Fischer Esterification Sample 2 PDF

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Fischer Esterification Sample 2...

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Lead Author: Kayla Hazelwood Reviewer: Ashlynn Swaney Editor: Rebecca Massey Organic Chemistry Lab 238-M5 March 27, 2016 at 12:20pm

Fischer Esterification Introduction: Esters are one of the most well-known, and common derivatives of carboxylic acids. Although, esters serve as an important functional group as well. Volatile esters are known for their flavor and fragrance found in flowers, fruits, artificial flavors and artificial aromas. Esters are commonly combined to create every-day items like perfume. For example the compound ethyl butyrate mimics the aroma of pineapples. On an industrial scale, esters prove useful in creating solvents, and starting materials used in polymers (plastic), and even used in pharmaceuticals.2 1

Esters are formed through Fischer esterification, the condensation reaction of a carboxylic acid and an alcohol. In order for this reaction to happen, an acid catalyst is needed to protonate the carbonyl group. Once protonated, the carbonyl carbon becomes more electrophilic and an alcohol can attack and attach.3 Thus the tetrahedral intermediate is formed. Next protonation is continued and a water leaving group is formed. When the oxygen falls back to reform the carbonyl, the water leaves. The catalyst used in this reaction is sulfuric acid, which is regenerated during the mechanism along with the production of a water, and ester shown in Figure 1. Figure 1: Mechanism of Fischer Esterification Reaction H O

O H3C

+

O

+

H HO

HO

S

O

OH

H3C

O

OH

H H H O+ O

R

H3C

O R

H3C

H

O H

O

+

+

O

+

R

OH

R

O O S O OH

H

H3C

O

OH

O R H3C

+

H2O

+

HO

S

O O

O

Table 1: Table of Reagents Name Acetic Acid Sulfuric Acid NaHCO3 Na2SO4

Molecular Weight (g/mol) 60.05 98.079 84.007 142.04

Boiling Point (°C)

Melting Point (°C)

Density (g/mL)

118 337 851 1429

60.8 10 50 884

1.05 1.84 2.20 2.66

Experimental: First 1.4 mL of acetic acid, 0.7 mL of unknown alcohol #427, and 3 drops of sulfuric acid were added to a 5 mL long neck round bottle flask. Next a condenser and stir bar was added. The mixture was then placed on a sand bath and hot plate to heat and stir for 65 minutes. A wet paper towel was placed on the condenser, and the mixture was checked every few minutes to make sure the condenser did not turn black. After completion the mixture was left to cool and transferred to a conical tube. Next 1.0 mL of NaHCO3 was added and stirred until no more carbon dioxide bubbles were produced. The organic layer was preserved, while the aqueous layer was discarded. The previous step was repeated and again the organic layer was preserved. The organic layer was then dried in a separate vial with Na2SO4. The final solution was placed into an NMR tube and NMR was generated. All organic waste was properly disposed of in waste beakers and glassware was returned.

Data and Results:

Table 2: Hydrogen-NMR Results Group A B C D E

Multiplicity Triplet Singlet Multiplet Multiplet Triplet

Graph Integration 1.00 1.85 0.92 0.92 1.48

Hydrogens 2 3 2 2 3

[ppm] 4.00 2.00 1.60 1.40 0.90

Graph 1: Hydrogen-NMR

A

C

B

D

Table 3: Carbon-NMR Results Group A B C D E F

Chemical Shift 170.9500 64.3976 30.6127 21.0095 19.1086 13.6989

[ppm] 120.0000 65.0000 30.0000 22.0000 18.0000 14.0000

E

Graph 2: Carbon-NMR

B

E

C

D A

Figure 2: Final Product A

D

B

E

C

Letter labels on the structure above correspond to Graph 1 Hydrogen-NMR.

Figure 3: Starting Unknown Alcohol #427

F

Discussion: The final product of the reaction underwent the Fischer esterification mechanism shown in Figure 1. Although, Figure 1 shows acetic acid and an alcohol attached to an “R” group, the actual reaction is with a specific “R” group attached to the alcohol. This sped up reaction was possible when the chemical mixture was refluxed. Reflux, a heating process, also helped reduce the amount of product lost. Further, the product was then separated and dried. Separation was done with a separatory funnel to filter out the product. The products were less dense than water, and this caused the ester to rise to the top and the water separated to the bottom. This made the product easy to be collected, pouring out the bottom layer into a conical vial. Finally, the product was dried with anhydrous Na2SO4. The purpose of this was to remove excess water, it was absorbed because it was polar and like dissolves like; this separates impurities from the product.4 The final product of acetic acid and unknown alcohol #427 is proposed in Figure 2 above. The unknown alcohol is identified in Figure 3. The produced product of an alcohol and acetic acid is in fact the desired ester. The product structure was determined by the analysis of hydrogen and carbon NMR in graphs 1 and 2. One unique type of NMR is known as Carbon 13-NMR in Graph 2 which identifies chemical shifts and the non-equivalent carbons. These are outlined in Table 3 and six different carbons are shown. The magnetic moment of a carbon nuclei is less than a hydrogen, therefore Carbon-13 NMR signals are weaker than in Hydrogen-NMR. For Hydrogen-NMR, seen in Graph 1, the letters labeled on the product structure in Figure 2 correspond directly with Table 2 and Graph 1 data. The structure of the unknown can be determined by nuclear magnetic resonance (NMR) and identified by non-equivalent hydrogen groups shown by differing signals. Other characteristics include splitting pattern, integration, and the position of signals on the x-axis measured by parts per million (ppm). The closer the hydrogen group is to the more electronegative atom, the more up-field it is pushed and it has a higher ppm (de-shielded), whereas the further away the group is from the electronegative atom the signal is more downfield and closer to 0 ppm (shielded). The chemical shifts help to identify the functional groups and where they are placed in the molecule. The splitting pattern is based on the n+1 rule, where the number of hydrogens on the neighboring carbons are counted up and then an addition +1 is added. The splitting pattern is the numbers of peaks in the signal. The NMR can also help identify how many hydrogens are on each carbon for their specific hydrogen connectivity, and the structure of the compound. The integration numbers reflect the number of hydrogens present on specific carbons.4 When the ester product structure is compared to Hydrogen-NMR and results in Table 2, group B is the most obvious match. B is shown as a singlet on the graph, which meant that this hydrogen group is not near any other hydrogens. Hence the n+1 multiplicity rule is 0+1 here, and can be identified on the original structure as the alpha methyl group on acetic acid. Next, group A is the other hydrogen group easily identified because of the chemical shift (ppm). This group of hydrogens is connected to the

oxygen of the functional ester group because A has the highest ppm, and is the most de-shielded. On the NMR group A is shown as a triplet which identifies on Figure 2 structure next to group C, next to two hydrogens and the multiplicity equals 2+1 which gives a triplet. For hydrogen groups C and D, they show multiplets on NMR and have at least four hydrogens surrounding the group. On the NMR graph, C is more de-shielded than D and would be closer to the oxygen of the ester. Lastly, group E is on the end of the structure as the most shielded methyl. It is the furthest from the oxygen. Its multiplicity is also a triplet, and it is next to two hydrogens. However, there are a few outliers to mention. On NMR, one unmentioned group in the table and structure, gives a negative value integration number. It is uncertain what group this could match to for the given structure. It is determined that the final product uncertainty is the cause of error. The final product is determined with just the use of NMR, and the structure is not completely clear from the graph analysis. Uncertainty is from the fact that the starting alcohol is unknown. Possible error could have been caused by a variety of factors such as drying, transferring, and refluxing. Mentioned above the final product was dried with anhydrous Na2SO4. The product was almost completely consumed by the drying reagent and was drained (laid on its side to drip) to receive a sample. This could have caused the product to contain impurities or not be fully dried. Further, transferring products has a liability for loss. When separating in the conical vial, it is possible not all of the organic layer was received and also that not all of the aqueous layer was removed. Finally, refluxing was a long process and there is no perfect set time for the reaction to fully go to completion. The reaction for Fischer could have happened within in the first few minutes of refluxing, but the reaction could have also needed more than 65 minutes to produce the ester. Esterification is a multiple step mechanism, which has several protonation and deprotonation steps. A shorter reflux time could have not allowed all steps to take place, or even let some of the product evaporate out.

Conclusion: The unknown alcohol #427 was identified as butanol, and the ester product was identified as (IUPAC NAME?). This ester could have been used as an ingredient in artificial flavoring, perfumes, and even have pharmaceutical applications. These structures were determined with the used of NMR clues to show connectivity. Although the final ester product and starting alcohol structures did, for the most part, match up to the NMR graphs, there are other ways that could have been utilized to determine structure. Future improvements for this experiment could be to take a boiling point to have a more certain identity. Another interesting approach to this experiment would be to take and IR and see the functional groups present. It would be quite obvious on IR if an alcohol is still present and the reaction did not take place. The only source of support for this laboratory was reliant on NMR spectra, which seen in the graphs do not ensure complete accuracy of identification because of the negative integration reading and the discussed errors of impurities.

References 1.

Carbaro, J.; Hill, Richard. Experiments in Organic Chemistry. Contempurary Publishing Company: Raleigh, 2005.

2. Barbaro, J.; Hill, Richard. Experiments in Organic Chemistry. Contemporary Publishing Company: Raleigh, 2005. 3.

Brown, William H.; Iverson, Brent; Anslyn Eric; Foote, ChristopherS. Organic Chemistry, 7th Edition. Brooks/ Cole: Cengage Learning, 2012/2014.

4. Hazelwood, K. (12/04/2015). Conversion of Alcohols to Alkyl Halides and Analysis by NMR and IR, and Individual Dry Lab. Chemistry 237M5, CH238-M5. Retrieved March 25, 2016, from Documents

Questions:

1. Write a detailed mechanism for this Fischer Esterfication. (see Figure 1 above) 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...