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 via Aldol
condensation.
To purify the product through a
recrystallization.
To identify and analyze purity through TLC
and melting point analysis.
To characterize the product using IR
spectroscopy.
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
H3CO
O
H
+
H3CO
3,4-dimethoxybenzaldehyde
cost: $18.20 per 100 g
NaOH
Sodium Hydroxide
cost: $24.60 per 100 g
1-Indanone
cost: $180.00 per 100 g
H3CO
+ H2O
H3CO
Aldol Product
MW: 278.4 g/mol
mp: 180-181oC
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
O
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.
OVERVIEW


Synthesize aldol product in test tube.
Recrystallize solid in propanol/water to
purify.

Suction filter to isolate solid.

Dry product in oven.

Perform TLC analysis on product.

Obtain product mass, calculate % yield.

Obtain experimental melting point.
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:
Experimental Results
Theoretical yield (g)
Determine limiting reagent first, then
calculate theoretical yield.
HINT: Don’t round off number too soon!
Actual yield (g)
% yield
Melting range
(oC)
Product Appearance
Start temperature dial on setting of 5.
Back down one tic mark ~ 150oC and
watch closely. Record Ti-Tf range.
Compare to literature value to determine
purity of product.
Record the physical state and color of
product.
Table 22.2:
TLC Analysis
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:
IR Analysis
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)
Table 22.4:
Green Chemistry Results
Atom Economy (%)
Experimental Atom Economy (%)
“Eproduct”
Cost Per Synthesis ($)
Cost per Gram ($/g)
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.
Costs of stock bottles of
compounds on p. 192 and in
PowerPoint!
Remember to use reactants
and catalyst in this
calculation!
CPS/actual yield (g)
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




Place all liquid waste in container labeled
“LIQUID ORGANIC WASTE”.
Place all solid waste in container labeled
“SOLID WASTE”.
Place all used TLC capillaries and melting
point capillaries in broken glass container,
NOT TRASHCAN!
Leave TLC chambers with lids off in your lab
drawer.
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!
In-lab Question
(The following question should be answered in laboratory notebook.)

CH3CH2
Draw the product, and a complete mechanism
for its formation, of the reaction shown
below.
O
C
+
H
H3C
O
C
CH3
OH
?
In-lab Question
(The following question should be answered in laboratory notebook.)
Give two specific examples of how this experiment
is “greener” than a traditional aldol condensation.
What is it about these changes that make the
experiment “green”?