Isolation and Purification of Cholesterol from Egg Yolk

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The purpose of this experiment was to isolate and purify cholesterol from egg yolks. Two hard boiled egg yolks were twice extracted with diethyl ether and methanol, with the filtrate collected via vacuum filtration. Potassium hydroxide pellets were added to the filtrate, the ether was distilled off, and the mixture was saponified by reflux. The crude cholesterol was isolated through a series of ether extractions and aqueous washes; then the ether was dried with MgSO4 and removed by rotary evaporation. The melting point of the yellow, sticky crude product was 91-119 oC. This crude product was then recrystallized from methanol, yielding 0.128g of pale yellow crystals with a melting point of 131-135 oC. This represents 0.33% of the original mass of the two yolks. The cholesterol was then dissolved in ether and further purified by bromination with a bromine/acetic acid reagent and debromination with zinc powder, a series of aqueous washes, and a final recrystallization from methanol. Here, a yield of 28% was recovered from an initial mass of 100 mg of recrystallized material. The melting point of the off-white crystalline final product was 146-148 oC, which is very close to the literature value for cholesterol of 148.5 oC. From both the melting points and the physical appearances, it is apparent that the final bromination/debromination procedure did in fact further purify the product. The percent of cholesterol in egg yolks was calculated using the mass of the recrystallized product. This calculation does not seem to be valid, as the melting points demonstrated that the recrystallized product was not as pure as the final product. The goal of the experiment was accomplished; cholesterol was isolated and purified from the egg yolks.
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