Lab 8.doc

LAB 8: REDUCTION Quiz what is the role of the first 0.5 mL of water that is added in the reduction reaction? To hydrolyze borate ester why is it possible to separate (1R, 2S) - hydrobenzoin from (1R, 2R) - and (1S, 2S)- hydrobenzoin? (be specific) because they are diasteriomers which have different physical propertie why can?t you separate (1R, 2R)-hydrobenzoin from (1S, 2S)-hydrobenzoin? (be specific) because they are enantiomers which have similar physical properties why is it possible to separate (1R, 2S) - hydrobenzoin from (1R, 2R) - and (1S, 2S) - hydrobenzoin? (be specific) because they are diasteriomers which have different physical properties why can?t you separate (1R, 2R)-hydrobenzoin from (1S, 2S)-hydrobenzoin? (be specific) because they are enantiomers which have similar physical properties why are reductions using sodium borohydride run in ethanol or methanol and NOT in ether? it is not soluble in ether In the reduction of benzyl rxn, the major product, meso-hydrobenzoin, is considered achiral even though it has 2 chirality centers, why? It has a plane of symmetry Why are reductions using lithium aluminum hydride run in ether and NOT ethanol or methanol? LiAlH4 reacts violently with protic solvent Why are reductions using sodium borohydride run in ethanol or methanol and NOT in ether? NaBH4 is insoluble in ether what is the role of the 1 mL of hot water, after the 1st addition of .5mL water, added to the clear solution in the reduction rxn? To saturate solution to begin crystallization why is it important to keep the benzyl in ethanol solution in ice when borohydride is added in the reduction rxn? To control rate of rxn or rxn is exothermic List 3 reasons an excess of borohydride is used in the reduction experiment. To ensure reaction goes to completion To account for NaBH4 reacting with solvent To account for decomposition Know the reduction of benzil to hydrobenzoin (R,R) ? Hydrobenzoin cannot be separated from (S,S) hydrobenzoin by recrystallization. Why is that? Enantiomers cannot be separated by recrystallization Why are reductions using sodium borohydrie run in ethanol or methanol and NOT in ether? sodium borohydride is not soluble in ether In the reduction reaction, the hote water and the 95% ethanol act as a solvent pair for the benzil. Which is the better solvent and which is the poorer one? Better: 95% ethanol Poorer: water Why is the hydroxide ion in the dibenalacetone synthesis considered a catalyst? It is not consumed in the reaction and regenerated at the end. Why are reductions using lithium aluminum hydride run in ether and NOT in ethanol or methanol? lithium aluminum hydride has a tendency to react in a volatile manner with alcohols Recrystallization techniques are used to purify meso-hydrobenzoin. What (give names of compounds) are you separating meso ? hydrobenzoin from? you separate the meso ? hydrobenzoin from the borate ester. (R,R) hydrobenzoin (S,S) hydrobenzoin, S-benzoin, and R-benzoin Suggested Questions How reduction is characterized in organic chemistry Increase of bonds to H and/or decrease of bonds to electronegative atoms How oxidation is characterized in organic chemistry Decrease of bonds to hydrogen Increase of bonds to electronegative atoms How to recognize reduction, oxidation and substitution Why NaBH4 used instead of LiAlH4 Milder, more selective in reducing aldehydes and ketones, does not react violently in contact with water or alcohols like LiAlH4 Why was ethanol used as solvent instead of ether in the reaction Unlike LiAlH4, sodum borohydride is insoluble in ether and soluble in methanol and ethanol Why ether must be used as a solvent if LiAlH4 is used Because liALH4 is soluble in ether Why was excess NaBH4 used in the reaction To ensure completeion of the rxn To account for NaBH4 reacting with solvent To account for decomposition of NaNH4 Specific side products and why they form RR or RS hydrobenzoin- carbonyl carbon is sp2 hybridized, attack could occur from above or below R or S benzoin- incomplete rxn Role of each addition of water 1st addition: to hydrolyze borate ester 2nd addition: to saturate solution to start crystallization Role of ice bath To control the reaction- addition of NaBH4 is exothermic Why recrystallization carried out To purify meso hydrobenzoin from its side products (This is possible because it has different physical properties from the major product) Why melting point was carried out Sources of error Loss from transferring of products/crystals (scooping crystals out of test tube, from filter paper) With regards to crystallization: Insufficient drying of crystals, adherence of crystals to filter paper, crystals dissolving during washing process on Hirsch funnel With regards to melting point: Moisture in sample, improperly calibrated thermometer of mel-temp, crystals not finely powdered, sample heated too quickly, more than 2-3 mm in capillary tube How IR, TLC and mixed melting point could be used IR: Can show presence of functional groups, and thus, show whether the product is pure. TLC: To determine the # of components in the product (would show purity) Mixed melting point: Can mix some of the product w/ the compound that the product is supposed to be. If melting point stays the same as that compound, the product is what it is supposed to be. If melting point is depressed and melting range is bigger, indicates presence of impurities Must be able to identify R and S stereochemistry on major and side products Prelab Questions Calculate the theoretical yield of meso-hydrobenzoin if 60 mg sodium borohydride were the limiting reagent. Provide answer to the nearest mg. Show calculations. For each of the following reactions (CLICK ON PDF. LINK BELOW), indicate if they are oxidation, reduction or substitution reactions. Postlab Questions NONE Introduction What is the general structure of a ketone? (Draw structure) What is the specific kind of product that is formed in the reduction of ketones? Secondary alcohols List the two ways reduction is represented in organic chemistry Are as an increase of bonds to hydrogen and decrease of bonds to electronegative atoms What specific kind of molecule (functional group) is benzil and what specific kind of molecule (functional group) is it forming upon reduction? The specific molecule (functional group) that benzyl is a diketone. Upon reaction it forms a diol. Apparatus NONE Reactions