Synthesis of Esters

Introduction:

The perception of flavor is a complicated physiological and psychological response that incorporates the sight, smell, taste, and texture of an object.  A large portion of the perceived flavor of a food actually comes from its fragrance.  Remember your last cold?  How did things taste?  Many foods, including spearmint gum and onions, do not have a 'taste' at all if your sense of smell is impaired.

The major components of most flavors and fragrances are a class of compounds known as esters.  Esters are derived from the reaction of carboxylic acids and alcohols.  Most aromas are not single compounds, but complex mixtures.  For example, over 200 esters have been identified in the 'rich aroma' of coffee.  However, several common fragrances have a major ester component.  The figure below shows the structure of some common flavors and fragrances:


The flavor and fragrance of many foods are 'enhanced' by adding these esters. 

Esters can be prepared through a wide variety of synthetic routes.  Several of the most common are shown below:





Purpose
:

The purpose of this experiment is to synthesis some common esters which are used as artificial fruit essences and components of perfumes.  You will choose an ester from the figure above and synthesis it using both the Fischer esterification and the acid chloride methods.  As you can see, the Fischer esterification is an equilibrium process while the acid chloride is not.  Finally, you will use your sense of smell, boiling point, IR, and TLC to help determine the purity of your product.
 

Procedure
:

Before you come into lab, make sure you have filled in your table of reagents and products.  You will need these values (particularly the molecular formula and molecular weight) to determine the identify of your products and to calculate your final yield.  You will also need to come to lab with IRs of your starting materials and expected product(s) already in your notebooks.  Finally, it is important that you know exactly what you are going to be doing so you can work more efficiently:


Fischer Esterification:

  1. Use your largest test tube, a one-holed stopper, and a Pasteur pipet to construct the reaction vessel (see figure above).  Be very CAREFUL inserting the pipet into the stopper.
  2. Add 25 mmol of the acid, 12.5 mmol of the alcohol, a boiling chip, and 8-10 drops of concentrated sulfuric acid to the test tube.
  3. Position the test tube in your aluminum bead bath and begin to heat.  Position the stopper and pipet on top of the test tube as an air condenser.
  4. Reflux for 30 minutes and adjust the heat as needed to maintain the reflux line about half way up the test tube.
  5. Cool the reaction mixture in an ice bath to below room temperature.
  6. Carefully add 5 mL of water to the mixture with stirring.
  7. Carefully add 5 mL of saturated sodium carbonate (drop wise) to the mixture.  Continue to add 1 mL portions of saturated sodium carbonate until no additional evolution of gas is seen.
  8. Add 3 mL of diethyl ether to the test tube and gently swirl the mixture.  Allow the layers to separate.
  9. Transfer the organic layer to a centrifuge tube with a Pasteur pipet.
  10. Dry the organic layer with magnesium sulfate and centrifuge.
  11. Transfer the dry organic layer into a clean, dry, PRE WEIGHED screw top vial.
  12. Place the vial in your aluminum bead bath at very low heat and evaporate the ether.  Place your snorkel hood to remove the ether vapors.
  13. Weigh the vial to determine your yield. 
  14. Carefully smell the product and describe.
  15. Determine the purity of your product by comparing your IR and boiling point to the literature.  Also conduct a TLC determination.

Acid Chloride Esterification:

  1. Carefully clean and dry your largest test tube.  You will need to CAREFULLY flame dry it to make sure all traces of water have been removed.
  2. Add 25 mmol of the acid to the test tube.
  3. Add an excess (a few drops more the required 25 mmol) of thionyl chloride to the acid in the test tube.
  4. Position the test tube in your aluminum bead bath and heat for 15 minutes.  Position the stopper and pipet on top of the test tube as an air condenser.
  5. Carefully add 12.5 mmol of the alcohol to the reaction mixture.
  6. Cool the reaction mixture in an ice bath to below room temperature.
  7. Carefully add 5 mL of water to the mixture with stirring.
  8. Carefully add 5 mL of saturated sodium carbonate (drop wise) to the mixture.  Continue to add 1 mL portions of saturated sodium carbonate until no additional evolution of gas is seen.
  9. Add 3 mL of diethyl ether to the test tube and gently swirl the mixture.  Allow the layers to separate.
  10. Transfer the organic layer to a centrifuge tube with a Pasteur pipet.
  11. Dry the organic layer with magnesium sulfate and centrifuge.
  12. Transfer the dry organic layer into a clean, dry, PRE WEIGHED screw top vial.
  13. Place the vial in your aluminum bead bath at very low heat and evaporate the ether.  Place your snorkel hood to remove the ether vapors.
  14. Weigh the vial to determine your yield. 
  15. Carefully smell the product and describe.
  16. Determine the purity of your product by comparing your IR and boiling point to the literature.  Also conduct a TLC determination.

TLC:

  1. Create a developing chamber by adding approximately 5 mL of developing solvent (hexanes) to a 150 mL beaker and covering it with plastic wrap.
  2. Dissolve a small amount of each starting material and each product in methylene chloride.
  3. Use a micro pipet to spot a sample of your starting materials and each of your products at the origin of the TLC plate.  (See this link TLC for more details).
  4. Develop the plate and record your observations (including drawings of the TLC plate) and Rf factors under UV light, and the iodine tank.


Conclusions
:

 (Updated 3/30/04 by C.R. Snelling)