Download The Acid-Catalyzed Reaction of Acetic

Exp't 88
The Acid-Catalyzed Reaction of Acetic Anhydride and
Isopropyl Alcohol
by Robert Minard and Kurt Rublein, Organic Instructional Laboratories, Revision 10/8/04
Thanks to James Irvine, Former Chem 36 Student, for Revision Suggestions
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.
TLC
You are required to run a TLC to monitor the progress of the reaction. Plates should have three spots
(or lanes) on the origin: one for the main organic starting material that is being transformed, one for a
cospot (starting material and the reaction mixture), and one for the reaction mixture.
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Procedure
Wear gloves while mixing the reactants. Measure 1.6 mL of acetic anhydride into the short-neck roundbottomed 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 12 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 round-bottom 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
In addition to TLC, the product can be analyzed by NMR, GC, IR, or by RI. Analyze your sample
according to your assignment sheet and the instructions on Sample Preparation in the Lab Guide.
Postlab 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?
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