Carboxylic Acids and Esters Synthesis of Various Esters and Determining their Fragrance Properties
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Fragrances and Odors The detection of fragrances and odors by
animals is a complex biochemical process. Fragrances and odors are detected through the interaction of small molecules in the air with olfactory receptors. Odor is a very important factor in the taste of food. An odor can sometimes trigger distinct memories from the past.
Olfactory System
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Odors of Carboxylic Acid Esters Most fragrances of fruits and flowers are a
result of a complex mixture of odorant molecules. However, many carboxylic acid esters alone give characteristic fruity odors. We will be using odor as our test of product formation in today’s experiment.
Carboxylic Acid Esters Carboxylic acid esters can be prepared by the
Fischer Esterification Reaction. This reaction involves the acid-catalyzed reaction of a carboxylic acid with an alcohol. O R C
+ OH
R'
OH
H3O +
O R C
OR' +
HOH
This esterification reaction is an equilibrium
reaction with starting materials and products containing about the same energy; Keq ≈ 1 - 4
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Carboxylic Acid Esters We will not be isolating products, so the incomplete
conversion is not a problem in this lab. The esterification reaction produces products that are higher molecular weight than the starting materials and have no -OH groups to participate in hydrogen bonding. O R C
+
R'
OH
H3O +
OH
O R C
OR' +
HOH
The low molecular weight carboxylic acids and alcohols
will be more soluble in the water layer than the ester layer. Thus, they will not be present in significant amounts to mask the fragrance of the ester product in the procedure we are using.
General Procedure You will work in teams of three students. Each
student will prepare four different ester products. Thus, each team will prepare a total of 12 (twelve) different esters - out of 36 total possible combinations. Some specific combinations are required in Tasks I and II.
You will need to record the fragrances for the
starting carboxylic acids, the starting alcohols, and the product esters for all 12 combinations.
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Procedure - Esterification You will conduct the reactions in test tubes. Concentrated
sulfuric acid will be used as the catalyst. The tubes will be warmed for 15 minutes, then water will be added to create two layers. The ester product will form the organic phase on top of the water. The water will contain most of the sulfuric acid as well as unreacted alcohol and acid. For each ester product, remove a few drops of the ester layer with a Pasteur pipette and place it on a watch glass. This increases surface area for evaporation. Wait a short time to let any starting alcohol evaporate, then determine the fragrance of the ester product. Rinse the watch glass and test another product.
SAFETY PRECAUTIONS! DO NOT try to smell the reagents directly from the
bottles. Waft your hand over an open bottle toward your nose. Sniff cautiously; don’t snort the odors! Team members should try to reach a consensus on each of the odors. If necessary, you may record two different odors that reflect your team’s impressions. Don’t try to smell all of the ester products at one time; take a short break to clear your odor receptors. Some people recommend inhaling coffee bean odor to clear other odors.
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SAFETY and Clean-up Sulfuric acid is toxic and corrosive. Clean up any spills
immediately. Use sodium bicarbonate to neutralize the acid after flushing with water. The lower molecular weight carboxylic acids are irritants and can cause skin burns at high concentrations. Vinegar is dilute acetic acid. Dispose of your products and glassware rinses in the appropriate waste bottles.
