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Chemistry_2510_Yeast_2008 - Chemistry 2510 Introduction to...

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Chemistry 2510 Introduction to Experimental Organic Chemistry Fall 2008 Experiment #7 Reduction of Ethylacetoacetate Using Yeast Laboratory Notebook : You need to summarize your experimental plan in your lab notebook. Be sure to include a table of all reagents and compounds that will be used in this experiment. Laboratory Observations: Be sure to record any observations during the laboratory. You must record exactly how much of the starting materials you started with and the weight of the isolated product. Results and Conclusions : Calculate a % yield for your reactions, and identify the products through IR spectroscopy and polarimetry. Relevant Reading: Technique 15, IR Spectroscopy, pp 163-178. Background: Reduction of carbonyl groups is an important chemical conversion in organic chemistry. The reduction of these the carbonyl group produces a hydroxyl group. O Reduction OH Reductions can be accomplished by a wide variety of reagents, including hydrogen with an active metal catalyst, sodium borohydride (NaBH 4 ), and lithium aluminum hydride (LiAlH 4 ). These reagents have differing selectivities in terms of which type of carbonyl group they will reduce (i.e. aldehydes, ketones, esters, etc.). These reducing agents, as they are known, also tend not to be stereoselective. This means that if the carbonyl carbon becomes a stereocenter upon reduction (i.e. if R 1 and R 2 are different carbon-containing groups in the reaction shown below), generally both stereoisomers will be produced in about equal amounts (a racemic mixture).
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R 1 R 2 O Reduction R 1 R 2 OH H R 1 R 2 OH H + Racemic Mixture Enzymes are biological catalysts that can lower the activation energy of reactions just like chemical catalysts (i.e. the active metal used with hydrogen to hydrogenate alkenes). A relatively common enzyme that assists in reductions is known as a dehydrogenase (enzymes are named, by convention, for one direction of the reaction they catalyze). Enzymatic catalysis of reactions often brings about a single stereoisomer as the product, when more than one stereoisomer is possible. Reactions with pure, isolated enzymes are possible today, as well as reactions that involve small organisms in the reaction mixture (and the enzymes they contain on their surfaces or inside their cells). In this lab, common baker’s yeast will be used as a chiral reducing medium to transform an achiral starting material, ethyl acetoacetate, into a chiral product. When a single stereoisomer is formed in a chemical reaction from an achiral starting material, the process is said to be enantiospecific. In other words, one stereoisomer (enantiomer) is formed in preference to its mirror image. In this
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