Organic Chem Lab 5: Synthesis of Methyl Benzoate by Fischer Esterification PDF

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Organic Chem Lab 5: Synthesis of Methyl Benzoate by Fischer Esterification...

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Synthesis of Methyl Benzoate by Fischer Esterification

Abstract: The objective of this experiment is to prepare an ester (methyl benzoate) using an acid catalyzed reaction between a carboxylic acid (benzoic acid) and an alcohol (methanol). The product was then characterized using Thin Layer Chromatography and Infrared Spectroscopy. Introduction: Fischer esterification is a common method of preparing esters. This reaction involves an acid-catalyzed reaction between a carboxylic acid and an alcohol. The acid catalyst, in this

experiment sulfuric acid, acts to protonate the carboxylic acid making the carboxylic carbon atom more susceptible to nucleophilic attack by an unpaired pair of electrons on the oxygen atom of the alcohol. This leads to the formation of a tetrahedral intermediate with two equivalent hydroxyl groups. One hydroxyl group is then eliminated after tautomerization and deprotonation which gives the final product, an ester, and water. In this process, alcohol and the carboxylic acid are heated together in the presence of the acid catalyst until equilibrium is reached. An equilibrium mixture gives an overall yield of 67% methyl benzoate, however with the addition of methanol in excess, the reaction is driven to the product side leading to an overall yield of 97% methyl benzoate. Table1. Physical and Chemical Properties of Chemicals Used Throughout the Experiment Compound

Molecular Weight (g/mol)

Density (g/mL)

Melting Point (°C)

Boiling Point (°C)

Benzoic Acid

122.12

1.316

122.4

250.0

Methanol

32.04

0.791

-98.0

64.0

Methyl Benzoate

136.15

1.087

-13.0

198-199

Experimental Five grams of benzoic acid were weighed out and transferred into a 100-mL round bottomed flask. Twelve and a half milliliters of methanol were added to the flask and the mixture was cooled in ice. One and a half milileters of concentrated sulfuric acid was slowly poured down the side of the flask and the flask was swirled to mix the reagents. Two boiling chips were added to the flask and the mixture was refluxed for one hour with water flowing slowly through the condenser. After one hour, the solution was cooled and decanted into a separatory funnel that

contained 25 mL of water. The reaction flask was rinsed with 20 mL of dichloromethane and the dichloromethane was added into the separatory funnel. The funnel was shaken and the layers were separated. The aqueous layer was discarded and the organic layer was washed with 15 mL of water in the sepratory funnel. The aqueous layer was discarded again and the organic layer was then washed with 15 mL of 5% sodium bicarbonate to remove any unreacted benzoic acid. The organic layer was washed twice with sodium bicarbonate to ensure that the aqueous layer was basic. Then the organic layer was washed with 15 mL of saturated sodium chloride. The organic layer was then dried over approximately 10 g of anhydrous sodium sulfate in a 125 mL Erlenmeyer flask. The dry dichloromethane was then decanted into a 100 mL round bottomed flask after ten minutes. The drying agent was rinsed with an additional 5 mL of dichloromethane and added into the reaction flask along with 2 boiling chips. The dichloromethane was removed by simple distillation. The water was removed from the condenser before the methyl benzoate began to distill because of the comparatively high boiling point of methyl benzoate. Thin layer chromatography and IR was then used to characterize the product formed.

Results: Yield Report Weight of benzoic acid

5g

Moles of benzoic acid

0.0409 mol

Volume of methanol

12.5 mL

Moles of methanol

0.31 mol

Limiting reagent

Benzoic acid

Theoretical yield of methyl benzoate

5.446 g

Theoretical yield of methyl benzoate

0.04 mol

Actual yield of methyl benzoate

4.348 g

Percent yield of methyl benzoate

78.00%

5 g benzoic acid x

1 mol benzoic acid 122.12 g benzoic acid

12.5 mL methanol x

0.791 g 1 mL

x

1 mol methanol 32.04 g

Theoretical yield = 0.0409 mol x Percent yield =

4.348 g 5.446 g

= 0.0409 mol benzoic acid (limiting reagent) = 0.31 mol methanol

1 mol methyl benzoate 1 mol benzoic acid

x

136.15 g methyl benzoate 1 mol methyl benzoate

= 5.446 g

x 100 = 78.00%

Thin Layer Chromatography Results Product

Rf (in)

Methyl benzoate

0.72

Benzoic Acid

0.25

Infrared Results: Characteristic peaks observed at 1601.44, 1718.00, 2952.51, and 2999.76 cm-1 -

The 1601.44 stretch represents the aromatic ring which is typically observed between 1400-1600 cm-1

-

The 1718.00 stretch represents the unsaturated ester which is typically observed between 1715-1730 cm-1

-

The 2952.51 stretch presents the carbon in the ester which is typically observed between 2850-3000 cm-1

-

The 2999.76 stretch represents the aromatic ring which is typically observed between 3000-3100 cm-1

Discussion: The yield of methyl benzoate expected with methanol added in excess is 97% and the percent yield obtained in this experiment was 78%. Some of the product could have been lost throughout the duration of the experiment. For example, some of the product may have been lost during the wash and extraction phase to cause a lower percent yield. Thin Layer Chromatography was used to assess the purity of a compound. Two spots were placed onto the plate, Benzoic acid, and methyl benzoate obtained. TLC plate was developed to measure the products to calculate the Rf values. Based on the results, the methyl benzoate made was more polar than the benzoic acid which is consistent with the expected characteristics of pure methyl benzoate. The peaks obtained in the IR correspond to the structural components of methyl benzoate. The main peak that was observed (1718) fell between the 1715-1730 region. This absorption range is characteristic of esters which confirms that an ester is present in the compound. The other observed peaks confirm the methyl group attached to the ester as well as the benzene ring.

Conclusion

The results obtained support the formation of methyl benzoate through Fischer esterification of benzoic acid and methanol under acidic conditions. This reaction took place through a nucleophilic attack to produce an ester and water. It was also done under reflux conditions to maximize the percent yield of the product. The percent yield obtained was 22% lower than the theoretical yield which may have been caused by experimental or human error. The product was characterized using two methods and the characteristics of the product were similar to the characteristics to pure methyl benzoate. The synthesis of methyl benzoate by Fischer esterification was successful based on the percent yield, TLC and IR.

References and Notes K. L. Williamson, Macroscale and Microscale Organic Experiments, 2nd Ed. 1994, Houghton Mifflin, Boston p385 Revised 10/15/03 Roberts, R.M. et al. “Experimental Organic Chemistry” Howard University Edition, Academx Publishing Services, 2014-2015. Pages 159-163....