Chemistry_2510_Yeast_2008 - Chemistry 2510 Introduction to...

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).

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