Experiment 22:
THE SOLVENTLESS ALDOL-TYPE
CONDENSATION
O
O
O
H3CO
H
H3CO
+
NaOH
H3CO
+ H2O
H3CO
Objectives:




To synthesize an a,b unsaturated ketone
from an aldehyde and a ketone using
solvent-free conditions.
To purify the product through a
recrystallization.
To identify and analyze purity through TLC
and melting point analysis.
To analyze the product using IR and NMR
spectra.
Before coming to lab…

Review these techniques:

Recrystallization

TLC Analysis

Melting Point Analysis
ALDOL CONDENSATIONS

Aldol condensations are nucleophilic
additions of an enolate ion, which are
strong nucleophiles, to another C=O
group under basic conditions.

Protonation gives the aldol product.

Once formed, the aldol product
undergoes dehydration in base.


Abstraction of an a proton gives an
enolate that can expel an –OH ion to
give a conjugated product.
Dehydration is usually exothermic
because it leads to a very stable
conjugated product.
H
H
C
O
b
C CH3
Ca
H
CHEMICAL EQUATION
O
O
O
H3CO
H
+
NaOH
H3CO
3,4-dimethoxy
benzaldehyde
H3CO
+ H2O
H3CO
1-indanone
Aldol Product
MECHANISM
Base
removes a
proton to
form
enolate ion.
O
O
resonance
-H2O
H
O
H
+ NaOH
H
H
O
Resonance
stabilized enolate
forms carbanion,
which attacks
C=O of aldehyde
OH
O
H
+
H
O
O
+ H2O
H
C
H
O
O
When new C-C bond forms,
C=O p bond breaks, electrons
stay with oxygen.
H
O
O
-OH
O
O
H
OH
-OH
O
+ NaOH + H2O
-H2O
C
H
O
O
O
H
Oxygen
anion
removes
proton
from
water,
releasing
–OH.
Acidic a
proton is
removed by
–OH.
Electrons
form new p
bond.
TYPICAL ALDOL CONDENSATION



A typical aldol condensation reaction is
carried out in an organic solvent, such as
ethanol, requiring eventual waste disposal.
Isolation of the product requires a much
more involved work up, not to mention the
hazards to the experimenter and the
environment due to waste disposal.
Another challenge is that solution phase
reaction is reversible, so low product yields
may result.
GREEN CHEMISTRY CONCERNS



One of the main themes of greener chemistry
is to cut down on the use of solvent, and
hence cut down on solvent waste. The best
way to do this is simply not to use any solvent
at all.
Workup is generally easier since as the
reaction proceeds, the product separates from
the melt as a solid, making work up much less
complicated.
Another benefit is that unlike a solution phase
reaction, the solid-state reaction is
irreversible, resulting in higher chemical
yields.
EXPERIMENTAL PROCEDURE
(Synthesis)




Place aldehyde and ketone in
large test tube.
Crush solids with glass rod until
liquefied.
Add NaOH. Mix and scrape
sides of test tube until pale
green solid forms.
Continue to react for 15 minutes
at room temperature.
EXPERIMENTAL PROCEDURE
(Purification and Product Isolation)






Place test tube in a 90oC water bath.
Add 90:10 PROPANOL/WATER and
heat until solid dissolves.
Suction filter.
Transfer solid to PREWEIGHED
WATCH GLASS AND PLACE IN OVEN
15 MINUTES to dry.
Obtain final product mass and calculate
% yield.
Proceed to PRODUCT ANALYSIS (TLC,
mp, IR).
Table 22.1
Determine limiting reagent
first, before calculating
theoretical yield.

Theoretical yield (g)
Actual yield (g)
% yield
Melting range
(oC)
Product Appearance
Record Ti-Tf range.
 Compare to literature value to
determine purity of product.

Record the physical state and
color of product.

Table 22.4
Atom Economy
Experimental Atom Economy
E product
Cost per Gram
Calculate based on reactants
ONLY!
 Review Experiment 13 for
calculation.

Calculate based on reactants
ONLY!
 Review Experiment 13 for
calculation.

Review Experiment 13 for
calculation.

Calculate COST PER
SYNTHESIS 1st based on
reactants and catalyst.

Table 22.2
TLC Rf values
Compound
1-indanone
3,4-dimethoxybenzaldehyde
Aldol Product
Standard
Sample
Rf values are UNITLESS!
 Rf values are 2 decimal places
ONLY!

Product Analysis
(IR)
Table 22.3
Functional
Group
Base
Values
(cm-1)
sp3 CH stretch 2800-3000
sp2 CH stretch 3000-3100
C=O stretch
1680-1740
C-O stretch
1000-1200
aromatic C=C 1500-1600
stretch (2)
aldehyde CH 2700-2800
stretch (2)
1-Indanone
3,4-dimethoxy
-benzaldehyde
Aldol
Product
Frequency
(cm-1)
Frequency
(cm-1)
Frequency
(cm-1)
SAFETY CONCERNS


Solid NaOH is very corrosive!
Immediately flush skin with water if
any gets on your skin!
Propanol is very flammable! Use
extreme caution during purification!
WASTE MANAGEMENT




After final mass is determined, and small
amount of product has been set aside for
further TLC and mp analyses, place product
in container labeled “Aldol Product”.
Pour all liquid waste into bottle labeled “
Organic Waste (Aldol)”.
Leave TLC chambers with lids off in your lab
drawer.
Place all used TLC capillaries and melting
point capillaries in broken glass container,
NOT TRASHCAN!
CLEANING


Clean all glassware with soap, water,
and brush, followed by a rinse with
wash acetone before returning to lab
drawer.
DO NOT return any glassware to lab
drawer dirty or wet!