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Organic Chemistry I

by: Delaney Stokes

Organic Chemistry I CHEM 2010

Delaney Stokes

GPA 3.77


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This 4 page Class Notes was uploaded by Delaney Stokes on Wednesday October 28, 2015. The Class Notes belongs to CHEM 2010 at Volunteer State Community College taught by Staff in Fall. Since its upload, it has received 56 views. For similar materials see /class/230706/chem-2010-volunteer-state-community-college in Chemistry at Volunteer State Community College.


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Date Created: 10/28/15
Purification of Cholesterol An Oxidative AdditionReductive Elimination Sequence From your lectures sessions in CHEM 2010 you have learned that elimination reactions may occur when alkyl halides are treated with strong base With strong base the mechanism is generally E2 as shown in the figure below C OHquot H Br The orbitals of the atoms involved in the bond breaking sequence must be aligned in an antiperiplanar arrangement to provide maximum orbital overlap in the transition state This reaction is neither an oxidation nor reduction according to the convention of electron counting since both an electropositive atom H and electronegative atom Br are lost Another important type of elimination reaction is known as reductive elimination In this reaction a reducing agent often powdered Zn metal is used as the electron source While the exact mechanism is not fully known it can be envisioned mechanistically as shown in the following figure Zn ZnBr Br Br gt ZnBr2 This reaction is considered an oxidation reduction reaction since the Zn metal is oxidized to Zn2 and the dibromocyclohexane is reduced two electronegative Br atoms are lost In this experiment you will use a reductive elimination reaction in the purification of crude cholesterol Cholesterol is a steroidal compound present in animal cells In spite of its bad reputation cholesterol is vital for life It is a structural component in cell walls and is also required for the formation the myelin sheath covering nerve bundles It is also a precursor for other important steroidal hormones Cholesterol isolated from natural sources is generally contaminated with 3 5 related compounds The structure of cholesterol and related impurities is shown in the following figure 7Chole5ten3 ol 57cholestadien3l3 ol Because of steric constraints imposed by the steroid ring structure only cholesterol reacts with bromine to form an insoluble diaxial dibromo compound as shown below Cholestanol does not react with bromine whereas the two remaining impurities are dehydrogenated by bromine to form soluble dienes and trienes by a mechanism we will not consider here The dibromocompound has limited solubility in the reaction solvent and precipitates as a solid It can be purified by solvent washing or crystallization techniques and separated from the impurities which remain soluble in the reaction solvent Once the dibromo compound has been purified it can be reacted with zinc to regenerate cholesterol free of the impurities As part of your prelab writeup include a reagent table which lists pertinent properties of each reagent molecular formula molar mass molecular weight boiling point melting point etc An IR of cholesterol should also be included An example of the reagent table is shown below Some data is lled in but you should complete the remainder Typically the density of pure solids is not particularly relevant to the laboratory procedure so na not applicable is used However the density of solvents and solutions should be included wherever possible References to toxicity and ammability should also be included On line MSDS material safety data sheets are a useful source of physical and chemical data Procedure Note Ethers are common solvents used in many organic synthesis preparations They have the general formula R O R Ethers dissolve many organic substrates yet are unreactive with many reagents these characteristics make them useful solvents A common ether solvent is diethyl ether It is commonly referred to simply as ether and was often used as a general anesthetic prior to the development of improved substitutes lts low boiling point 35 C and its propensity to form explosive peroxides on standing are safety concerns This procedure uses t butyl methyl ether as the solvent It has a higher boiling point than diethyl ether and is less prone to form unstable peroxides This compound is generally known as MTBE and was added to certain gasoline blends to provide more complete combustion However it dissolves in water to the extent of 48g MBTE per 100g water Concerns about groundwater contamination from leaking gasoline storage tanks have led to a phase out of this use A Bromination of cholesterol In a 50mL Erlenmeyer ask dissolve approximately 1g recorded to three decimal places of crude cholesterol in 7mL t butyl methyl ether MTBE by gentle warming on a hot plate setting 4 5 Using a graduated pipet add 5mL of the brominesodium acetate reagent this has been prepared from 90g bromine IOOmL acetic acid and 0 8g sodium acetate to the Erlenmeyer ask in step 1 Record your observations over several minutes After the reaction appears to stop place the ask in an ice bath while stirring with a glass rod for approximately 10 minutes Collect the solids by Buchner filtration and wash with a cold solution prepared from 3mL MTBE and 7mL acetic acid to remove the entrained mother liquors from the solid Wash the solids with cold methanol and suck dry for about 5 minutes Transfer the solids to a pre weighed 50mL Erlenmeyer ask and record the weight of the dibromo compound Reductive Elimination Add 20mL MTBE to the dibromo compound in step 6 followed by 5mL of acetic acid Swirl the contents to mix the solvents Add 02g Zn dust and continue swirling Record your observations After swirling for 5 10 minutes add water dropwise until any solids other than the zinc dissolve no more than 05mL water should be necessary Decant the solution from the zinc into a small separatory funnel and add 20mL water Shake and drain the water layer into a 150mL beaker Add an additional 20mL water and repeat 11 Add 20mL 10 NaOH solution to the separatory funnel shake settle and drain the aqueous layer into the same 150mL beaker in step 10 12 Add 20 mL saturated sodium chloride solution to the MTBE layer in the separatory funnel shake settle and drain The use of saturated salt solutions will lower the water content of the M TBE layer by drawing the water into the highly ionic salt solutiona process similar to osmosis 13 Dry the MTBE layer with a small amount of magnesium sulfate filter through a Chem wipe tissue into a 50mL beaker to remove the hydrated magnesium sulfate to D l 000 gt155 oxu 4gt m O


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