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Fischer Esterification

Lead Author: Ashlyn Kenwright Reviewer: Ryan Salazar Editor: Amber Crim

Introduction Esters are a common derivative of carboxylic acids1. Fischer esterification is a basic condensation reaction between a carboxylic acid and an alcohol to form an ester. This reaction requires an acid catalyst which is used to protonate the carbonyl group making it a better electrophile for the attachment of the alcohol2. The catalyst commonly is sulfuric acid or tosic acid3. The final product shows a hydroxyl group on the carboxylic acid getting replaced with the R group of the alcohol. Esters are fairly common, though few people realize it. Esters are responsible for smells from certain flowers and fruits and are commonly used in commercial perfumes1. This reaction can be used to produce the specific esters needed to produce these scents by using particular carboxylic acids and alcohols, along with an acid catalyst. One thing to note for the reaction is that Fischer ester synthesis is reversible and an equilibrium is reached between the products and reactants. Some ways to increase product yield are by increasing the concentration of one or more reactants or removing one of the products from the reaction. This occurs from Le Chatelier’s principle1. The mechanism for this reaction involves a proton donor, typically in the form of an acid, to initiate it. When the reaction is complete, the acid is regenerated, and a water molecule is produced. This mechanism is shown below in Figure 1.

Figure 1. The mechanism for the Fischer Esterification reaction.

H O H3C

O

+

HO

S

O

+

O H3C

O

OH

H

R

H3C

O R

H3C

O H

H HO

OH

H H O+ O

H

+

+

O

H

R

OH

R

O O S O OH

O

H3C

+

O

OH

O R H3C

+

H2O

+

HO

S

O

O O

Looking at only the initial reactants and final products in Figure 1, it may seem that only the alcohol group is affected on the initial carboxylic acid. However, the initial hydroxyl group becomes the carbonyl group in the ester, while the original carbon on the carbonyl group attaches to the R group on the alcohol being introduced. Also at the end of the reaction, to reform the carbonyl group for the product, a hydrogen must be removed from the oxygen. This hydrogen

returns to the acid catalyst and reforms the acid. The final molecule present at the end of the reaction is an ester and water.

Table 1. This table consists of the reagents used in this experiment and the chemical properties. Name Acetic Acid Sulfuric Acid NaHCO3 Na2SO4

Molecular Weight (g/mol) 60.05 98.079 84.007 142.04

Boiling Point (°C) 118 337 851 1429

Melting Point (°C) 60.8 10 50 884

Density (g/mL) 1.05 1.84 2.20 2.66

Experimental To begin this experiment, 0.7 mL of unknown alcohol #461, 1.4 mL of acetic acid, and 3 drops of sulfuric acid were added to a 5 mL round bottom flask. A stir bar was also added, along with a condenser. This was placed in a sand bath and heated (the sand bath was on top of a stir plate). The mixture was then stirred and heated for 75 minutes. When the time was complete the solution was allowed to cool to room temperature, the condenser was removed, and the mixture was transferred to a conical tube. 1.0 mL of NaHCO3 was added to the tube and the solution was stirred until no more CO2 was produced. The aqueous layer was then removed and discarded. Another 1.0 mL of NaHCO3 was added and again stirred and the aqueous layer was removed and discarded. The organic layer was removed and transferred to a sample vial, and a small amount of Na2SO4 added to remove any remaining water. The final solution was transferred to an NMR tube and the NMR was performed. The glassware was cleaned and returned to the drawer.

Results Graph 1. The graph below shows the H-NMR results for the final product.

B E

A

D C

Table 2. This table shows the chemical shift, multiplicity, and integration of each labeled peak on Graph 1. Label A B C D E

Multiplicity Triplet Singlet Septet Quartet Doublet

Integration 2 3 1 2 7

Chemical Shift 4.1 2.0 1.6 1.5 0.9

Figure 2. Below, the final product is drawn with the hydrogens labeled according to Graph 1. E CH3

O

H HD

H3C

B H

H A

C

O

CH3

H

Graph 2. The graph below shows the carbon-NMR results for the final product.

E

B C

A

D

F

Table 3. The table below shows the chemical shift of each labeled peak on Graph 2. Label Chemical Shift A 171.4 B 63.2 C 37.3 D 25.1 E 22.5 F 21.1 It should also be noted that the peak at approximately 78 ppm is the chloroform that was used to run the NMR analysis.

Figure 3. Below is the final product with the carbons labeled according to Graph 2.

B E

CH3 H3C

D

O O

C

F CH3

A

Based on both of these graphs, the final product was isoamyl acetate, and the alcohol used in the reaction was 3-methyl-1-butanol.

Discussion The first step of this mechanism is vital. It is the protonation of the carbonyl oxygen on acetic acid by sulfuric acid. This allows the reaction to proceed and lets the oxygen on the alcohol of the unknown bind to the carbon of the carboxylic acid. Towards the end of the reaction, sulfuric acid removes the hydrogen on the oxygen that has a double bond to the carbon in the ester. Sulfuric acid also helps remove water which is a product of the reaction. Reflux was used in order to speed up the reaction. The use of reflux, on top of having water removed helped increase the amount of ester being made. Since this is a reversible

reaction, removing unwanted products as they are made will help the formation of the desired product to reach completion. Also, there was a lot of acetic acid at the start of the experiment. This large amount of reactant will help push the reaction forward and make a lot of the desired ester product due to Le Chatelier’s principle. The addition of Na2SO4 removed any left over water and should have left the ester as the final product. This would give a very accurate reading for NMR. For the H NMR, there is a slight discrepancy with the molecule. Hydrogens labeled E have an integration of 7 on the graph, but in the molecule, there are only 6 hydrogens. This may be due to the fact that there are 6 equivalent hydrogens and since there are so many, the NMR misreads the actual integration number. On the carbon NMR, carbon A is surrounded by two electronegative oxygens which is why it is the carbon that produces the most downfield peak. The final product at the time of the reaction was uncertain. This was because the alcohol initially used was unknown. To determine the structure and chemical formula of the final product and the initial alcohol the carbon NMR and proton NMR spectra were used. Each of the peaks on either NMR was observed. Possible structures were drawn out and combined until the correct structure was built, matching all of the peaks on both spectra. Using the carbon NMR and proton NMR spectra, the final product in this reaction was discovered to be isoamyl acetate. By determining this, it was established that the unknown alcohol #461 was 3-methyl-1-butanol. Conclusion The unknown alcohol #461 was 3-methyl-1-butanol. There is a couple improvements could be made to this lab. First, if there was another way to confirm the identity of the final molecule, such as boiling point, we would be able to be more certain about the identity. Second, there should be a way to guarantee that 100% of the aqueous layer was removed from the conical tube to ensure the purity of the organic layer. In this particular experiment, due to the nature of determining the final product, there were no values to be compared to. There were no chemical structures or molecular weights given. Due to this, nothing could not be compared to ensure accuracy, only NMR spectra could be used.

Questions 1. Write a detailed mechanism for the Fischer esterification? H

H3C

+

O

O

+

HO

S

O

H

O

OH

H3C

O

OH O

H H H O+ O

H3C

O H

O R +

OH

+

O

O O S O -

R O

H3C

OH

O

+

O R

R

R H3C

H3C

H HO

H

2. What is the IUPAC name of isoamyl alcohol? Of isoamyl acetate? a. 3-methyl-1-butanol b. 3-methylbut-1-yl ethanoate 3. Write an equation for the Fischer preparation of a. Benzyl isobutyrate

H3C

O

H2SO4

O OH

+

OH

H2O

+

O

CH3 CH3

CH3

b. Isobutyl benzoate O O

HO OH

CH3

+

H2SO4 O

CH3

CH3 CH3

+

H2O

Works Cited 1. Carbaro, J.; Hill, Richard. Experiments in Organic Chemistry. Contempurary Publishing Company: Raleigh, 2005. 2. Brown, William H.; Iverson, Brent; Anslyn Eric; Foote, ChristopherS. Organic Chemistry, 7th Edition. Brooks/ Cole: Cengage Learning, 2012/2014. 3. Halligudi, S.B.; Joseph, Trissa; Sahoo, Suman. “Bronsted acidic ionic liquids: A green, efficient and reusable catalyst system and reaction medium for Fischer Esterification.” Journal of Molecular Catalysis A 234 (2005). 107-110. 21 Nov 2015....