The Bromination of Cholesterol, An Electrophilic Addition Reaction of a Natural Product PDF

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Experiment VIII: The Bromination of Cholesterol, An Electrophilic Addition Reaction of a Natural Product

Abstract: The purpose of this experiment was to perform a chemical reaction with cholesterol through an electrophilic addition reaction known as halogenation, specifically bromination. Cholesterol bromide is created as the product. This is completed by mixing Br2 with cholesterol and Methyl tert-butyl ether (MTBE). This creates an ionic mechanism where the Carbon Carbon double bonds are broken. It is then dissolved in HOAc with NaOAc. The mixture is cooled and crystalizes in an ice bath and then ran through a vacuum filtration with a wash solution to obtain the product. 102mg of cholesterol dibromide was recovered with an acquired melting point range of 111.8 ºC – 119.7 ºC. The cholesterol dibromide obtained has a percent yield of 65%. Procedure: The bromination and vacuum filtration to obtain cholesterol dibromide were completed with the given steps from the Poulos, Z.J., “The Bromination of Cholesterol32 An Electrophilic Addition (AdE) to a Natural Product”, in Organic Chemistry I- A Laboratory Manual for the Health Science Major,” Fall 2019; Hayden-McNeil: Plymouth, MI, 2019, pp 132136. The only change made during this experiment was that the solution of 30% MTBE in glacial acetic acid to wash the crystals was already provided, so step 6 of the procedure was skipped. Reactions:

Cholesterol

Cholesterol Dibromide

Data and Calculations: Molecular Weight (Cholesterol)

386.66mg

Molecular Weight (Cholesterol Dibromide w/ Acetic Acid) Literature Melting Point (Cholesterol Dibromide w/ Acetic Acid) Mass of Cholesterol

606.52mg 112.0 ºC – 115.0 ºC (0.102g x 1000)

Tared Aluminum Dish

=102.00mg (0.411g x 1000)

Mass of Aluminum Dish with Product

=411.00mg (0.513g x 1000) =513.00mg

Recovered Mass of Cholesterol Dibromide (Actual Yield) Melting Point Range (actual) Theoretical Yield

(513mg – 411mg) =102.00mg 111.8 ºC – 119.7 ºC mmol 100.00mg x = 0.26mmol 386.66 mg C27H46O 0.26mmol

x

606.52 mg = 158.00mg mmol

x

1 x 100 = 65% 158 mg

C27H46Br2OHOAc Percent Yield

102mg

C27H46Br2OHOAc *Molecular weight and melting points of cholesterol dibromide and molecular weight of cholesterol were taken from the literature of Poulos, Z.J., “The Bromination of Cholesterol32 An Electrophilic Addition (AdE) to a Natural Product”, in Organic Chemistry I- A Laboratory Manual for the Health Science Major,” Fall 2019; Hayden-McNeil: Plymouth, MI, 2019, pp 136. Discussion and Conclusion: In this experiment, halogenation was the main chemical reaction that cholesterol had to go through, specifically bromination. Cholesterol is an alkene. When reacted with bromine, the pi bond is broken to form two sigma bonds, and with the addition of

bromine to the Carbon Carbond double bond, cholesterol creates vicinal dibromide. Since the product has only one specific stereoisomer, this reaction is stereospecific. Throughout this experiment, color and smell were important observations that helped make this a successful experiment. The solution became a yellow color with a strong vinegary scent indicating that it is saturated with bromine and the mixture is undergoing crystallizations rapidly. When running it through the vacuum filtration, the product was observed to change yellow to white immediately. The final product of cholesterol dibromide is a white powdery substance. The experiment was carried out with an initial 102mg of cholesterol that was dissolved with MTBE and then added with a solution of bromine drop by drop to achieve the yellow color. The mixture was then cooled in an ice bath for crystallization. The mixture was filtered through a vacuum filtration apparatus to obtain the product. 102mg of the product was obtained. Since cholesterol is the limiting agent, the number of moles of cholesterol should equal the number of moles of cholesterol dibromide. The experiment has a calculated theoretical yield of 158mg and a percent yield of 65%. The melting point range was determined to be 111.8 ºC – 119.7 ºC which is very close to the literature melting point of cholesterol dibromide being 112.0 ºC – 115.0 ºC, which means the product is quite pure. Although the recovered mass of the product is 102mg, the same as the starting mass of cholesterol, the percent yield was only 65%. This could mean the during the crystallization process, the mixture was not properly cooled. When analyzing the IR Spectrum of Cholesterol, there are three major bonds from the two functional groups of alcohol and alkene: Oxygen Hydrogen bond (O–H), Carbon Hydrogen bond (C–H), and Carbon Oxygen bond (C–O). The Oxygen Hydrogen bond has a range of 3200cm-1 to 3600cm-1 with a broad peak at 3302cm-1. Carbon Hydrogen bond has a range to the right of 3000cm-1 with multiple peaks shown at 2928cm-1, 2905cm-1, 2865cm-1, and 2850cm-1.

Lastly, the Carbon Oxygen bond has a range of 1000cm-1 to 1200cm-1 with a strong peak at 1052cm-1. The IR Spectrum of Cholesterol Dibromide has four major bonds: Oxygen Hydrogen bond (O–H), Carbon Hydrogen bond (C–H), Carbon Bromine bond (C–Br), and Carbon Oxygen double bond (C=O). A There is a broad peak at 3390cm-1 for Oxygen Hydrogen bond. Carbon Hydrogen bond has multiple peaks at 2942cm-1, 2904cm-1, 2866cm-1, 2849cm-1. At 590cm-1 is the Carbon Bromine bond that has a range to the right of 667cm-1. There is also a strong peak at 1706cm-1 from the Carbon Oxygen bond, which is a result of the acetic acid’s carbonyl group....