Aldol Condensation


The acidity of organic compounds is often determined by neighboring groups because they can help
stabilizing the resulting anion (i.e., halogen, nitro, etc.) because of their electronegative character
Functional group
pKa
Alkane
~50
Ester
~25
Aldehyde/ketone
~18-20
Alcohol
~15-19
For instance, the presence of a carbonyl group greatly increases the acidity of neighboring hydrogen
atoms (a-protons) because of the resonance stabilization in the resulting enolate ion (the numbers in
parentheses below are from acetone, AM1)
127.8 pm
(123.5 pm)
137.4 pm
(149.5 pm)

Many of the carbonyl compounds can be deprotonated with moderately strong bases i.e., hydroxide,
alcoholates, etc.
 Ketones and aldehydes can be reacted with each other in
Aldol or Claisen-Schmidt condensation
H
+
O
Ph
H
O
H
O
O
OH
O
T
H
H
+
O
[OH-]
Aldol
H
Self condensation of acetaldehyde to
O alsoOH
form[OH
crotonaldehyde
called "aldol"
T
]
H
Ph
Formation of cinnamaldehyde
O
H
Ph
 In Chem 14CL, acetone is reacted with two equivalents
of benzaldehyde using sodium hydroxide as catalyst
O
CHO
O
2
NaOH(aq.)/EtOH
+ 2 H2O
+
 The first step is the formation of the first enolate ion
O
C
O
C
+
–OH
C
O
C
C
C
+ H2O
H
en olate ion
 Note that water is one of the products in the enolate
formation  water has to be excluded from the reaction
mixture as much as possible (dry glassware, absolute
ethanol) in order to optimize the amount of enolate
 The enolate ion acts as the nucleophile, which attacks the
carbonyl group of the benzaldehyde to form the new C-C bond
 The trans, trans-isomer is primarily formed and isolated in the
reaction
 Acetone is reacted with two equivalents of benzaldehyde
in a small Erlenmeyer flask
 An aqueous ethanolic solution of sodium hydroxide is
added slowly (drop wise) while stirring
 The mixture is stirred for 30 minutes
 The crude product precipitates as a yellow solid from the
solution during the reaction
 The solid is isolated by vacuum filtration
 The filter flask has to be clamped
 The neoprene adapter (rubber adapter) has to be placed






between the porcelain funnel and the filter flask to have
a better seal
A filter paper is placed on the plate of the funnel so that
all holes of the porous filter plate are covered
The stir bar is removed from the reaction mixture and
rinsed with water
After the suction is turned on, the filter paper is wetted
with water to ensure that the filter paper adheres to the plate
The suspension is carefully poured into the funnel
The solids that are stuck to the walls is scraped off and suspended
in a small amount of water to transfer it to the funnel as well
The filter cake is rinsed with small portions of water until the filtrate
is neutral (test with pH-paper)

The crude contains benzaldehyde, benzalacetone (mono-addition product), dibenzalacetone,
mesityl oxide (self-condensation product of acetone), etc.
 The crude product is purified by recrystallization


The literature suggest ethyl acetate or ethanol as solvent for the recrystallization
In Chem 14CL, 95 % ethanol is used for this purpose because dibenzalacetone is
the least polar compound in the mixture and therefore displays the lowest solubility
in 95 % ethanol
 Slow crystallization yields nicer crystals and a purer product

Procedure

The crude is dissolved in a MINIMUM amount of the boiling solvent







Place the solid and a stir bar in a small Erlenmeyer flask
Add a small amount of solvent to the solid
While stirring, heat the mixture up to a gentle boil to dissolve the solid
If the solid does not dissolve completely, add a little more solvent and boil again
Any solids that do not dissolve in the hot solution are removed by filtration
The hot, saturated solution is allowed to cool down to room temperature slowly
The solution is then placed in an ice-bath for 15 minutes to reduce the solubility
of the compound in the solution
 The product is isolated by vacuum filtration and rinsed with a small amount of water
 Air is sucked through the solid for 15 minutes to pre-dry the solid
 Transfer the product to a vial and leave it open during the week to dry the product
 The melting point (m.p.) of a pure substance is defined as the
temperature at which both the solid and liquid phases of that
substance has the SAME vapor pressure i.e., both phases are
in equilibrium
 Melting points are used:
 to identify compounds (by comparison with literature values)
 to establish purity
 the purer the sample the higher the melting point
 the narrower the melting point range
 impure samples display a melting point depression and a broad melting point
range while a pure substance usually possesses a very sharp melting point
(T=0.5-1 oC)
 Exception: Eutetic mixtures possess a sharp melting point as well
 Acetone and benzaldehyde are colorless liquids while the product of the
reaction is bright yellow. Why?
 Acetone only possesses an isolated carbonyl group
 In benzaldehyde, the carbonyl group is conjugated to a benzene ring
 In the product, the carbonyl group is connected to two alkene functions
that are attached to benzene rings
 The conjugation decreases the HOMO-LUMO gap resulting in
a bathochromic shift for the transition (HOMO-LUMO gap calculations
with DFT, EDF2, 6-31**)
Compound
l (nm)
e(M-1*cm-1)
CH3COCH3
188
900
6.01
PhCHO
244
14100
4.90
trans-PhCH=CHCOCH3
286
32360
4.27
trans, trans-PhCH=CHCOCH=CHPh
330
34300
3.84
cis, trans-PhCH=CHCOCH=CHPh
295
20000
cis, cis-PhCH=CHCOCH=CHPh
287
11000
HOMO-LUMO
GAP (eV)