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
1
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.
2
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
3
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
4
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
5
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.
6
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.
7
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.
8
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.
9