Advance Preparation

Set the reagents in a fume hood—if one is available—to keep the background room odor to a minimum. Otherwise, open your windows to help circulate the room air. It is not necessary to provide the reagents for all the esters to be synthesized. A subset of three of four esters can be used (e.g., pineapple, banana, wintergreen, orange.) Label both reagent containers and dispensers to avoid contamination. A good way to keep the reagent container and dispenser together is to tape a test tube to the reagent bottle and place the polyethylene pipette used to dispense the reagent in the test tube. The cork stoppers used in this laboratory need to accept the polyethylene pipette with a tight fit. Polyethylene pipettes with tapered ends work nicely. You probably will have to dispose of corks after each laboratory because they pick up the odor of the synthesized ester and will confuse the next set of students if the corks are recycled. If your cork borer does not drill a small hole, you can clamp the solid cork stoppers in a vise and drill holes in them with an electric drill fitted with a small drill bit. Have a copy of the following chart posted to identify functional groups.

Click here for a printable version of this chart.

Pre-Lab Discussion

It is important to demonstrate the proper method for detecting odors. In one hand, hold the open test tube about 15 cm from your nose. Using your free hand sweep the air space above the test tube toward your nose. Discuss the concept of organic synthesis, particularly the synthesis of esters (called esterification). The mechanism for ester synthesis is well established but discussing this mechanism may not be appropriate for beginning students. You might have students try to determine the systematic name for the esters that they make. This would help determine which carboxylic acid and alcohol to use for the synthesis. You will notice that some compounds have common names. To avoid confusion, be careful in labeling the compounds. For basic or general chemistry students, tell them which carboxylic acid to mix with which alcohol to produce a particular ester.

Alternatively, support student learning by having students predict in the pre-lab which ester is produced by particular combinations of carboxylic acid and alcohol, and to compare the structure to the table of esters. Once students have predicted the structure and odor of particular combinations, assign combinations to student laboratory groups to synthesize.

Teacher-Student Interaction

Help students observe physical and chemical changes that take place when the reaction mixture is being heated. Discuss the purpose of the boiling stone [provides a site for bubbles to form during boiling]. Discuss the concepts of catalysis, equilibrium, and Le Chatelier's principle.

Anticipated Student Results

Students should recognize odors like banana and wintergreen. Other odors may be less readily identified. Students' odor palates are easily overwhelmed and, thus, they may confuse odors. Students should realize that natural odors are mixtures. Many students will be able to write the reaction equation for any ester after writing the equation for their ester; others will need to see more than one example before they can generalize.

Implications and Applications

1. In some cases the odor of the synthesized ester does not exactly match the odor of the fragrance found in nature. What might be a possible explanation?
   Answer: The fragrances found in nature are most often due to a mixture of compounds rather than a single one. Fresh ground coffee contains at least 200 identifiable esters.

2. Some of the compounds in the list of fragrances have functional groups in addition to the ester group. Identify these compounds and determine the types of functional groups present with the help of the functional group chart posted.
   Answer: –OH hydroxyl, –NH₂ amine, –C₆H₅ phenyl group (aromatic compound).

Post-Laboratory

This activity is designed to provide students with hands-on experience in conducting an organic synthesis. Students should be able to discuss the industrial role of organic synthesis in making compounds that are difficult or expensive to isolate from naturally occurring sources. Vanilla is an example of a synthesized flavoring. Aspirin is a familiar example of a synthesized drug (see the Aspirin Adventures module). Most of the vanilla and all the aspirin used today are synthesized. Students should also be asked to discuss the benefits and hazards of preparing synthetic compounds not found in nature. Various pesticides, herbicides, and polymeric substances are everyday examples. Discuss benefits and abuses of these substances.

Extension Options

You may choose to challenge your students with the following assignments.

1. The ester functional group is very important in the synthesis of polymers. Look up the molecular structure of polyester polymers and determine the compounds used in their synthesis.
   Useful link: The Macrogalleria — comprehensive polymer resource from the University of Southern Mississippi.

2. Look on the label of common household products to see if you can identify the esters.

3. Suppose you looked on a bottle of fingernail polish remover and found it was composed of ethyl acetate. How would you know it was an ester? How would you prepare this ester? (Smell. Can be prepared from ethanol and acetic acid.)

4. Make molecular models of a carboxylic acid, an alcohol, and the ester that can be synthesized from them.

For more information, at other Web sites...

Medicines by Design: The Biological Revolution in Pharmacology — from the National Institute of General Medical Sciences, National Institutes of Health.

Relevant National Science Education Standards

Unifying Concepts and Processes — A chemical reaction is a system, and this activity lets the students alter the behavior of a system (that is, its products) by selecting the system's components (that is, its reactants).

Physical Science — The atomic and molecular nature of matter is central to this activity, as is the concept of chemical reactions.

Science and Technology — The activity lets students carry out their own technological design, allowing them to design an ester and synthesize it in the same way that chemists design new molecules. This reinforces understanding of the technology of designed materials.

Relevant New Jersey State Science Education Standards

5.4	The activity demonstrates the technology of designed materials and its roots in the knowledge of chemistry by letting students design and synthesize their own esters.
5.6	The atomic and molecular nature of matter is central to this activity, as is the concept of chemical reactions. In addition, this activity lets the students alter the behavior of a chemical reaction by selecting its reactants.