Lab Report 8 - Grade: A - Chemistry (Chem 238 - G5): Exp 8: Acid-Catalyzed Fischer Esters PDF

Preview

Summary

Lab Report 8...

Description

Fischer Esterification

Lead Author: Bradley Wurth Reviewer: Elijah Marsh Editor: Hannah Strickland

Chemistry 238 Section G5

Experiment 8

Introduction: Esters are a common functional group. They can also be a derivative of carboxylic acids. One way to prepare esters is through Fischer esterification. Fischer esterification is a method for making esters from carboxylic acids and alcohols in the presence of an acid catalyst. Acid-catalyzed Fischer esterification is reversible. Usually, at equilibrium the carboxylic acid and alcohol used in the reaction are still present along with the ester formed.1 This reaction can be controlled to produce esters in large yields. This can be done by removing water or by adding more alcohol.1 In this reaction, the carbonyl group is protonated. Once the carbonyl group is protonated, it becomes more electrophilic. Then, the oxygen on the alcohol acts as a nucleophile and attacks the carbonyl carbon. This pushes the electrons from the carbon-oxygen double bond onto oxygen, removing the positive charge. This is referred to as the tetrahedral intermediate. Then, oxygen in the alcohol group is protonated which causes it to form a good leaving group. The other alcohol group forms a double bond with the carbon, causing water to leave. The carbonyl group is de-protonated, and the ester is formed.1 This mechanism is shown in figure 1. All chemicals used in this experiment are shown in table 1.

Figure 1: Figure 1 shows the mechanism for Fischer esterification. Table 1: Table of Reagents2 Compound

Molecular Weight (g/ mol)

Boiling Point (°C)

Melting Point (°C)

Density (g/ cm3)

acetic acid

60.05

117.9

16.6

1.05

carbon dioxide

44.01

-78.46

-56.6

0.11

sodium bicarbonate

84.01

851.0

50.0

2.20

142.04

1429.0

884.0

2.66

sulfuric acid

98.07

337.0

10.3

1.83

water

18.02

100.0

0.0

1.00

sodium sulfate

Experimental: A 5 mL round bottom flask was obtained, and 0.7 mL of unknown #571 was added to the flask. Then, 1.4 mL of glacial acetic acid was added to the flask along with 3 drops of sulfuric acid and a stirring bar. A condenser was attached to the round bottomed flask, and a wet paper towel was wrapped around the condenser. The round bottom flask was then placed into a sand bath. The power mite was set to 30. The sand bath apparatus was then placed on a magnetic stirring plate. The stirring plate was turned on. The reaction was allowed to stir and heat for 60 minutes. Right away bubbles formed and the mixture turned a light yellow color. After 60 minutes, the solution was a darker yellow color. The flask was then removed from the sand bath and allowed to cool to room temperature. Then, the mixture was transferred to a conical tube. Next, 1 mL of sodium bicarbonate. Carbon dioxide bubbles formed. The solution was stirred until there were no bubbles remaining. The aqueous layer, the bottom layer, was removed with a pipette. This process was repeated two more times. Next, sodium sulfate was added and the solution was covered for five minutes. Finally, 3 drops of the product were added to deuterated chloroform. This mixture was placed in an NMR tube, capped, and submitted for NMR analysis. Results: In this experiment, an acid-catalyzed Fischer esterification reaction was conducted to react acetic acid with an unknown alcohol to form an ester. A liquid product mixture was obtained from this reaction. Proton NMR spectroscopy was conducted. The proton NMR spectrum obtained is shown in figure 2. The peaks in the spectrum were then analyzed. This analysis is shown in table 2.

Figure 2: Figure 2 shows the proton NMR for the unknown alcohol #571. The 5 major proton peaks are labeled, as well as impurity peaks. Table 2: Proton NMR for Unknown #571 Label

Multiplicity

Integration

Chemical Shift (PPM)

A

3

2

4.0 - 4.2

B

1

3

2.0 - 2.1

C

7

1

1.5 - 1.6

D

4

2

1.4 - 1.5

E

2

6

0.8 - 1.0

Discussion: Acetic acid and an alcohol, unknown #571, were reacted to form an unknown ester. This was done through the process of acid-catalyzed Fischer esterification. During this reaction, acetic acid, unknown #571, and sulfuric acid were added together and allowed to react for 60 minutes. The sulfuric acid was the acid catalyst. During this time, an unknown ester was forming. This mechanism is shown in figure 1. A condenser was used during this reaction to aid in reflux. In this reaction, refluxing helps drive the reaction to completion. Refluxing allowed the mixture to be heated without much loss of product. When vapor reaches the condenser, it is condensed back into a liquid, and then falls back into the flask.3 The wet paper towel helped aid in this process. Sodium bicarbonate was used in this reaction to aid in the purification of the ester. Because the reaction was prepared under acidic conditions, washing it with basic sodium bicarbonate helps neutralize the solution, and forms salts and alkaline materials that can then be removed in the aqueous layer. Sodium bicarbonate also aids in the separation of the ester and aqueous layers.3 Sodium sulfate was used in this reaction to dry the product. Drying in this reaction is necessary because water is a byproduct. Le'Chatellier's principle states that if more product is added to a reaction, then the reaction will be driven backwards, thus forming more reagents. In this case, if more water is added, then more reagents will form and less ester products will be formed.3 In this experiment, the unknown alcohol was analyzed by using proton NMR spectroscopy. The proton NMR spectrum indicated 5 different areas of hydrogens, as well as 2 impurity peaks. The spectrum was analyzed. The impurity peak at 2.1 ppm was most likely acetic acid. The impurity peak at 4.7 ppm was most likely isoamyl alcohol.4 Because there was an integration of 6 at peak E, it can be inferred that there were two equivalent methyl groups in the product. Because these methyl groups had a low chemical shift, they were most likely farther away from the carbonyl group. The integration of peak C indicates

that it is the carbon that is bonded to the two methyl groups as well as the rest of the molecule because there is only one hydrogen present and the multiplicity is 7. Due to it’s multiplicity, this would indicate that a carbon is bonded to it. This carbon is most likely peak D because it has an integration of 2 and a multiplicity of 4. The multiplicity indicates that it is bonded to another carbon. This carbon is most likely peak A. Peak A has an integration of 2 and a multiplicity of 3. The multiplicity indicates that it is not bonded to another hydrogen containing molecule besides the carbon labeled peak D. The only peak left is peak B. Due to its far downfield location on the proton NMR spectrum, it is most likely bonded to a carbonyl carbon. This agrees with the multiplicity which is 1 and the integration which is 3. After the analysis, it was confirmed that ester was most likely isoamyl acetate. In order for isoamyl acetate to be formed from acetic acid, the alcohol used must have been isoamyl alcohol. This concludes that unknown #571 was isoamyl alcohol. The structure for isoamyl acetate and isoamyl alcohol is shown in figure 3.

Figure 3: Figure 3 shows the structure for unknown #571, which is isoamyl alcohol. This figure also includes the ester product, isoamyl acetate. The structure also includes the corresponding proton NMR peak labels. Conclusion: During this experiment, a Fischer esterification reaction was done with the reactants acetic acid and isoamyl alcohol. This reaction formed isoamyl acetate. Sulfuric acid was used as the acid catalyst during this reaction. Isoamyl alcohol was the unknown. The unknown was identified through the process of proton NMR spectroscopy. The hydrogens of isoamyl acetate correlated with the hydrogen peaks on the proton NMR spectrum, thus confirming the unknown. More analysis could have been done on the final product. IR spectroscopy could have been done to confirm the functional groups. Boiling point could have also been conducted to help aid in the process of identifying the unknown. A way to improve this experiment would be to use a reflux column with water instead of a condenser with a wet paper towel wrapped around it. This could have aided in the process of more efficiently condensing the mixture.

References: 1Brown,

W. H.; Iverson, B. L.; Anslyn, E. V.; Foote, C. S. Organic Chemistry; Wadsworth Cengage Learning: Australia, 2014. (accessed Mar 28, 2017). 2The PubChem Project https://pubchem.ncbi.nlm.nih.gov/ (accessed Mar 11, 2017). 3Structural Biochemistry/Organic Chemistry/Method of Fischer Esterification https:// en.wikibooks.org/wiki/Structural_Biochemistry/Organic_Chemistry/ Method_of_Fischer_Esterification#Reflux_through_a_Condenser (accessed Mar 28, 2017). 4Fulmer, G. NMR Trace Impurities http://www.sas.upenn.edu/~marisa/documents/ OrganoMetSolv.pdf (accessed Mar 28, 2017)....