Chem 102b Experiment 14: Part II Revised Preparation of Esters

Transcription

1 Purpose of the Experiment: Chem 102b Experiment 14: Part II Revised Preparation of Esters Students will be given alcohols and carboxylic acids to synthesize various esters. Their products will be purified using extraction techniques. The esters will be characterized by determining their boiling points using simple distillation techniques and by Infrared absorption spectroscopy analyses. Safety Considerations: Students should note that acetic acid is corrosive. Also, take care not to overload olfactory receptors in the nose when comparing the odors of the esters. The following procedure is to replace the one given in your lab manual. Carboxylic esters are synthesized by the condensation of carboxylic acids with alcohols. Carboxylic acids usually have pungent unpleasant odors. Esters, on the other hand, often have pleasant odors and are used as additives in artificial flavors and perfumes. The small structural change in the carboxylic acid yielding a carboxylic ester yields a noticeable change in the physical properties of the compound such as smell, boiling point, etc. Duplication of a naturally occurring flavor or smell often requires a mixture of 10 or more esters. Rarely is an artificial flavor composed of just one ester. Some common flavors are listed below: Ester Ethyl acetate Isopentyl acetate Benzyl acetate Octyl acetate Ethyl butanoate Isopentenyl acetate Flavor/Odor Fruit Banana Pear Orange Pineapple "Juicy Fruit"

2 Fischer esterification yields equilibrium between the reactants and products. As Le Chatelier s principle states, the equilibrium system will adjust to minimize any stress applied to the system. If the concentration of either of the reactants is increased or if the concentration of either of the products (i.e. carboxylic acid and ester) is decreased, the reaction will "shift to the right." Thus, the yield of the products will be increased. Anhydrous sodium sulfate is a drying agent that absorbs water. If anhydrous sodium sulfate were added to the product collection flask, water would be removed and the reaction would be shifted to the right. The yield of the ester would increase. The experiment is to be performed by a group of 4 students. Each member of the group will make, isolate, and characterize one of the esters listed below using the Fischer esterification techniques. The product will be a liquid that will be purified via a two-step process. Purity of the product will be determined using Fourier Transform Infrared absorption Spectroscopy. Ester Alcohol Acid Boiling Point Ethyl Acetate Ethanol Acetic Acid 77 C Benzyl Acetate Benzyl Alcohol Acetic Acid 213 C Isopentyl Acetate Isopentanol Acetic Acid 142 C Octyl Acetate Octanol Acetic Acid 199 C IR spectra will be collected for each of the esters synthesized and compared within each group. IR spectra are specific for every compound and can be considered to be a kind of "fingerprint" for that compound. As you learned in the qualitative analysis experiment, compounds that share the same functional groups share some of the same properties. After the esters have been synthesized and characterized, the groups should compare boiling points and IRs of the products. You will be given spectra of the reactants. Please compare your analysis to the IRs of the reactants and make some assessments about the completion of your reaction. Please note that only the alcohol used to make the ester has changed. Consequently, the spectra for all the esters will have many similarities take care to note the differences. Glassware: You will need to check out a macrokit from the preproom. In the macrokit there will be 2 round bottom flasks (a 250-mL and a 100 ml); a condenser, a vacuum adapter, a distillation adapter, and a thermometer adapter. You will also need 2 lengths of tygon tubing per student and a mercury thermometer that will measure about 400 C. Taped to the inside of the macrokit lid is a list of glassware that should be in the kit. Please make certain all items are there and are not broken. The kits cost approximately $65.00 apiece. You need to be very cautious with this glassware.

3 NOTE: Begin the esterification at the beginning of week 7 before the pre-lab lecture and recrystallization of your aspirin product. The Reaction: 1. The following alcohols will be available for the reaction: ethanol, isopentanol, benzyl alcohol, and octanol. You will use one of these according to your TA s instructions. 2. Obtain an aluminum dish and fill - full with sand. Place on a hotplate and heat moderately (turn dial to 3 or 4). 3. Check out macrokit from preproom. 4. Place 2-3 boiling chips in the 100-mL round bottom flask (reaction flask). 5. Place 5 ml of your assigned alcohol into the reaction flask. 6. Place 10 ml of acetic acid into the reaction flask. 7. Add 10 drops of concentrated H 2 SO 4 to the reaction flask. Cut a cotton ball amount of glass wool and place the plug loosely in the end of the condenser. 8. When the reflux apparatus is ready to go, have your setup checked by your instructor. 9. Lower the reflux apparatus into the sandbath moving the sand until the baseline of the sand covers the bottom 1/3 of the flask. Increase the heat to 5-6. Reflux for minutes. Cool the reaction mixture. B: The Extraction 1. While stirring slowly add 5 ml of the 5% sodium bicarbonate into the reaction vessel. Swirl the flask until CO 2 formation ceases. 2. Transfer contents of the reaction mixture to a centrifuge tube. Shake the mixture vigorously for 2-3 minutes. 3. Allow the two layers to separate. Using a micropipet, remove the lower layer (the water layer) and put it into a small beaker. 4. Add another 5 ml of the sodium bicarbonate solution and shake again for another 2-3 minutes. Again remove the bottom water layer. 5. Repeat one more time for a total of three bicarbonate extractions. The third time, try to remove as much water as possible. 6. Add 1 g of anhydrous sodium sulfate. This is a drying agent and will remove remaining water in the flask. Let the flask stand for at least minutes. This is a good place to stop if time is short. Start cleaning your glassware. Be certain to return the macrokit intact to the preproom.

4 C: The Distillation 1. Transfer the dry ester to the round bottom flask and assemble a distillation apparatus. Remember to clamp the round bottom flask, the condenser, and the vacuum adapter to a ring stand. When inserting the thermometer into the thermometer adapter, use extreme caution. The position of the thermometer needs to be at the "y" junction of the distilling adapter to correctly determine the boiling point of the ester. 2. Use a clean 250-mL round bottom flask (which is clamped to ringstand) to receive the distillate. 3. Have your instructor check your setup before proceeding. 4. Raise the sandbath/hotplate (seated on a ring that is clamped to a ringstand) to the reaction flask. Be certain to cover the lower 1/3 of the reaction flask with sand. 5. Turn the hotplate on high. Make a note of your ester s boiling point. When you are within 20 degrees of your bp, lower the heat to a moderate heating level. Record temperature every minute and make observations in your lab notebook. 6. Note the temperature when material begins to come out of the adapter. Note the temperature again when it stops coming out. This is your boiling point range for your ester. 7. The material you have collected should be your pure ester. You will take an IR spectra of this material. Please label your flask according. Note the smell of your group s esters in your lab notebook. D: Infrared Spectroscopy 1. Check out an IR card from the 3 South preproom window. 2. Your instructor will demonstrate how to run a background on the instrument. 3. Using a micropipet, place 1-2 drops of your ester onto the card s surface. 4. Run a spectrum of your ester. 5. Print the spectrum. 6. You will have IRs of the starting materials: ethanol, benzyl alcohol, isopentanol, octanol, and acetic acid. 7. Compare the IR of your ester with the starting materials and the other esters in your group. Which peaks are shared; which peaks are unique to your spectra; do you have any starting materials in your sample; do you have any water in your sample? Remember that it is not efficient or effective to try to identify all the peaks of the spectrum. The peaks that should interest you most are at Ester cm -1 (C=O) strong absorption, cm -1 (C-O) usually gives two or more absorptions with one stronger than the others do. Acetic Acid will have a very broad O-H peak at cm -1, the C=O peak will be a broad stretch at cm -1, and the C-O peak will be strong in the cm -1 range.

5 8. NOTE: Water is a product of the esterification reaction. It has an O-H bond that will be a broad stretch at cm -1. If you have a stretch in that range, note the cm -1 range to identify if you have starting material (acetic acid) or water in your sample.