Experiment 7: Acidity of Alcohols Williamson Ether Synthesis of Download

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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
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