Organic Chemistry Lab 6 PDF

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Sodium Borohydride Reduction of a Ketone Kanye West TA: Jeff Regier Completed On: Monday, February 24, 2020

Introduction

An increase in the number of hydrogens in a molecule is generally indicative of a reduction reaction, whereas a decrease in the number of hydrogens is usually associated with an oxidation reaction. Thus, reactions between carbonyl groups, such as those found in ketones and aldehydes, and reducing agents, such as sodium borohydride (NaBH4), are examples of reduction reactions. The general reaction and mechanism for the reduction of benzophenone, a ketone, with sodium borohydride to form the alcohol, benzhydrol, is depicted in Figure 1 and Figure 2, respectively. In regards to the mechanism of the reaction, the hydride ion, H-, from the nucleophilic borohydride reagent is transferred to the electrophilic carbonyl carbon of benzophenone. This results in the formation of an alkoxide and BH3. The alkoxide oxygen then gets protonated, by taking a hydrogen ion from the proton solvent, ethanol, and the benzhydrol product is formed.

Sodium borohydride, which is effective for the reduction of aldehydes and ketones to alcohols, serves as the reducing agent, as it reduces benzophenone, exhibits a loss of electrons and, experiences an increase in its oxidation state. By contrast, benzophenone is the oxidizing agent, as it is reduced by sodium borohydride, exhibits a gain in electrons and, experiences a decrease in its oxidation state. The oxidation and reducing agents can additionally be characterized based on the oxidation numbers of the reactants and the products. For instance, the oxidation number of benzophenone carbonyl carbon is decreased from +2 to +1, it is being reduced and thus identified as the oxidizing agent. In the case of the sodium borohydride, the oxidation number of boron is increased, so it is oxidized and thus identified as the reducing agent.

The required apparatuses for this experiment include beakers, thermostats and a stirrers. As the reduction reaction will be contained inside sample vials that will placed in an ice bath and then outside the ice bath, the reaction temperatures will be around 2 - 3 ºC (Ice Bath Temperature) and then 20–22 °C (Room Temperature). Ethanol will be serving as the reaction solvent, it will provide the hydrogen ion necessary for the protonation of the alkoxide oxygen.

Once the ethanol sample has been dissolved with the benzophenone, the sample vial containing the mixture will placed in an ice bath whilst being stirred. After adding the sodium borohydride sample into the solution, the sample vial will be kept in the ice bath for approximately 5 minutes, after which the reaction will be removed and placed under room temperature, whilst being stirred. At the 3, 10 and, 16 minute intervals, spots of the reaction mixture will be applied to the TLC plate (Figure 3). After 20 minutes, the reaction will be quenched by

being combined with ice cold distilled water. The crude product will be present as the white precipitate and will be isolate through vacuum filtration with a Buckner funnel. The product will then be further purified via hot petroleum ether recrystallization. The product will be characterized by H-NMR spectroscopy, IR spectroscopy and melting point determination. Procedure The procedure given in the ON Tech CHEM 2120 February 24, 2020 laboratory manual, Sodium Borohydride Reduction of a Ketone, was followed without any significant changes.

Data and Results

The primary data observations were made during the product characterization phase of the experiment, durning which the recrystallized product was tested for a melting point range, IR spectroscopy and, H-NMR spectroscopy. The course of the reaction was monitored using thin layer chromatography, in which a TLC plate (Figure 5) was spotted at various timed intervals in order to track the formation of benzhydrol. The corresponding retention factor values were calculated for each of the TLC spots and presented in a tabular manner (Table 2). Additionally, calculations were performed to determined the percent yields of the crude product (81.36% Yield) and the recrystallized product (22.11% Yield) (Figure 4).

After collecting the crude product through vacuum filtration, it was observed that the crude product was powder-like in terms of consistency and white in appearance. The recrystallized product was also white in appearance but had a complete different consistency, as it was quite fibrous in terms of texture and was similar to glass wool borosilicate. During the reaction, it was observed that a gaseous product was evolved, this gaseous product was hydrogen gas, which likely formed when the NaBH4 reacted with the polar protic solvent, ethanol.

Once the product had been recrystallized, the MelTemp was used used to determine the melting point ranges of both the crude product (54-56 °C) and the recrystallized product (63-66 °C) (Table 3). Subsequent IR and H-NMR spectroscopies were then performed on the recrystallized product (Figures 6 - 11).

Discussion

The objective of this experiment was to reduce the ketone, benzophenone with sodium borohydride to form the alcohol, benzhydrol, which was then characterized through melting point determination, IR spectroscopy and, H-NMR spectroscopy. The progress of the reduction reaction was monitored using TLC, where a TLC plate (Figure 5) was spotted at three timed intervals in order to track the formation of benzhydrol. After the completion of the reaction, the crude product, which was present as a white precipitate, was isolate through vacuum filtration and then be further purified via hot petroleum ether recrystallization.

An analysis of the TLC chromatograph indicates that the benzhydrol product formed in less than three minutes. This is evident by the fact that at the three minute timed interval, two spots were present on TLC chromatograph, the spot with the greater retention value, is that of benzophenone and the spot with the lower retention value is that of benzhydrol. As the reaction proceeded, the benzophenone spots faded and reduced in size, whereas the benzhydrol spots became larger and more prominent, indicating greater formation of the benzhydrol product and thus, a greater product yield. Based on the TLC chromatograph, the reaction likely finished in the first 5-10 minutes, as by the 16 minute time interval, the benzophenone spot largely stopped changing.

As the recrystallized product had a fairly narrow melting point range (~3 0C) that closely resembled the literature melting point range (63-66 °C compared to 65-67 °C), it can be stated that the benzhydrol product was of a relatively high purity. If the benzhydrol product had contained soluble impurities, it would have melted at a temperature that was much lower than that of the literature melting point.

The IR spectrum of benzophenone showcases that the ketone can be identified due to the presence of the carbonyl (C=O) band at 1659 cm^-1 and the absence of the hydroxy (O-H) broad band at ~3400 cm^-1 (Figure

6). Upon being reduced by sodium borohydride, benzophenone is reduced to benzhydrol, which can be identified by the disappearance of the carbonyl (C=O) band at 1659 cm-^1, and the appearance of the hydroxyl (O-H) broad band at ~3400 cm-^1 (Figure 7). Additionally, a C-H stretch can also be observed. By comparing the relative peaks, bands and, stretches of the pure benzhydrol IR spectrum with the peaks, bands and, stretches of the student IR spectrum, it can be determined wether or not the student sample exhibited any benzhydrol formation.

When comparing the IR spectra of the student products with the IR spectra of the pure benzhydrol, it can be seen that the student sample certainly showed product formation, however, not to the same degree as the IR spectrum of the pure benzhydrol. In the student sample, the presence of a hydroxyl (O-H) broad band at ~3400 cm-^1 can be observed but whereas the IR spectrum of the pure benzhydrol exhibited a complete absence of the carbonyl (C=O) band at 1659 cm-^1, the student sample does not. The presence of the carbonyl (C=O) band at 1659 cm-^1 shows that the student sample is not completely pure, but rather that it contains a mixture of unreacted reactants and products.

In addition to the IR spectroscopy, NMR spectroscopy was also used to characterize the benzhydrol product. Three aromatic multiplet signals at a range of 7-8ppm, with two of the multiplets being overlapped at 7.3-7.4 ppm, were observed in 1H-NMR of the benzhydrol product. The broad peak at 2.3ppm represents the OH in benzhydrol, however it disappears in benzophenone and therefore the OH-group is absent in the benzophenone. The peak at 5.85ppm in benzhydrol, also disappears in benzophenone H-NMR spectra, as the hydrogen atom is no longer prevalent in benzophenone’s structure.

As depicted is Figure 12, benzhydrol contains five chemically different C-atoms (1, 2, 3, 4 & 5) (Figure 12). The chemically (symmetrically) equivalent C-atoms are depicted in Figure 13, the chemical equivalence is

due to the presence of the vertical plane of symmetry (σv ) (Figure 14). As a result of the five chemically different C-atoms, there will be five peaks expected in the 13C-NMR spectrum. In the benzophenone molecule, the peak at the C-1 atom is due to the presence of carbonyl group and in the benzhydrol molecule, the peak at C-1 atom is due to the presence of the hydroxyl group. Additionally, no significant side products or impurities were observed in the student sample, “1H_13C NMR benzhydrol student product W16”.

The overall objective of this lab experiment was to reduce the ketone, benzophenone with sodium borohydride to form the alcohol, benzhydrol. Once the product had been attained, it was then to be characterized through melting point determination, IR spectroscopy and, H-NMR spectroscopy. As the various characterization tests confirmed the formation of benzhydrol, the lab objective was successfully accomplished.

Figures and Tables

Figure 1 - General Reaction for the Reduction of Benzophenone with Sodium Borohydride to form Benzhydrol.

Figure 2 - Mechanism for the Reduction of Benzophenone with Sodium Borohydride to form Benzhydrol.

Figure 3 - Reaction Progress Timeline

Figure 4 - Sample Yield Calculations for the Crude Product (81.36%) and the Recrystallized Product (22.11%)

Figure 5 - TLC plate with the Timed Interval Spots

Figure 6 - IR Spectrum of Benzophenone

Figure 7 - IR Spectrum of Benzhydrol

Figure 8 - IR Spectrum of Benzhydrol (Student Sample)

Figure 9 - 1H-NMR Spectrum of Student Benzhydrol Product in CDCl3 Solvent

Figure 10 - Expansion of 7.0 - 8.0 Region

Figure 11 - 1H-NMR Spectrum of Student Benzhydrol Product in CDCl3 Solvent

Figure 12 - Five Chemically Different C-atoms In Benzhydrol

Figure 13 - Four Chemically Equivalent C-atoms In Benzhydrol

Figure 14 - The vertical plane of symmetry (σv ) of Benzhydrol

Compound

Amount

MW (g/mol)

Moles

Benzophenone

0.25 g

182.217 1.371 x 10^-3

Sodium borohydride 0.13 g

37.83 3.436 x 10^-3

Stoichiometry/ Comments

Solute Limiting Reagent

Ethanol

5 mL

46.069 8.563 x 10^-3

Solvent

Pet ether

30 mL

86.178 2.38 x 10^-4

Recrystallization Solution

Crude Product Obtained

Amount

Benzhydrol

1.03 g

Recrystallized product

Amount

Benzhydrol

0.28 g

MW (g/mol)

Moles

Yield

184.2 5.5917 x 10^-3

MW (g/mol)

81.36%

Moles

Yield

184.2 1.5200 x 10^-3

22.11%

Table 1 - Reagent Table

Starting Benzophenone

TLC Spot #2 (3 mins)

TLC Spot #3 (10 mins)

TLC Spot #4 (16 mins)

10

16

Label On TLC Plate

SM

Distance Travelled by Solvent Front

4.4 cm

4.4 cm

4.4 cm

4.4 cm

Distance Travelled by Component (s)

3.9 cm

2.85 cm & 3.8 cm

2.8 cm & 3.9 cm

2.9 cm & 3.8 cm

0.63 & 0.88

0.65 & 0.86

Retention Value (Rf)

Table 2 - TCL (Rf) values

3

0.88 0.64 & 0.86

Product

Melting Temperature Range

Literature Melting Temperature Range

Crude Product

54-56 °C

65-67 °C

Recrystallized Product

63-66 °C

65-67 °C

Table 3 - Melting Temperature Ranges

References

Brown, W. G., & Chaikin, S. W. (1949, January). Reduction of Aldehydes, Ketones and Acid Chlorides by Sodium Borohydride. Journal of the American Chemical Society, 71(1), 122-125. doi:10.1021/ja01169a033

R. M. Silverstein, G. C. Bassler, T. C. Morrill, Spectrometric Identification of Organic Compounds, John Wiley & Sons, New York, 5th Edition, 1991, chapter 3, 129.

R. Sanz, J. Escribano, R. Aguado, M. R. Pedrosa, F. J. Arnáiz, Synthesis, 2004, 10, 1629

Rickborn, B., & Johnson, M. R. (1970, April). Sodium borohydride reduction of conjugated aldehydes and ketones. The Jornal of Organic Chemistry, 35(4), 1041-1045. doi:10.1021/jo00829a039

Ward, D. E., & Rhe, C. K. (1989, January 5). Chemoselective reductions with sodium borohydride. Canadian Journal of Chemistry, 67(7), 1206-1211. doi:10.1139/v89-182 Sodium bhoride...