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Questions 1,2,3 : based on your knowledge Questions 4 and 5: you can put in any number to answer it 9:48 PM bbhosted.cuny.edu Lab Report dibromination of cholesterol 1. How does bromination of cholesterol work to separate pure cholesterol from other molecules that are similar in structure to cholesterol? Are impurities present in large amounts? (see lab book) 2. Instead of cutting and pasting an image into your lab report, leave space and draw in the cholesterol molecule by hand and then draw the sequential steps of the mechanism for bromination of cholesterol by Br. Take care to indicate whether the bromine addition is "syn" or "anti." 3. We brominate cholesterol in the experiment but do not complete the final purification step. What would we have to do to complete the process? 4. You should calculate percent yield you obtained by first calculating the theoretical yield of the reaction based on your starting material of 0.1 g or 100 mg and the molecular weight of cholesterol (if you want to be even more accurate you will take into account the approximate percent impurities). The reaction stoichiometry is a 1:1 ratio BUT REMEMBER YOUR PRODUCT HAS ANEW MOLECULAR WEIGHT (dibromination). Once you have the correct theoretical yield in grams you ean proceed to calculate percent yield as usual. 5. Even if you got a high yield, is this completely satisfactory as a result? Why or why not?

Explanation / Answer

1. Bromination is one of the processes to purify cholesterol from the impurities which are present in it. In the process of bromination, the solvent used in reaction is ether. Cholesterol dissolves in ether but other impurities present in it are insoluble. Cholesterol reacts with bromine but others do not react and thus gets separated easily.

2. In the bromination of cholesterol, initially electron rich alkene of cholesterol will attack on bromine molecule as a nucleophile and take Br+ and leaves Br-. The cholesterol is now +ve charged. The bromide ion will now act as a nucleophile and cholesterol acts as an electrophile. This nucleophile then attacks on electrophile and forms dibromide. The bromine adds in anti fashion because the bromine atoms like to lie in opposite faces due to steric hinderance. Anti addition is preferred here.

3. After the reaction, the dibrominated cholesterol forms crystals when the reaction mixture is kept in ice bath. After that, the crystals were separated by filteration through funnel. The crystals were given washing with ether to remove the unreacted materials. The crystals obtained is pure dibrominated cholesterol.

4. mass of cholesterol taken = 0.101 g

molar mass of cholesterol = 386 g/mol

Moles of cholesterol used = mass/molar mass = 0.101/386 = 2.62*10-4 mol

According to reaction, 1 mol of cholesterol produces 1 mol of dibromide cholesterol.

So, 2.62*10-4 mol of cholesterol produces 2.62*10-4 mol of dibromide cholesterol.

molar mass of dibromide cholesterol = 546 g/mol

Mass of dibromide cholesterol = moles *molar mass = 2.62*10-4 * 546 = 0.143 g

Mass produced experimentally = 0.263 g

% yield = (experimental yield/theoretical yield)*100

= (0.263/0.143)*100 = 184%

% yield = 184 %

5. The yield is more than 100 %, this means that the compound is not completely pure. There are some impure compounds also in the mixture. Or it is also possible that the product is not completely dry. So, the results are not satisfactory.

3.

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