Ester Synthesis and Saponification

Preparation: McMurry, Ch. 21.4 and 21.7.

PART 1: Synthesis of an Ester

Using the weights given below, calculate the volumes in microliters needed of 2-methyl-1-propanol and propanoic acid for the reaction above.

Procedure: To 5 mL vial add the volume equivalent of 125 mg of 2-methyl-1-propanol (isobutyl alcohol) and 150 mg of propanoic acid (= propionic acid). Make a note of the odors of each of these reagents. Then add 50 mg of cation ion exchange resin in the hydrogen form and a spin vane. Attach the condenser (no water circulation is necessary), monitor the temperature frequently and heat the reaction just to reflux temperature (observed by the barest signs of condensation above the liquid). Maintain the reflux for 1 hr and cool it to room temperature (make another note of the odor of the product at this stage.)

Chromatography: Vertically clamp a Pasteur pipet with a very small wad of glass wool, 1/2 g of silica gel (no solvent) and 1/2 g of potassium carbonate; just before use, run ca. 5 mL of dichloromethane through the column and recycle it until most of the air bubbles are gone. Use a pipet bulb as a cap to prevent the top of the dichloromethane from falling into the potassium carbonate layer.

Preweigh a clean 25 mL beaker or flask to serve as a collector when you run the product through the column. Remove the chromatography cap and using a Pasteur pipet, transfer the product from the vial, (leaving behind the ion exchange resin) onto the top of the column. Rinse the vial with another 0.5 mL of dichloromethane and transfer to the column; repeat with three more 0.5 mL portions of dichloromethane.

Why is the reaction mixture passed over potassium carbonate?

Analysis of the Product:  Evaporate the dichloromethane gently on the steam bath (use a boiling splinter until the liquid can no longer bump.) Once the dichloromethane has been definitely removed make a final note of the odor of the product. Determine the weight of the ester and its boiling point and calculate the percent yield of theoretical (set up the calculation in a Reaction Table).  Obtain an IR spectrum of the product to identify it.  Fully interpret the IR spectrum.

How does the material removed relate to the change in odor before and after chromatography?

Write a detailed mechanism for the reaction you carried out.

PART 2: Saponification of Methyl Benzoate

What does saponification mean?  You may want to reconsult McMurry.

Place 2.0 g of methyl benzoate, 20 mL of 2 M NaOH, and a boiling chip in a 50 mL round bottom flask and attach a condenser with circulating cold water. Reflux the mixture until it becomes homogeneous (one-half hour or so). After the solution has cooled somewhat, transfer it to a 125 mL Erlenmeyer flask. Wash the round bottom flask with about 5 mL of water and add this to the Erlenmeyer.

Acidify the mixture with concentrated HCl.  How do you know enough HCl has been added? What is the white precipitate that is forming?

Heat the mixture to boiling and add enough water to just dissolve the solid (Why are you doing this?)

Allow the solution to slowly cool to room temperature. Collect the crystals by suction filtration after the crystallization is complete.  Allow the product to air dry, then determine its weight and melting point. Submit the product in a sealed plastic bag.

What are the products of this reaction?  In your notebook write a complete mechanism for the reaction you carried out. Be certain to write Lewis structures (molecular structures) for all the reactants and products involved in this reaction.

References:

"Mayo et al.": Mayo, D.W., Pike, R.M., Butcher, S.S. and Trumper, P.K. Microscale Techniques for the Organic Laboratory; Wiley: New York, 1991.

"McMurry": McMurry, John, Organic Chemistry, 4th ed., Brooks/Cole Publishing Co.: Pacific Grove, CA, 1995.

Williamson, K.L. Macroscale and Microscale Organic Experiments, 2nd ed., Heath: Lexington, MA, 1994. p. 391.

Revised by SLP on 1/5/99