Post lab 7 - Grade: A PDF

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Summary

lab 7...

Description

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Melting point of crude was 134-139°C Melting point of recrystallized 135-137°C %yield found of crude: 97.4%

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%yield found of recrystallized: 57.6%

Reaction Scheme: 1.NaBH4, EtOH 2. HCl/ H2O

O

OH Benzoin

OH

OH Hydrobenzoin

Experimental Procedure: Benzoin (0.501 g) and Ethanol (4.0 mL) were added to a 25 mL Erlenmeyer flask and swirled gently at room temperature until the benzoin was fully dissolved. NaBH4 (0.101 g) was added in small portions over 5 minutes using a microspatula. The mixture was then swirled gently at room temperature for ~20 minutes. Then, the mixture was cooled in an ice bath and water (5.0 ml) and HCl (0.3 mL) were added. After 15 minutes water (2.50 mL) was added to “quench” the mixture. The product was then collected via vacuum filtration on a Hirsch funnel and the solid was washed with ice cold water. The solid was allowed to dry for 15 minutes and then the mass of the cruse product and yield was found. Crude product (~1-2 mg) was set aside for TLC analysis. The crude solid was then recrystallized with acetone in a 25 mL Erlenmeyer flask and the crystals were left to dry overnight. Once dried overnight the mass, percent recovery, and melting point of the purified substance 1,2-diphenylethane-1,2-diol was obtained. Next was the TLC analysis. Two TLC plates were spotted; SM: starting material (benzoin), R: recrystallized product, SMP: coproduct (starting material + recrystallized product),

C: crude product, SMC: coproduct (starting material + crude product). The starting material, crude product, and recrystallized product were dissolved in three separate vials of ethyl acetate to be used for the spotting. The TLC plates were added to a TLC developing chamber containing 9:1 CH2Cl2:ethanol, and they were run until they were ~1 cm from the top. Once dry, the spots were visualized using UV light and circled in pencil. Rf values are obtained as well.

Results (modify table as needed). Delete this and the parenthetical text Table 1 Reagent

Final Mass (g)

Melting point (°C)

% yield

Characterization methods

Crude product

0.495 g

134-139 °C

97.4 %

TLC and MP

Recrystallized product

0.285 g

135-137°C

57.6 %

TLC and MP

Figure 1

Discussion: In this experiment, benzoin was reduced by sodium borohydride in order to produce a crude yield of hydrobenzoin (0.494 g) being 97.4%, which was then recrystallized to obtain a 57.6% yield of 1,2-diphenylethane-1,2-diol (0.285 g). the chemical yield for the first reaction was very good because it means that 97.4% (majority) of the product was obtained in the experiment. However, the second reaction with a yield of 57.6% was not as high which means that a good amount of product was lost. The reaction product was identified through Thin Layer Chromatography (TLC) and its melting point. As shown in Figure 1, the TLC analysis indicated that there were two separate compounds and the solvent traveled up the place a distance of 4.6 cm in plate 1 and 4.3 cm in plate 2. The starting material spot (SM), contained benzoin and traveled up the TLC plate a distance of around 3.2 cm in plate 1 and 3 cm in plate 2 with Rf values of 0.696 and 0.698 respectively. Comparatively, the crude product (CP) in Plate 1 only traveled up the plate 2.4 cm with an Rf of 0.521 indicating that it was a different compound from the starting material, furthermore, when both the CP and the SM were spotted together, two separate spots showed up on the plate with two separate Rf values indicating that there were two separate compounds in the Co-spot. Likewise, the recrystallized product (R) in plate 2 had a spot distance of 2.3 cm and an Rf of 0.535 with the co-spot of R and SM showing two different spots similar to the SM and C co-spot. This signaled that the R and SM were different compounds. Since both the C and R had only one spot, it could be concluded that there was no benzoin in both these products. Different isomers of 1,2-diphenylmethane-1,2-diol have different melting points ranges with meso-hyrdobenzoin, (+/-)-hydrobenzoin, (R,R)-(+)-hydrobenzoin and (S,S)-(-)hyrdrobenzoin having melting point ranges of 137-139°C, 118-123°C, 146-149°C, and 148-

150°C respectively. The melting points of the C and R were 134-139°C and 135-137°C respectively. Comparing this with the known melting points of the different hydrobenzoin isomers, the C and R had melting points closer to that of meso-hydrobenzoin, therefore, both the C and R were most likely in the meso isomer. The highly polar silica gel plate allows for more polar molecules to have greater interaction with the plate, therefore, causing them to not travel as far up the plate. Based on Figure 1, benzoin traveled further up both plates than the C and R indicating that it has a lower polarity compared to those two compounds. In order to obtain a greater yield and/or purity of the product, an increase in temperature could be used during the combination of reagents in order to improve reagent reactivity. Furthermore, a different recrystallization solvent from acetone could be used in order to obtain recrystallized product yield. Postlab Questions: Let’s say you have 3 UV active spots in the crude material and co-spot TLC plate. One has the same Rf value as the starting material and the other 2 are very different. What could these 2 nonstarting material spots be? Use structures and words. Hint: think about the nucleophilic addition of the hydride. The other two spots could be the polar products of the reaction. The starting material is nonpolar, so it moves more, whereas polar groups on the TLC bond with the coating and move less, so those other two spots are compounds formed; both being major. O R C R’

NaBH4

OH R C R’ H makes stereochemistry

non-polar C O

OH group is polar...