Aldol Condensation and Transfer Hydrogenation
O
O
O
NaOH
+
H
aq. EtOH
p-tolualdehyde
4'-methylacetophenone
chalcone product
NH 4+HCO 2Pd/C
MeOH
O
Background: The Aldol Condensation
Under acidic or basic conditions, an aldehyde or ketone may react with itself to form a carboncarbon between two molecules. The α-carbon of one molecule acts as a nucleophile and attacks the
carbonyl group of another molecule. The mechanisms differ depending on whether acid or base is
used; the mechanism for the base-catalyzed aldol condensation of ethanal is shown below:
O
O
H
CH 2
CH 2
CH 2
H
H
O
O
enolate anion
OH
O
H O
O
H OH
H
H
OH
aldol product
- HO
The product of the above reaction is both an aldehyde and an alcohol, and thus can be described as
an aldol.
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In organic synthesis, it is more common to direct the enolate of one carbonyl compound to attack
the carbonyl of another compound, which can provide more structurally elaborate products:
O
O
acid or base
+
R1
R3
H
HO H
R1
R2
O
* *
* = potential stereogenic carbon
R3
R2
Note that a pair of stereogenic carbons can result from this reaction. The aldol condensation has
been developed to a high level of art, and diastereo- and enantioselective variants are powerful tools
in organic synthesis. Know this structural motif! If a compound has a hydroxyl β- to a carbonyl, or
is related to this structure, it could potentially be synthesized via the aldol reaction.
This motif is very common in natural products. For example, a key step in fructose biosynthesis is
the aldol condensation of two 3-carbon sugars, catalyzed by an aldolase enzyme:
OH
2-O
OH
3PO
H
+
OH
aldolase
2-O
OPO32-
O
3PO
O
glyceraldehyde
3-phosphate
(GAP)
OH
OPO32-
OH
dihydroxyacetone
phosphate
(DHAP)
O
fructose-1,6-bisphosphate
This motif is also seen in a class of compounds called polyketides. A classic example of polyketide
biosynthesis is the production of the antibiotic erythromycin A in the bacterium Saccharopolyspora
erythraea (Figure 1, next page). A factory-like cluster of enzymes extends a chain three carbons at
a time (making some chemical modifications along the way) to arrive at a linear precursor that then
cyclizes into a lactone. Further enzymatic modifications convert the lactone to the final antibiotic.
Humans have learned what the various “machines” (enzymes) in this “factory” (assembly) do, and
through genetic modification “retooled” organisms to produce new molecules never before seen in
nature.
When care is taken, the aldol product can often be isolated after its synthesis. However, when harsh
acidic or basic conditions are used, aldols can dehydrate to form an α,β-unsaturated carbonyl
compound:
HO H
R1
O
R3
R2
H
acid or base
O
R1
R3
+ H 2O
R2
This is another motif that you should be familiar with: α,β-unsaturated carbonyl compounds can
possibly be synthesized via an aldol reaction. In the case of the reaction that you will be
performing, the strongly basic conditions, plus the thermodynamic favorability of forming a highly-
S2
O
S
OH
ACP = "Acyl Carrier Protein"
ACP
O
S
ACP
O
OH
OH
O
S
HO
ACP
OH
OH
NMe 2
HO
O
O
OH
OH
O
O
O
S
O
ACP
O
OMe
O
OH
Erythromycin A
OH
OH
O
O
S
OH
OH
O
OH
ACP
further
enzymatic
modification
O
S
OH
OH
O
OH
OH
ACP
O
O
S
ACP
OH
cyclize
O
O
polyketide chain growth
via aldol-like condensations
OH
OH
lactone
Figure 1: Biosynthesis of the Polyketide Antibiotic Erythromycin A
conjugated C=C double bond, favor formation of the dehydration product. Such diaryl enone
compounds are called chalcones; many natural products such as flavonoids are synthesized from
chalcone precursors.
OH
HO
O
a chalcone
O
OH
O
naringenin
a flavonoid found in grapefruit
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Background: Chemoselective Reduction of the C=C Bond
One common, “textbook” method for the reduction of alkenes is hydrogenation using a transition
metal catalyst (e.g. H2, Pd/C). However, the use of hydrogen gas can be cumbersome. One way to
maintain a hydrogen atmosphere is to equip the apparatus with a hydrogen balloon. The hydrogen
gas gradually diffuses out of the balloon, so it has to be replenished periodically. Alternatively, a
closed, high-pressure system (“bomb”) may be used. High pressures of flammable gas are a safety
concern, and often special rooms with blow-out walls are required for such systems.
In today’s experiment, the hydrogen atom in the formate ion (HCO2 ) will act as a source of
hydrogen. Because the hydrogen is being transferred from formate to the alkene, this type of
reduction is known as transfer hydrogenation. The exact mechanism of its reaction with Pd/C is not
known for sure, but the net result is as if hydride (H: ) and carbon dioxide are produced:
O
H
Pd/C
O
"H: "
+ CO 2(g)
The hydride could then possibly add to the alkene directly, or indirectly, or be protonated to form
H2 gas in situ.
Both the C=C and the C=O double bonds of your chalcone are capable of being reduced under these
reaction conditions. Sometimes, when another functional group in a reactant is sensitive to the
reaction conditions, a protecting group strategy can circumvent problems. However, the
protection/deprotection reactions add steps to the synthesis, as well as additional reagents. Instead,
you will be using a set of reaction conditions that are chemoselective for reduction of an alkene in
the presence of a carbonyl. The alkene will be reduced faster than the carbonyl; however, the level
of chemoselectivity is not great, and long reaction times will result in loss of product via carbonyl
reduction.
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Procedure: Aldol Condensation
SAFETY INFORMATION
Wear nitrile gloves while conducting this experiment.
The aldehydes, ketones and chalcones encountered in this project are potential skin and eye
irritants.
Contact with solid sodium hydroxide pellets causes severe burns and a concentrated aqueous
sodium hydroxide solution is corrosive and causes burns. Sodium hydroxide is hygroscopic: keep
containers closed when not in use, and immediately dispose of any excess in the aqueous waste
container.
Diethyl ether is extremely volatile and flammable. Use it in a fume hood.
Ethanol is flammable and toxic.
Combine 0.41 mL of p-tolualdehyde and 0.47 mL of 4’-methylacetophenone, plus 2 mL of
95% ethanol, in a vial. Add a stir bar and stir the solution. While stirring, add 0.6 mL of 44 wt.%
NaOH(aq). Stir at room temperature for 30 minutes. During this period a pale yellow solid should
form. While stirring the reaction mixture, cool it to 0 °C in an ice-water bath, add 2−3 mL of water,
and cool again to 0 °C. Collect the product by vacuum filtration, washing the product twice with
5−10 mL of cold water to remove residual sodium hydroxide.
Recrystallize the crude product from 95% ethanol. Allow the product to dry until next lab
period, then obtain its mass, melting point, and IR spectrum. The expected melting point is 126127.5 °C. Also reserve ~10 mg of product in a small glass vial; this will be used as a TLC reference
for the hydrogenation reaction. Do not start the next step until your TA has chosen one student’s
product for NMR analysis.
Cleanup: The filtrate can be disposed of in the aqueous waste carboy.
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Procedure: Transfer Hydrogenation
SAFETY INFORMATION
Carry out this experiment in a hood and wear nitrile gloves.
Keep palladium on activated carbon away from heat and sources of ignition. Do not breathe the
dust. If dry Pd/C comes into contact with methanol vapor, it can ignite the methanol solvent. Be sure to
add the Pd/C to your reaction flask before adding the methanol. Be sure to sweep up any spilled
catalyst with a wet tissue and then discard the tissue in the solid waste container.
Ammonium formate is an irritant. Avoid contact with skin, eyes and clothing. Ammonia is an
irritant.
Methanol is flammable and toxic. Use it in a hood.
Flammable Hydrogen gas can be generated in the reaction. No flames should be used in the
laboratory.
Hexane and ethyl acetate are flammable and irritants.
Diethyl ether is extremely volatile and flammable. Use it in a hood.
Clamp a 50-mL round-bottomed flask (without grease) to a ring stand in your hood. To
protect the inside of the ground-glass joint during the addition of solid reagents, roll a small piece of
weighing paper into a cone and insert it into the flask so that it fits snugly in the neck of the flask
and just covers the inside of the joint. Add 200 mg of the chalcone and 7.5 mmol of solid
ammonium formate per mmol of chalcone through the opening in the weighing paper. Weigh 0.040
± 0.005 g of 10% palladium on carbon on a weighing paper and add the Pd/C carefully to the flask.
Remove the rolled weighing paper and swirl the flask to coat the surface of the catalyst with the
other reagents. (Safety precaution: Add the methanol only after swirling the reagents together in
the flask.) Add 18 mL of anhydrous methanol and again swirl the flask to mix the contents. Add a
water-cooled reflux condenser to the flask and place the flask in a heating mantle. Begin heating the
flask.
The reaction mixture will start bubbling even before it reaches the boiling point, due to the
gas produced in the decomposition of ammonium formate. (Reminder: Do not begin to time the
reflux period until the methanol is boiling and recondensing from the condenser). Heat the
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reaction at reflux for 18−20 minutes. Shorter heating can leave unreacted chalcone and longer
heating can produce more complex product mixtures.
Recovery and TLC Analysis of the Product
At the end of the reflux period, cool the flask containing the reaction mixture in an ice-water bath
for 5 min. Prepare a filter pipette by tamping down a small wad of Kimwipe into a Pasteur pipette,
and then adding 1-2 cm of Celite on top of the Kimwipe plug. Clamp the pipette with the tip inside
a 100-mL round-bottom flask. Pass 1-2 mL of Methanol through the pipette to rinse the Celite and
pack it down.
Use another Pasteur pipette to transfer the cooled reaction mixture into the filter pipette.
When the filtration is complete, rinse the reaction flask with 1 mL of methanol and pass it through
the filter pipette into the round-bottom flask containing the product solution. Replace the roundbottom flask containing the product mixture with a small beaker and rinse the pipette with 3 mL of
water in order to be able to dispose of the palladium safely. Dispose of the entire filter pipette into
the solid waste container.
Concentrate your filtrate on the rotary evaporator. Add 5 mL of water and 15 mL of ether to
the crude product in the flask. Swirl the flask to mix its contents and then transfer the mixture to a
separatory funnel. Rinse the flask with 5 mL of ether and add this rinse to the separatory funnel.
Mix the two phases to produce an efficient extraction and then drain the aqueous phase into a
labeled flask. Pour the ether layer into a clean, labeled Erlenmeyer flask. Extract the aqueous
phase again with an additional 10 mL of ether. Combine the two ether solutions and wash this ether
layer with 10 mL of water. Drain the aqueous phase and pour the ether solution into a clean 50-mL
Erlenmeyer flask. Dry the ether solution with anhydrous sodium sulfate.
While your product is drying over sodium sulfate, analyze the product by TLC. Prepare the
reference TLC solution by dissolving your ~10 mg of reserved chalcone in 1.0 mL of ethyl acetate.
Use 30/70 ethyl acetate/hexane (v/v) as the TLC developing solvent, and visualize the spots using
UV as well as p-anisaldehyde stain.
Filter the dried ether/product solution into a clean, dry, and tared 100-mL round-bottom
flask and concentrate in vacuo. Record the mass of your product. Obtain an IR spectrum for your
product. Your TA will select one student’s product for NMR analysis.
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Cleanup: Place the Pd/C catalyst-filled filter pipette and any other materials containing Pd into the
solid waste container in the hood. The aqueous layers from your extraction and spent drying agent
can be disposed of in the aqueous waste carboy. Place the remaining TLC solvent in the carboy for
flammable organic waste.
Interpretation of Experimental Data
IR: The more extensively conjugated a carbonyl is, the lower its frequency for the C=O
stretch in the IR. An aliphatic ketone such as acetone has a C=O stretch around 1720 cm-1. In an
aryl ketone such as acetophenone, the C=O stretch is shifted down to ca. 1665-1684 //, and in a
chalcone it is shifted even further to 1650-1664 //. Analyze your IR and determine not only if the
spectrum is consistent with what you expect for your product, but also if impurities (e.g. solvent;
unreacted starting material; byproducts) are present.
NMR: Analyze the NMR spectra your TA obtained for the aldol and the reduction products.
Determine if the expected product was obtained. Also determine what impurities may be present.
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