Chirality of Ethyl-3-Hydroxybutanoate Generated from a Biological Pathway
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Chirality of Ethyl-3-hydroxybutanoate Generated from a Biological Pathway
Jake Zimny
LaSalle University
Philadelphia, PA 19141
Submitted February 10, 2006
Abstract: The reaction being studied is a reduction of a ketone into an alcohol with a chiral center. Because a biological agent, bakers yeast, is being used to drive this reaction, the optical purity that results in the product is so stereo-selective that the major product, (+), is formed for 89% of the product.
Introduction: This experiment was preformed to study the chiral selectivity of the reduction of the ketone portion of Ethyl acetate to a secondary alcohol. This reaction was enzyme catalyzed with a common batch of bakers yeast. The reaction conditions of this process are therefore much less harsh and therefore more environmentally friendly. The main focus of this experiment will not be on the inherent “greenness” or the energy considerations of this mechanism versus a comparable method, but instead focus on the stereo-selectivity. The stereo-selectivity of the product using a biological catalyst is important when contrasted to other methods of generating the product. The most common reaction that would mimic the reduction taking place in this experiment is hydride addition using sodium borohydride, NaBH4. Normally an alternative to this oxidizing agent would be lithium aluminum hydride, LiAlH4, however this reagent would also react with the ester portion of our starting reagent. The effect of reducing the ketone via hydride addition is that product is entirely a racemic mixture (McMurry 696). After the initial reaction the product was isolated using a series of vacuum filtrations, ether extractions, and rotorary evaporations, before being analyzed via proton NMR spectroscopy.
Results and Discussion: The table listed below as Table 1 lists the starting amounts of each of the chemicals present in the reaction flask, along with their formula mass.
Compound:
Moleculear Mass (g/ mol)
Mass Added (g)
Moles Added:
Sucrose
342.2992
80.26
0.234473233
Ethyl Acetoacetate
130.1432
0.038419218
Disodium Hydrogen Phosphate
141.9588
0.501
0.003529193
Sodium Chloride
58.44277
Saturated Soln
Table 1
From the data in Table one and the mass of the final product (2.481 g) a yield for this reaction can be determined. The resulting yield is therefore 51.14%, assuming that the ethyl acetoacetate is the only reagent in this experiment. This assumption is based upon the fact that the sucrose sugar is only present in the reaction flask as a priming agent for the yeast, the sodium hydrogen phosphate is present as an acid base buffer and sodium chloride as an agent in separation. It is a relatively low yield, however that may not be a negative depending on the enanteomeric excess, e.e., of the product. The e.e. of this reaction must be determined via NMR spectroscopy.
The IR spectrum, located in the appendix, of the product suggests two important points. One, that the sample does not contain water. And two, that the spectrum has some signals that match the characteristics of the suggested product. These are the alcohol stretch that is present between 3650 and 3400 (McMurray 408, 410) and the ester stretch at around 1735 (McMurray 802). It also lacks a stretch that would be present for an alkene or alkyne.
The data from the table listed below is taken from the NMR that was taken of the product with the LSR added. This spectrum, for which the table titled Table 2 was generated, is located in the appendix.
Triplet:
Base Halve (cm):
Middle Peak Height (cm):
Area (cm2):
Ratio:
J (mm):
J (Hertz):
Plus (+)
14.82
40.755
9.202
14.021
Minus