Lab 6 debromination PDF

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Summary

To utilize the processes of bromination and debromination in order to purify a sample of cholesterol through modification of the chemical structure. ...

Description

Bromination and Debromination of Cholesterol March 13th, 2018 Aim: To utilize the processes of bromination and debromination in order to purify a sample of cholesterol through modification of the chemical structure. Chemicals: Name Bromate

Structure

Sodium bromide

Cholesterol

Dibromocholesterol

Solid, 113°C (mp), 586.493 g/mol

Reagents: Acetic acid, zinc powder Set up: Vacuum filtration

Physical Solid, 350°C (mp), 127.901 g/mol solid,747 °C (mp), 102.894 g/mol solid, 148°C (mp), 386.664 g/mol

Mel Temp Apparatus

Equipment: Mel-Temp

Bunsen Burner

Hirsh Funnel

Procedure: A literature procedure was followed. The following is the actual sequence followed in the lab: First, a test tube containing dissolved cholesterol was filled with a 100 mg sample of bromate and a 500 mg sample of bromide. Acetic acid was added to this mixture, and the test tube was stirred. Then it was placed in an ice bath and the cholesterol dibromide was allowed to crystallize. This precipitate was filtered via vacuum filtration and the mass taken. For the next step of the experiment, the entire sample was transferred to a test tube and zinc dust was added to remove the bromide. The now isolated cholesterol was heated on a hot plate. After being allowed to settle, the sample was separated from the zinc solid and heated. Then after being allowed to cool in an ice bath, the precipitated pure cholesterol was utilized to determine the melting point as well as tested for the presence of halogens using the Beilstein test. Reaction mechanism:

ZINC

+

Br2

Data: A sample of 0.4053 g (0.004 mol) sodium bromide and 0.0958 g (0.0007 mol) bromate were combined in a test tube with 0.0467 g (0.0001 mol) cholesterol. Acetic acid was added to produce 0.2017 g (0.00012 mol) of dibromocholesterol, but due to the presence of excess water the yield has less validity. After the addition of zinc dust, the isolated sample of purified cholesterol had a mass of 0.0418 g (0.0001 mol). Table one: Data

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Cholesterol Dibromocholesterol Sodium bromide Bromate

Initial Mass (g)

Final Mass (g)

0.00467 0.4053 0.0958

0.0418 0.2017 -

Theoretical melting point (◦C) 148 113 -

Experimental melting point (◦C) 146-148 105-114 -

Figure 1:

Figure 2:

Figure 3:

Final cholesterol sample

Final dibromocholesterol sample

IR Spectrum results:

Green-cholesterol exp., red-cholesterol std., black-dibromo exp Discussion: The aim of this experiment was to purify a sample of cholesterol through the technique of bromination and debromination. The acetic acid was used to produce dibromcholesterol, which formed once the color of the liquid inside the test tube was no longer a light-yellow tint. The addition of zinc dust successfully removed the bromides from the cholesterol structure to produce a purified cholesterol. Based off IR spectrum analysis (see figure 3), it was confirmed by the CI1 peak at 3417 that cholesterol was the compound we started with and also what we finished with. It was expected for the alcohol group to appear at 3300 and C1 marks its appearance at 3417, thus we concluded that it was the cholesterol compound; based on the symmetry of red and green lines at C1. Peak CI2 and CI3 mark the place where C-H bonds appeared on the initial cholesterol sample, and their disappearance confirms a successful bromination reaction. CF1 occurred at 3363 and marks the purified sample of cholesterol’s OH location, very near to the peak of the sample initially. At peak D1 and D2 it was predicted to see two separate peaks appear around 1000 and 800, indicators that a carbon-bromide bond formed in two places. The experimental sample had peaks at 956 and 893, confirmation that the substance was dibromocholesterol. The carrying out of the Beinstein test was utilized to analyze the validity of the purified cholesterol following debromination and vacuum filtration of the sample. Based on the color of flame that burns as you raise a copper wire coated with the sample over a Bunsen burner, the test determines if the product is halogen containing. During the experiment the final sample of cholesterol burned with a normal distribution of orange coloring, indicative of a lack of bromide in the structure (see figure 1). The sample of dibromocholesterol burned with green coloring, which is what was to be expected as the structure was brominated

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(see figure 2). Lastly, the melting points shown in figure 1 confirmed the identity of the structures. However, due to excess water in the product the yield was invalid. Conclusion: The aim of this experiment was to purify a sample of cholesterol through the method of bromination and debromination. Upon vacuum filtration of the resulting product, the final yield was 0.0418 grams but was declared invalid based on over hydration of the sample. The melting point range of the product was determined to be 146-148◦C compared to the theoretical melting point of 148◦C, confirming the structure of the compound was cholesterol. Reference: 1.

Organic laboratory Manual, Shanbhag & Veliz, Page 40-41, Nova Southeastern University, 2012

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