Benzoin reduction formal report PDF

Preview

Summary

lab report...

Description

Name: Alice Duong

Report Submission Date:

3/30/2020

Experiment Title:  Sodium Borohydride Reduction of Benzoin Section Day and Time Friday AM:Wednesday AM Teaching Assistant: Yousef Bagheri Lab Technique: Leave this section blank in your report. But, make sure to tell Gradescope it is on page one of your lab report. Your TA will assign the technique points when they grade the post-lab. Do not delete this text. Purpose: Synthesize 1,2-diphenylethane-1,2-diol from benzoin using a sodium borohydride reduction reaction. Utilize TLC to assess product identity and purity. Pre-Lab: Leave this section blank in your post-lab report. But, make sure to tell Gradescope it is on the appropriate page. Your TA should have graded the prelab before you began the experiment. They will assign the awarded points for the prelab with the report. Do not delete this text. In-Lab Observations and Recordings: -

-

The heating block was set to 200 °C and 0.501 g of benzoin were added to a 25 mL flask 4 mL of ethanol was added to the flask, the benzoin began to dissolve slightly. The solution was stirred for 3 minutes above a heating block and the benzoin had not dissolved so 5 more mL of ethanol were added to the flask 2 more mL of ethanol were added and the flask was stirred for 20 minutes until all of the benzoin had dissolved. Retrieved 0.102 g of sodium borohydride and added it to the solution over a span of 5 minutes After swirling for 20 minutes, 5 mL of H2O and 0.3 mL of 6M HCl were added to the solution and it began to bubble indication an exothermic reaction Performed vacuum filtration on the solution which left 0.319 g of product

-

1 mL of boiling acetone was added to the remaining solution

-

Melting point of the saved crude product was found to be 134 – 135 °C

-

Total weight of the crystals was found to be 0.008 g

-

→ Recrystallized yield percent: .008 / .501 = 1.59% Changes in experiment: -

Problem: Too much solvent (Acetone) was added because it dissolved all the solids and no crystals could be formed Solution: boiled off acetone to let it evaporate and added hexanes to help with precipitation for recrystallization

Duong 1

Weighing data before reaction Flask

Flash with benzoin

Sodium borohydride

Crude product

Reserved for tlc

26.946g

27.423g

0.102g

1.373g

0.002g

Melting point data Crude (

)

134-135

Recrystallized (

)

138 -140

TLC Analysis Crude TLC plate analysis: Spots order: A (benzoin), B (crude product), and C (benzoin and crude product) Total length of solvent: X = 5.9 cm A length: Y1 = 4.8 cm → Rf1 = 0.814 (Y/X) B length: Y1 = 3.4 cm → Rf1 = 0.576 C length: Y1 = 3.5 cm, Y2 = 4.7 cm → Rf1 = 0.593, Rf2 = 0.797

Figure 1: Crude TLC plate

Duong 2

Recrystallized TLC plate analysis: Spots order: A (benzoin), B (recrystallized product), C (benzoin and recrystallized product) Total length of solvent: X = 5.7cm -

A length: Y1 = 4.8 cm

→ Rf1 = 0.842 (Y/X) -

B length: Y1 = 3.5 cm

→ Rf1 = 0.614 -

C length: Y1 = 3.6 cm, Y2 = 4.7 cm

→ Rf1 = 0.632, Rf2 = 0.825

Figure 2: recrystallized TLC plate

Reaction Scheme:

Experimental Procedure: A heating block was turned on and set to 200 °C. To a 5 mL flask was added 0.501 g (0.00236 mol) of benzoin. To the flask, 4 mL (0.0685 mol) of ethanol was added. The solution was then stirred for 2 minutes. After the initial stir, the flask was again stirred above a heating block for 3 minutes. 7 more mL of ethanol were added to the solution. Solution was then stirred above the heating block for 30 minutes until the entire benzoin content had dissolved. 0.102 g of sodium borohydride were retrieved in a flask. The sodium borohydride was added to the benzoin flask over a span of 5 minutes to prevent high energy interactions. Towards the end of the addition, the solution began to bubble. The solution was stirred for 20 minutes. 5 mL of water

Duong 3

were added by a graduated cylinder along with 0.3 mL of 6M hydrochloric acid by a small test tube. The solution began to bubble after this addition. The solution was left to sit in an ice bath for 15 minutes. A piece of filter paper was weighed out to 0.469 g. A vacuum filtration apparatus was set up. The chilled solution was added to the filtration apparatus, rinsed with water, and allowed to dry for 15 minutes. After, the product was weighed along with the filter paper to be 0.788 g, putting the crude yield at 0.319 g. 0.002 g of product were put aside to be used at a later point. The melting point of the crude product was tested and found to be 122 °C – 138 °C. A small test tube of acetone was set to boil. 1 mL of the boiling acetone was added to the crude product in a beaker. The solution was then sat in the ice bath again. Another piece of filter paper was weighed out to be 0.696 g. The chilled solution was found to have crystals that are clear and chunky. The solution was then put through the filtration apparatus again to purify the crystals. The product on filter paper was found to weigh 0.704 g, giving the yield of purified product to be 0.008 g. 2 mg of benzoin, recrystallized product, and crude product were added to 3 separate vials. 1 mL of ethyl acetate were added to each of the vials. A developing chamber was set up using 9:1 CH2Cl2:ethanol. 2 plates were spotted, 1 with crude product and the other with crystallized product. The plates were added to the developing chambers using tweezers. When the plates had solvent 1 cm from the top, they were removed and allowed to sit for 1 minute. The plates were then analyzed under uv light to compare the functional groups found in each sample. The recrystallized product was tested for its melting point which was found to be 138 °C to 140 °C. The station was finally cleaned up and the lab finished.

Results (modify table as needed). Delete this and the parenthetical text Reaction Melting Product Point (°C) Crude 1,2 134-135 diphenylethane -1,2-diol

Percent Yield (%) 63.4

Characterization Method

Recrystallized 138-140 1,2 diphenylethane -1,2-diol

15.9

Thin Layer Chromatography

Thin Layer Chromatography

Discussion: In this lab 0.319 g of crude 1,2 diphenylethane-1,2-diol were synthesized from 0.501 g of benzoin, returning a 63.4% yield. From the 0.319 g of 1,2 diphenylethane-1,2-diol, 0.008 g of recrystallized 1,2 diphenylethane-1,2-diol were purified, returning a 1.59%. 0.102 g of sodium borohydride along with 11 mL of ethanol were added to a solution of benzoin which was then

Duong 4

stirred to reduce the benzoin to 1,2 diphenylethane-1,2-diol. The solution was purified by washing it with 5 mL of water and 0.3 mL of 6M hydrochloric acid. Vacuum filtration was used

on the solution after it was chilled in an ice bath to purify the crystals. These 1,2 diphenylethane1,2-diol crystals were further purified using acetone and further vacuum filtration. The crude and recrystallized 1,2 diphenylethane-1,2-diol were both tested in a developing chamber filled with 9:1 CH2Cl2:ethanol. 2 plates containing the initials benzoin, crude 1,2 diphenylethane-1,2-diol and recrystallized 1,2 diphenylethane-1,2-diol were set up and TLC was performed in the chambers. The two plates were viewed under uv light which showed that each plate had similar movement in every sample when compared. This means that the initial crude 1,2 diphenylethane1,2-diol was actually well purified as the further purification did not change the product much. The TLC analysis showed that the along with the benzoin, there was at least one other group which could be seen, meaning the proper product was received. Melting points of both the pure and recrystallized 1,2 diphenylethane-1,2-diol were found. The crude sample had a boiling point range of 122 – 138 °C whereas the recrystallized sample had a boiling point range of 138 – 140 °C. Yield could be increased by decreasing the amount of acetone that was used when recrystallizing the crude product. Too much acetone leads to a lower yield percent of recrystallized product since some product will be retained in the solvent through recrystallization.

Postlab Questions Answered: The two nontarting material spots would be the diastereomers of 1,2 diphenylethane-1,2-diol which are (1R,2S) 1,2 diphenylethane-1,2-diol and (1R,1S) 1,2 diphenylethane-1,2-diol. These products would have different Rf values than benzoin since they have different levels of polarity due to their different groups found along the chain (OH rather than a double bonded O).

Duong 5...