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To carry out the sodium borohydride reduction, 1.0g benzoin previously synthesized was combined in a 125mL Erlenmeyer flask with 15mL 100% ethanol and 250mg sodium borohydride. The flask was stirred on a magnetic stir plate for 15min. After 15 minutes, a thin-layer chromatography (TLC) analysis was performed to check for reaction completeness. The TLC developing chamber was prepared using a wide-mouthed capped bottle saturated with 4.5mL ethyl acetate (2-3mm solvent present in the chamber). A solution was prepared with a small amount of the benzoin reactant dissolved in acetone. A TLC plate was prepared with a 2mm long mark made on the long edge of the plate, 1cm from the bottom. Clean micropipettes were used to spot the compounds; the reaction mixture was spotted at 1/3 the TLC plate width, the benzoin solution was spotted at 2/3 the TLC plate width. The plate was placed in the developing chamber, which remained closed until solvent rose to 1cm from the top of the plate. Once the solvent reached this height, the plate was removed from the chamber and the solvent line was immediately marked with a pencil. The TLC plate was placed in the fume hood for 2-3min for solvent evaporation. A UV light (254nm) was for visualization and the spots were outlined with a pencil. Appropriate measurements were taken and the Rf values were calculated. When the reaction was completed the flask was placed in a 400mL beaker, surrounded by ice, and the reaction mixture was diluted with 15mL dH20. The beaker was stirred using a magnetic stir plate while 2mL 6M hydrochloric acid was slowly added, followed by 5mL dH20. The mixture was stirred for an additional 15mn. The precipitate was collected through vacuum filtration using a Buchner funnel and filter paper, and then washed with 25mL dH20. A small amount of the crude product collected was saved for melting point determination. Recrystallization was not performed. The product was placed on a watch glass of known mass and allowed to dry over a 2-day period.
Synthesis of the cyclic acetal derivative was carried out after the drying period; 0.4g of the dried 1,2-diphenyl-1,2-ethanediol was added to a 25mL Erlenmeyer flask with 5mL 2,2-dimethoxypropane and 10mg p-toluenesulfonic acid monohydrate. The flask was stirred on at room temperature for 60min. The reaction mixture was poured into a separatory funnel containing 50mL dichloromethane, followed by 25mL dH20. The organic phase was drained from the funnel into a clean Erlenmeyer flask. A second extraction was completed; 25mL saturated sodium bicarbonate was added to the separatory funnel and the organic phase was combined with the first organic phase collected. A third extraction was completed with 25mL dH20; after the third organic phase was extracted, the combined organic product was dried with anhydrous magnesium sulfate for 10min. The dried solution was filtered into a clean 50mL Erlenmeyer flask of known mass with a conical funnel and filter paper. The product was evaporated in a fume hood for a 5-day period. After evaporation was completed, the mass and melting point of the final product were determined.
Sodium Borohydride Reduction (4/4/16)
Benzoin = 1.0g
Ethanol = 15mL
Sodium Borohydride = 250mg
Monitoring Reaction by Thin-Layer Chromatography
Developing Chamber: Ethyl Acetate = 4mL
Benzoin Solution = <10mg Benzoin Product + 1-2mL Acetone
Plate #1 – RXN @ 15min
Distance Travelled Solvent = 4.6cm
Distance Travelled RXN Mixture @ 15min = 0.2cm
Distance Travelled Benzoin Solution = 1.0cm
Rf (RXN Mixture @ 15min) = 0.043478
Rf (Benzoin Solution) = 0.2173913
dH20 = 15mL
6M Hydrochloric Acid = 2mL
dH20 = 5mL
dH20 (Wash) = 25mL
Melting Point = 134-136°C
Watch Glass = 49.77g
Watch Glass + Product = 50.3g
Recrystallized Product = 0.53g
1,2-Diphenyl-1,2-Ethanediol = 0.4g
2,2-Dimethoxypropane = 5mL
p-Toluenesulfonic Acid Monohydrate = 10mL
Dichloromethane = 50mL
dH20 = 25mL
Saturated Sodium Bicarbonate = 25mL
dH20 = 25mL
Anhydrous Magnesium Sulfate = 5 micro-spatula scoops
Melting Point = >90°C (DID NOT MELT)
Watch Glass = 41.08g
Watch Glass + Product = 41.57g
Product = 0.49g
Benzoin (C14H12O2) MW = 212.3 g/mol
mp = 137°C
1,2-Diphenyl-1,2-Ethanediol (C14H14O2) MW = 214.3 g/mol
mp: meso- = 136-137°C
(±) = 121-122°C
Cyclic Acetal of 1,2-Diphenyl-1,2-Ethanediol (C17 H18 O2) MW = 254.3
mp: meso- = 57-59°C
(±) = 44-46°C
Percent Yield, Recrystallized 1,2-Diphenyl-1,2-Ethanediol
Theoretical Cyclic Acetal of 1,2-Diphenyl-1,2-Ethanediol
Experimental Cyclic Acetal of 1,2-Diphenyl-1,2-Ethanediol
Percent Yield, Cyclic Acetal of 1,2-Diphenyl-1,2-Ethanediol
The synthesis of 1,2-diphenyl-1,2-ethanediol through sodium borohydride reduction of benzoin was successfully completed during this experiment. The percent yield of the 1,2-diphenyl-1,2-ethanediol product was calculated to be 52.09%. The occurrence of the completed reaction was indicated by TLC analysis; the Rf value for the reaction mixture and the benzoin reactant were calculated to be 0.0435 and 0.2174, respectively. The significant difference in Rf values suggested there was relatively little, if any, benzoin remaining in the reaction mixture. The melting point after the evaporation period was determined to be 135-136°C. This falls between the known melting points for the meso- and (±) stereoisomers, which are 121-122°C and 136-137°C, respectively; however, the melting point is much closer to the (±) isomer. The results of the melting point determination suggest that the 1,2-diphenyl-1,2-ethanediol product was comprised of relatively pure (±)-1,2-diphenyl-1,2-ethanediol. The low yield may be due to the composition of the benzoin reactant of unknown purity, which was synthesized during a previous experiment.
The synthesis of the cyclic acetal of 1,2-diphenyl-1,2-ethanediol was not completed successfully during this experiment. The percent yield of the dried product was calculated to be over 100%, which is not possible. This was likely caused by the presence of excess impurities in the mixture due to inefficient extraction techniques and/or glassware contamination. The melting point determination of the cyclic acetal derivative indicated that the product contained none of the intended cyclic acetal; the product from this experiment was not observed to melt at temperatures reaching 90°C, it was expected to melt below 60°C. This suggests the product obtained is largely composed of unreacted 1,2-diphenyl-1,2-ethanediol, which has a melting point greater than 100°C. One major problem that occurred during this experiment was the loss of potential product due to a spill; the dichloromethane/water/reaction mixture from the separatory funnel was spilled and an unknown amount of the mixture was lost. A NMR analysis of a cyclic acetal product from the meso- stereoisomer would reveal a single methyl peak, while NMR analysis of the cyclic acetal product from the (±) stereoisomer would reveal two methyl peaks.
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