Reflux the mixture for 25 min. After the reflux period, let the mixture cool to room temperature. Once the solution is cool, add 2 mL of 5% sodium bicarbonate solution to the flask DROPWISE. Initially the bubbling will be very strong. Once the bubbling has subsided, transfer the mixture to one of your 13 X 100 mm test tubes. Notice that there are two layers of immiscible liquids. (Which layer is which?--you should be able to say from the density data in your Chemical Data Table, but you can do a drop test to be sure!) Remove the aqueous layer and add another 2 mL of 5% sodium bicarbonate solution. Gently mix the two layers by pipet mixing, and again remove the aqueous layer. Make sure all of the acid has been neutralized by adding a few more test drops of sodium bicarbonate; if there is still bubbling, add another 1 to 2 mL of sodium bicarbonate, mix, let separate, and discard the aqueous layer. In the last rinse, remove as much of the aqueous layer as possible. Add a small amount of sodium sulfate to remove traces of moisture, let stand for 5 min, then remove the organic layer by pipet filtration, leaving the drying agent in the tube. Transfer the organic liquid into the long-neck roundbottom flask from the microscale kit. Distill the liquid using simple distillation, being sure to record the temperature range over which distillate is collected, and transfer the distillate to a tared shorty vial. Cleaning Up The aqueous layers from the sodium bicarbonate washings can go down the drain with flushing water. The distillation residue should be discarded in the organic waste container. Analyses The product can be analyzed by NMR, GC, GC-MS, IR, or by refractometry. See your assignment strip for your assigned analysis. Questions Answers to the following should be included in your Final Report 1. Write balanced equations for all neutralization reactions that take place when the sodium bicarbonate solution was used to wash the reaction mixture. 2. At what step of the purification would unreacted isopropyl alcohol be removed from the product? 2 Synthetic Experiment PreLab Grading Sheet Name(s): TA: Date: PreLab For Exp't #: 88 Title: Mystery Product: The Acid-catalyzed Reaction of Acetic Anhydride and Isoopropyl Alcohol Possible Points Date, Name, Desk #, Experiment # & Title(abbreviated after 1st pg), Section & TA Name 8 Summary 10 Goals 10 Reactions, structures, diagrams 16 Completeness of Chemical Data Table(s) 14 PreLab Exercise 16 Spectral Features Comparison (NMR, GC, RI) 14 Work-up - Explanation of the product isolation and purification process 12 TOTAL FOR PRELAB 100 Missed Points Date Handed in: Total Points: General Comments: 3 Synthetic Experiment Final Report Grading Sheet Name: TA: Date: Final Report For Exp't #: 88 Title: Mystery Product: The Acid-catalyzed Reaction of Acetic Anhydride and Isoopropyl Alcohol Possible Points Name, Date, Experiment Title (abbreviated after 1st page) and every page numbered 4 OBSERVATION and DATA - Overall organization, readability, completeness 8 Data: Weighing data, molecular weights, moles, density, volumes, analysis conditions. (Examples: NMR solvent, IR – KBr and sample weight, GC analysis conditions sheet) 12 Yield: Show % yield calculations with limiting reagent clearly stated. 12 Missed Points Purity: boiling point, color, or other indicators of purity. RESULTS AND DISCUSSION - Overall organization, readability, completeness 8 16 Results; Achievement of goals, discussion of product Product Analysis Data: Quality and Interpretation – Structure(s) drawn on each Spectrum or Chromatogram and peaks interpreted. 24 Assignment and discussion of all NMR peaks, or major IR peaks, or GC peaks or RI. Unknown Determination: Correct ID of the “Mystery” Product See Lab Guide Chapter 3, Section 3.4 for guidelines in annotating spectra and Ch 11 for help with interpretation. POSTLAB PROBLEMS 16 TOTAL POINTS 100 Date Handed in: Total Points: General Comments: 4