The Aldol Condensation. Synthesis and Analysis ... 7 In condensation reactions,
two molecules ... 7 In today's experiment, we are going to do a crossed aldol in ...
The Aldol Condensation Synthesis and Analysis of 2,3,4,5-Tetraphenylcyclopentadienone O
“Yakety Sax” Bennie Hill theme song
TPCP
Reactions of Aldehydes and Ketones O
R 1. 2. 3. 4.
O
H
R
R'
Nucleophilic Addition. Substitution at the -Carbon Condensation Reactions Oxidation/Reduction Reactions
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Condensation Reactions In condensation reactions, two molecules are condensed to form one larger molecule. In many condensations, a small molecule such as water or an alcohol are eliminated. In carbonyl condensation reactions, one molecule acts as a nucleophile while the other acts as an electrophile at the carbonyl carbon.
Acetaldehyde
Aldol
Treatment of acetaldehyde with base generates 3-hydroxybutanal (“aldol”) O
OH
O
OH C H3C
C
CH H
H3C
CH 2
H
This reaction will also occur under acidic conditions, but we will not discuss that mechanism here.
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Aldol Mechanism O
O O OH H
CH
CH
C
C
C
CH
H
H 2O
+
H
H R
R R
Protons on a carbon adjacent to a carbonyl group are relatively acidic because the resulting anion is stabilized by resonance that delocalizes the negative charge onto an electronegative oxygen atom.
Aldol Mechanism O
O
C
C
O
HC
C RCH2
HC
H
R
R
H
OH
O
O
CH RCH2
H
C CH
H
H
OH RCH2
O
CH C CH
+
OH
H
R
R
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Aldol – Dehydration Step The dehydration step results in the overall
equilibrium favoring product OH
RCH2
O
CH C
H +
C H
H
R
RCH2
O
OH RCH2 OH
O
CH C
C
C
C H
C
+
OH
R
H
+
H2O
R
Aldol Reactions with Ketones Ketones also undergo the aldol reaction, and
dehydration is even more important here. O
CH3
O
OH C H3C
C CH3
H3C
C CH3
CH
O OH
O
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Crossed Aldols If we have two carbonyl reactants, A & B, and
both can form enolates, there are four possible products. A-A B-A B-B A-B Therefore, the yield of either B-A or A-B is not likely to be high. If one of the carbonyl components (e.g. B) cannot form an enolate there are only two possible products - A-A and B-A and the yield of B-A may be reasonable.
Tetraphenylcyclopentadienone (TPCP) In today’s experiment, we are going to do a crossed
aldol in which only one component (dibenzyl ketone) can form an enolate and the other component (benzil) is more reactive towards enolates. In addition, a second intramolecular aldol step gives a cyclic product with two -unsaturated bonds O adjacent to the ketone. Ph O PhCH2CCH2Ph dibenzyl ketone
O PhC
O CPh
benzil
Ph
Ph
Ph
TPCP
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Today’s Procedure A. Synthesis and Characterization of TPCP B. Spectrometric Analysis of TPCP C. Computer Visualization of the TPCP
Structure
Synthesis of TPCP You will prepare an ethanolic solution of
potassium hydroxide and add it to an heated ethanolic solution of benzil and dibenzyl ketone. The solution will be heated for 15 minutes, cooled and filtered to isolate deep purple crystals. The crystals will be washed with cold ethanol, and dried by drawing air through the filter.
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Characterization of TPCP You will determine the weight of your
product and its melting point (above 200 oC) You will also obtain a UV-Vis spectrum of the product.
Spectrometric Analysis of TPCP The TPCP product has a very high epsilon
value, so a very low concentration of TPCP must be used. For quantitative analysis, we would need to do multiple dilutions of a weighed amount of TPCP. Since we are only wanting to see the wavelengths absorbed by TPCP, we will just dissolve a small amount in a solvent into which TPCP dissolves slowly.
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Computer Visualization of TPCP The final component of this experiment is to
go to one of the two computers in the lab room to record some observations on the computergenerated structure for TPCP. Not everyone can wait until the end of lab to do the computer part. Some students will need to do this first (no waiting) or while the reaction mixture is heating. Follow the steps on the Instruction Sheet to complete the Worksheet.
Safety Potassium hydroxide and the ethanolic
solution of KOH are toxic corrosives. Eye hazard! BE CAREFUL in ‘crushing’ the KOH pellets in making the KOH solution. Don’t break the beaker! Hexanes are flammable irritants.
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Clean-up Dispose of filter papers and product in the
appropriate waste bottles in the hood. Solvents from filtration go into the “aqueous” waste bottles in hood. The aqueous ethanol filtrate goes into a waste bottle in the hood. The hexane solutions go into an organic waste bottle in the hood. Clean all glassware with acetone.
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