Sodium Borohydride Reduction of Benzoin lab PDF

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Reduction of Benzoin lab...

Description

Sodium Borohydride Reduction of Benzoin

Purpose The purpose of this experiment is to reduce benzoin using sodium borohydride and ethanol. Recrystallization and TLC analysis techniques are used to purify and evaluate the product.

Reaction Scheme

Experimental Procedure 1. Reduction of Benzoin To the 25-mL Erlenmeyer flask, benzoin (0.505 g, 2.379 mmol) and 95% ethanol (4mL) were added and swirled until dissolved. Sodium borohydride (0.105 g, 2.776 mmol) was added in small amounts over 5 minutes then swirled for 20 minutes. Once cooled in an ice water bath, water (5 mL, 227.4 mmol) and 6M HCl (0.3 mL, 8.22 mmol) were added, then after 15 minutes more water (2.5 mL, 138.7 mmol) was added (a process called quenching). The product was then collected using vacuum filtration, washed with a small amount of ice water. The crude product was then weighted; a percent yield and melting point were obtained. 1-2 mg was reserved for TLC analysis. A 25-mL Erlenmeyer flask was then used to recrystallize the crude solid from

acetone. The crystals were let to dry, a weight and melting point was obtained, and the percent yield of the recrystallized product was determined. 2. TLC A small amount of starting material (benzoin), reserved crude product (1-2 mg), and recrystallized product was each dissolved with ethyl acetate into three vials. Two TLC plates were spotted. One plate was spotted with starting material, crude product, and a mixture of both the starting material and crude product. The second plate was spotted with starting material, recrystallized product, and a mixture of the starting material and recrystallized product. Each TLC plate was run in a developing chamber of 9:1 CH Cl :ethanol until ~1 cm from the top of the 2

2

plate. The plates were then dried in the fume hood and the spots were visualized under UV light.

Results and Discussion Figure 1: Weights of Reactants Used Amount compound used

Molecular Weight

Amount used

(g)

(g/mol)

(mmol)

Benzoin

0.505 g

212.248 g

2.379 mmol

Sodium

0.105 g

37.83 g

2.776 mmol

Compound

Borohydride Figure 2: Crude and Recrystallized Product Results Product

Crude 1,2-diphenylethane-

Molecular Weight Weight of Product

Percent

MP

(g/mol)

Obtained

Yield (%)

(℃)

----

0.465 g

92.08%

135137 ℃

1,2-diol Recrystallized 1,2-

214.26 g/mol

0.422 g

83.56%

137-

diphenylethane-1,2-diol Figure 3: TLC Plates Analysis Plate

Key

1: Crude

A: starting material (benzoin) B: crude product C: starting material + crude product

2: Recrystallized A: starting material (benzoin) B: recrystallized product C: starting material + recrystallized product Figure 4: TLC Plates

139 ℃

The starting materials benzoin, ethanol, and sodium borohydride were reacted and purified using recrystallization techniques to obtain the product pure 1,2-diphenylethane-1,2-diol in 83.56% yield at the conclusion of this experiment. Once the reaction was completed, TLC analysis was carried out to determine the number of components in a mixture, and investigate

how different solutions differing polarities affect R values. The results of the TLC analysis were f

shown above (Figure 3 and 4). The starting material (A) resulted in large splotches seen in both A and C of both TLC plates. Both the crude product and recrystallized product resulted in smaller spots and are also more polar than the starting material because the spots did not travel as far up the TLC plate. Lane C on both TLC plates showed two components which is expected since the solution in C was a mixture of the starting material and the crude/recrystallized product. In lane B, of plate 1 (crude), the smaller spot is about halfway in between the spot of the recrystallized product and the spot of the starting material. Therefore, recrystallization resulted in obtaining the desired product because lane B of plate 2 shows an even more polar product with no trace of starting material. It is apparent in lane C of plate 2 that there are only two components which was the desired result. The MP of the crude product was 135-137 ℃, and the MP of the recrystallized product was 137-139 ℃. The reason why the MP of the product was lower was because when there were impurities in a product, it lowered the the true MP. When the crude product was purified through recrystallization, the MP increased as expected because the impurities were no longer present. The identity and purity was assessed, producing a good yield. To improve yield and purity of a compound, more physical purification processes could be used such as distillation and a second recrystallization. If distillation were performed, this would however result in a lower percent yield due to having to leave some original volume in the round bottomed flask that is not distilled. To improve the yield of a compound, using the least amount of solvent possible to dissolve the compound in recrystallization should be used. In this experiment, sodium borohydride (NaBH ) is used as a mild reducing agent. However, alternate 4

reagents could be used such as a strong reducing agent LiAlH . This is not advised in this 4

experiment though due to the dangerous hazards of LiAlH being extremely flammable and 4

reactive. The addition of hydrogen changes a carbon-oxygen double bond to an alcohol. Stoichiometry can differ in two diastereomers: (1R, 2S)-1,2-diphenylethane-1,2-diol and (1S, 2S)-1,2-diphenylethane-1,2-diol. A catalyst, or raising the temperature of the reaction can make the reaction proceed faster over a shorter period of time. A change in solvent can either increase or decrease the rate of the reaction depending on how reactive the solvent is with the reagent. For example, a more polar solvent can increase the rate of the reaction. The molar ratio of the reactant to the product is 1:1, so we do not need to worry about it. If there were 3 UV spots, one of the spots has the same R value of the starting material f

but the other two are different, these two non-starting material spots could be diastereomers with differing polarity. The sodium borohydride attacks by nucleophilic substitution....