Lab 6 Benzil Reduction - Report for organic chem PDF

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Introduction The objective of this experiment is to reduce benzil using sodium borohydride in a catalytic hydrogenation reaction. In a benzil reduction, there are five possible products that can occur. Specifically a racemic benzoin, racemic hydrobenzoin, and meso-hydrobenzoin. The purpose of this experiment is to use melting point, IR, and TLC to determine whether the product is racemic benzoin, racemic hydrobenzoin, and meso-hydrobenzoin. Brief Discussion of Stereochemistry Benzil contains an aldehyde and a ketone. The sodium borohydride will attack from either side of the aldehyde or ketone producing a racemic product of the R and S enantiomers. If only one carbonyl group is reduced, the product will be a racemic mixture of benzoin. If both carbonyl groups are reduced, the product will either be meso-hydrobenzoin or a racemic mixture of hydrobenzoin. After the first reduction, the molecule contains a chiral center, which causes the second reduction to take place on one side of the ketone. This would then form the racemic or meso hydrobenzoin. Therefore, it can have five possible products. Mechanism Attached on the second file. Key experimental details and observation We started the experiment with a yellow powder form of benzil. We obtained 1g of benzil and placed it in a 125ml erlenmeyer flask. Then, we add 10ml of 95% ethanol and a stir bar. After the ethanol was added, it started to form a yellow liquid. We heated the solution with the stirring until the benzil dissolved completely. Now that the benzil has fully dissolved, we take the solution off the hot plate and let it cool down to room temperature. The benzil will start to recrystallize. After it is done recrystallizing, put the solution back on the hot plate. Add 0.207g of sodium borohydride to the solution over a 2 minutes period. Stir for another 2-3 minutes, then let it stand at room temperature for 10 minutes. As the solution is dissolving, it appears that the color of the solution will change from a darker yellow to a lighter yellow. Next we add 10ml of hot water (~70-80 C) to the reaction flask with stirring. Heat the mixture to boling. Add another 15-20ml of hot water with stirring over a 1 minute period. Remove the stir bar and let the solution cool to room temperature, then place it in an ice bath for about 5 minutes. After 5 minutes has passed, we set up a vacuum filtration to collect our product. Before collecting, we clean the Buchner funnel and add a filter paper to the funnel. Then we carefully pour our solution into the filtration. We collect and dry our product in the oven. After it is done drying, we weigh our product. We take the melting point and IR of our product. We take a TLC of our product with the meso-hydrobenzoin standard and racemic benzoin standard. Ethyl acetate was used to dissolve the solids and 2:1 mixture of hexane:ethyl acetate as the developing solvent. Results Final product: 0.619g

Melting Point of the Product: 134.0-136.0 C Literature Melting Point: Compound

Melting Point

Meso-Hydrobenzoin

138.0C

Racemic Benzoin

134.0C

Racemic Hydrobenzoin

122.0C

Mixed Melting Point: Mixed

Melting Point Range

Product + Racemic Benzoin

117.0-125.0 C

Product + Meso-hydrobenzoin

133.0-136.0 C

Percent Yield: 60.7% TLC:

Spot

Distance of the spot Distance of the Rf value from baseline solvent front

Experimental product

1.40 cm

4.80 cm

0.29

Racemic benzoin

2.30 cm

4.80 cm

0.47

Meso hydrobenzoin

1.50 cm

4.80 cm

0.31

IR Spectrum:

Color

Bond

Yellow

O-H Stretch

Red

C-H Stretch

Purple

C-H Stretch

Green

Overtones

Blue

C=C Stretch

Discussions: The purpose of this experiment was to identify the product. There were several different techniques used to identify the product. One of the techniques used was melting point. The two possible products were racemic benzoin and meso hydrobenzoin. The literature melting point for racemic benzoin is 134.0 C and the melting point for meso hydrobenzoin is 138.0 C. The melting point range of the product was 134.0-136.0 C. The mixed melting point of the product and meso hydrobenzoin is 133.0-136.0 C, while the mixed melting point of the product and racemic benzoin is 117.0-125.0 C. The product melting point range is 134.0-136.0 C and the mixed melting point of meso hydrobenzoin is 133.0-136.0 C, which is extremely close. In fact, they are almost identical. That means that the product was very pure because there is a small narrow melting point range. It also means that the product could be a meso hydrobenzoin. Based on the IR spectrum, we concluded that the identity of our product was meso hydrobenzoin. This was proven by the O-H alcohol bond at 3372 cm-1 and 3306 cm-1. The double O-H bond is yielding two alcohol groups in the product. This proves that our product is meso hydrobenzoin. There is also a C-H alkene bond at 3061 cm-1 and 3033 cm-1. At 2900 cm-1 there is an alkane C-H bond. On the TLC plate, there were four different sections. The sections were the experimental product, meso hydrobenzoin, racemic benzoin, and co spotting. The distance of the experimental product from the baseline was 1.40cm, the racemic benzoin distance was 2.30cm, and the meso hydrobenzoin was 1.50cm. The Rf value for the product was 0.29 and the meso hydrobenzoin is 0.31. The Rf value of the product is very close to the meso hydrobenzoin, which means that the identity of the product could be meso hydrobenzoin. The purity based on the TLC plate is very high. There are no other spots on the TLC plates beside the ones that were supposed to be there. After conducting TLC, melting point, and IR, our product identity is meso hydrobenzoin. The percent yield is 60.7%. There were definitely products lost in our experiment. There were also many errors in the experiment. The products could have been lost during recrystallization, vacuum filtration, and there were impurities in the solution....