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Experiment 7: Acidity of Alcohols Williamson Ether Synthesis of Methyl Propyl Ether Alcohols, like water, are weak acids. The hydroxyl group can act as a proton donor to form an alkoxide ion. Alkoxide ions dissolved in alcohol, like hydroxide ions in water, are strong bases which can be prepared by the reaction of an alcohol with sodium or potassium metal or with a metal hydride. In this experiment, you will be examining the acidity of alcohols by reacting the alcohols with sodium metal. 2 ROH + 2 Na 2 RO-Na+ + H2 sodium alkoxide Once formed, alkoxide ions are useful strong nucleophiles (and strong bases) that can be used in a variety of SN2 and E2 reactions. In this experiment, sodium methoxide will be prepared and used as a nucleophile in SN2 reaction with bromopropane to synthesize methyl propyl ether. This is another variation on the Williamson ether synthesis used previously in experiment 5. Procedures: A. Reactivity with sodium Safety Precaution: Be extremely careful when handling sodium. Be sure that all containers are dry, and that no water can come into contact with the metal. Take the smallest piece of sodium that you can handle with a spatula or a pair of tweezers. Avoid skin contact. DO NOT DISPOSE OF ANY REACTION MIXTURES UNTIL THE SODIUM IS COMPLETELY CONSUMED. IF THERE IS A NEED TO DISPOSE OF UNREACTED SODIUM METAL, IT IS TO BE PLACED IN THE BEAKER IN THE HOOD MARKED "SODIUM WASTE". In separate, dry test tubes, place 2 mL each of methanol, ethanol, 2-propanol (isopropyl alcohol) and 2-methyl-2-propanol. Using tweezers, add a small (ca. 2 mm across) piece of sodium to each test tube. Record all observations in your notebook. What do you observe? What gas is formed from the reaction? Compare the rate of gas evolution relative to each compound. Arrange the alcohols in order of increasing acidity based on your results and explain why. It will be helpful for you to draw the structures of each compound in your notebook. What effect does an electron-donating group like a methyl (-CH3) or an alkyl group have on the resulting alkoxide produced from the reaction with sodium? Use pH paper to measure the approximate pH of each alcohol solution after the sodium has reacted. Record the pH's. Add 1mL of methanol to each of the test tubes to react with any residual sodium before disposing of each solution down the sink. B. Williamson Ether Synthesis of Methyl Propyl Ether CH3CH2CH2Br CH3O Na CH3CH2CH2OCH3 Na Br The first step in this synthesis is preparation of sodium methoxide by reaction of anhydrous methanol with sodium metal. Anhydrous methanol (10 mL) is added to a clean, dry 25 mL round bottom flask containing a stir bar. The flask is fitted with an air condenser and a water cooled condenser as shown in Figure 1. About 10 mL of dry xylenes is added to a clean, dry 50 mL beaker. The beaker containing the xylenes is weighed on a balance in a fume hood. Small pieces of sodium metal (stored under xylenes) are added to the beaker obtain a net weight of about 0.75 grams of sodium. (Note: the weight of sodium doesn't have to be exact but should be in the range 0.60 to 0.90 grams.) The sodium pieces are carefully transferred from the beaker to the round bottom flask containing methanol and allowed to completely react. The resulting sodium methoxide solution is cooled to room temperature and 2.00 mL (2.41 g) of bromopropane is added and the flask is fitted for fractional distillation as shown in Figure 2. With the receiver flask cooled in an ice bath, the reaction mixture is distilled, collecting as one fraction all distillate coming over below 50 oC. The product is weighed to determine % yield and analyzed by 1H NMR. Figure 1 Figure 2