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29. The Fischer Esterification
A. Background
Esters are an incredibly important functional group in organic chemistry. Esters are typically very pleasant smelling molecules and are therefore frequently used in the fragrance industry and as flavoring ingredients. Some representative esters are shown in figure 1 below. Additionally, esters are found in biological systems in the form of triacylglycerols (triglycerides), which are derived from glycerol and three fatty acid units. Triglycerides make up both fats and oils. A representative triglyceride along with its glycerol and fatty acid constituents are shown in figure
- The ester units contained within the structures in figures 1 & 2 are highlighted in red.
Figure 1. Some Representative Esters and their Fragrances
Figure 2. A Triglyceride Containing Three Ester Units
O O O (CH 2 ) 14 CH 3 O^ O O (CH^2 )^7 CH^3 HO OH OH glycerol O (CH 2 ) 7 CH 3 HO fatty acid
There are a number of different reactions that allow for the preparation of esters. In the previous laboratory experiment, you prepared aspirin by acylating salicylic acid, forming the molecule’s ester bond. Another frequently used reaction to form esters is the Fischer esterification, which was first reported by Emil Fischer in the late 1800s. The Fischer esterification involves reaction of a carboxylic acid with an alcohol. The products of the reaction are an ester and water. The reaction is catalyzed by addition of an acid such as sulfuric acid or phosphoric acid. In this laboratory experiment, you will prepare banana oil (isopentyl acetate) by the Fischer esterification of acetic acid and isopentyl alcohol (figure 3). The reaction will be catalyzed by the addition of sulfuric acid. Once the reaction is complete, a distillation will be performed to isolate the pure organic ester, which will be a liquid.
Figure 3. Fischer Esterification to Prepare Isopentyl Acetate
Figure 4 shows the mechanism for the acid catalyzed Fischer esterification. First, the carbonyl oxygen of acetic acid is protonated by the acid catalyst. This generates a highly activated form of the carbonyl electrophile. Next, the alcohol adds to the activated carbonyl carbon. Through proton transfer, the proton is removed from the ether oxygen and subsequently added to the alcohol oxygen to give a tetrahedral intermediate containing – OH 2 +, which is a great leaving group. Water then is lost from the molecule along with formation of the C=O double bond. Finally, the proton is removed from the carbonyl oxygen providing the ester product and regenerating the acid catalyst.
Figure 4. Mechanism of the Acid Catalyzed Fischer Esterification
The Fischer esterification is an equilibrium process. Energetically, the products have nearly identical stability to the starting materials and there is no significant driving force for the reaction. We must take advantage of Le Châtelier’s principle to generate and isolate a usable quantity of product. There are a few methods by which one can take advantage of Le Châtelier’s principle in order to shift the reaction equilibrium to the right. The first method involves removing one of the products from the reaction mixture as it is formed. Water is one of the reaction products and there are straightforward methods that will allow one to remove water from a reaction as it is O OH HO^ O O
H 2 SO 4
- H 2 O O OH HO H O OH H O OH OH (^) H Proton Transfer O OH 2 OH O O H
- H 2 O O O
- B-H B
Hickman still, suspend a thermometer into the very top of the vial using a septum containing a hole (do not use a screw cap). Continue heating until the aluminum block reaches a temperature of ~210 °C. You should observe the ester product collecting in the Hickman still. Be sure to record the temperature range at which the product collects in the still. Discontinue heating when the distillation temperature begins to drop off due to the fact that ester is no longer being collected. Transfer the distilled ester to a pre-weighed vial. Determine the yield and record an IR spectrum of the product. If time permits, determine the refractive index of your product. Using the wafting technique, observe the odors of acetic acid, isopentyl alcohol, and the isopentyl acetate product. Record these observations in your notebook. Note: During heating, be sure to remove the thermometer from the heating block well before the temperature limit of the thermometer is reached.
Figure 5. Distillation Apparatus
C. Prelab Questions
- When the reaction is complete and you cease heating, your reaction mixture contains mostly acetic acid, and the isopentyl acetate product. There may also be a small amount of isopentyl alcohol remaining, which is more soluble in ether than in water. Complete the extraction scheme below to show how the product is isolated during the extraction. You may want to take a look at post-lab question 1 to help you answer this question. Hickman still septum with hole resting on top of the Hickman Still heating block hot plate thermometer Note: You may need to put a small piece of wire between the septum and Hickman still to prevent formation of a sealed system
- Explain why it is necessary to distill your organic product?
- When setting up the distillation apparatus, you are instructed not to use a lock nut to tighten down the septum. This prevents the possibility of heating a closed system. Why should you avoid heating a closed system?
- Do you expect the ester that you form in the reaction (isopentyl acetate) to be water-soluble? Explain. D. Postlab Questions
- During the workup, you added NaHCO 3 to the reaction mixture. This base reacts with the leftover acetic acid. This reaction is the vinegar and baking soda reaction that you may have performed as a child. The final products of the neutralization reaction are CO 2 and water. a. Write a balanced equation for this neutralization reaction. b. What product is initially formed? This product decomposes to give CO 2 + H 2 O.
- What change, if any, would be observed if you increased the amount of sulfuric acid added to the reaction mixture?
- Considering the reversibility of the Fischer esterification, what would the reaction product be if isopentyl acetate were reacted with a large quantity of water in the presence of an acid catalyst?
- Describe the odor of your ester product (isopentyl acetate) compared to the odor of the reactants (acetic acid and isopentyl alcohol).
- Considering the boiling points listed in table 1 , would it be reasonable to attempt collecting your ester product as it is formed by a continuous distillation during the reaction?
- How does your experimental boiling point range compare to the literature boiling point value listed in table 1? HO O OH O O Ether
Aqueous NaHCO 3 Ether Layer Basic Aqueous Layer
