Exp't 19
Mystery Product: The Reaction of Cinnamaldehyde and
Acetone
by Kurt Rublein, Organic Instructional Laboratories, 2/1/95 – Rev 3/17/00
Introduction
The aldol condensation reaction is one of the most important synthetic routes that give large molecules
through the formation of carbon-carbon bonds. The first step involves the acid-base reaction between a
strong base such as hydroxide ion and a hydrogen located a carbon alpha to a carbonyl group. The
acidity of the organic species is large relative to most hydrogens bonded to carbon by virtue of the
resonance stabilization of the conjugate base (an enolate) that is formed (see Step 1)
In the subsequent step (see Step 2), the enolate attacks the carbonyl group of either another molecule of
the same aldehyde or ketone or that of another reagent also present (a crossed aldol). In designing a
crossed aldol condensation, the number of possible products is limited by carefully selecting one
member that has no acidic alpha hydrogens; therefore, one of the two carbonyl compounds will not form
an enolate but will react with enolate ions that are generated from the other carbonyl compound.
The aldol (the product contains the functional groups of aldehydes and of alcohols) usually is not
isolable, unless its molecular weight is so small that it can be quickly removed from the reaction
mixture. The Mystery Product you are synthesizing here is a solid at room temperature, so that is not
likely to occur! Rather, the aldol undergoes an acid-base reaction with the remaining acidic alphahydrogen, followed by the loss of OH- as a leaving group (a true oddity in organic chemistry) to give an
enal, which has the alkene and the aldehyde functional groups (see Step 4). The net loss of H+ and OHrepresents the loss of water, therefore the term aldol condensation (formation of a larger molecule from
two smaller molecules with the loss of some by-product).
Step 1
O
O
C
H
R
+ OH -
C
H2
C
R
Step 2
O
'R
H
R
CH2
-
C
-
+ H2 O
C
-
CH2
resonance-stabilized enolate
O
C
-
O
H2 C
R
O
O
C
C
C
H2
R'
R
H
Step 3
H
-
O
O
C
C
C
H2
R'
O
O
H
+
C
O
R'
R
R
H
H
H
+ OH -
C
C
H2
Step 4
H
O
C
R'
H
O
C
C
CH
O
H
OHR'
R
H
C
H
OH-
1
+ H2 O
C
R
Precautions
Sodium hydroxide and the base solution that you prepare is extremely caustic and gloves should be worn
during its preparation and handling. Remember that the hydroxide is behaving as a catalyst, so the
NaOH used in the reaction is still present when the reaction is complete.
PreLab:
Be sure to include all sections of the PreLab that accompany the Synthetic Experiments. In the
Chemical Data Table, you should include you best guess for the structure. (Hint: Take into account the
fact that you are using a 2:1 molar ratio of cinnamaldehyde to acetone.) If you are correct, the data for
your product is in Aldrich (use the formula index).
Questions to be Answered in the PreLab
1. What product would be expected if cinnamaldehyde and acetone reacted in a 1:1 molar ratio? Draw
the structure of this product.
2. Why is there a possibility that some of this 1:1 product will be present in the isolated product?
3. Include this 1:1 product in your Chromatographic Behavior discussion.
Procedure
Add 2.0 mmol of cinnamaldehyde, 1.0 mmol of acetone, and a stir bar to a 10-mL Erlenmeyer flask.
Add 4.0 mL of ethanol to the flask and stir to completely dissolve. With stirring, add 2 mL of 3M
NaOH in a dropwise fashion. A solid will start to form, but continue to stir the mixture for an additional
20 min.
During the reaction period, prepare 5 mL of a 1:2 solution of ethanol:water. When the reaction is
complete, add half of the solution to the reaction mixture and stir well. You may need to use a glass stir
rod to break up large chunks of solid. Filter the solution using suction filtration. Wash the solid with
the other half of the 1:2 ethanol:water mixture and then wash with water. Air dry the solid by
continuing to draw air through the Hirsch funnel for 10 min. Reserve a small sample of the crude
product for TLC analysis.
Recrystallize the product from either an ethanol/water mixture or by dissolving the solid in a minimum
of warm ethanol, cool, and adding water DROPWISE with intermittent scratching with a glass stir rod to
induce crystallization. Dry the solid thoroughly before weighing and obtaining a melting point of the
product.
Analyze both the crude and the recrystallized product by TLC. On the same plate, cinnamaldehyde
should also be spotted for comparison. Be sure to include all of your labeled TLC plates with your Final
Report.
Cleaning Up
The ethanol/water filtrates should be placed in the non-halogenated waste container.
Analysis
The product can be analyzed by NMR, UV-VIS, or IR spectrometry. See your assignment strip for the
analysis you are assigned. If you are assigned UV-VIS, compare your longest wavelength l max with
that of cinnamal acetone (not dicinnamal acetone) which has a l max at 332 nm. If your observed lmax is
different, how is your proposed structure compatible with your measured l max.
Exp't 19
Answer the following questions in your Final Report
1. Given that the cinnamaldehyde you used is mostly the trans-isomer, draw all the possible isomers of
the product. How could you quantify the relative amounts of each isomer?
2. Why does the enolate of acetone attack the carbonyl of cinnamaldehyde rather than the carbonyl
of another acetone molecule?
2
Synthetic Experiment PreLab Grading Sheet
Name(s):
TA:
Date:
PreLab For Exp't #:19
Title: Mystery Product: The Reaction of Cinnamaldehyde and Acetone
Possible
Points
Date, Name, Desk #, Experiment # & Title(abbreviated after 1st pg), Section &
TA Name
4
Summary- aldol condensation,. Formation and isolation of product.
Recrystallization, analysis by TLC, UV/Vis or other spectral technique.
12
Goals
12
Reactions, structures, conditions, diagrams
14
Completeness of Chemical Data Table(s)
10
Missed
Points
- Include your best guess for the structure of the “Mystery” product (4 pts)
Chromatographic Behavior Comparison (less polar is higher on plate, elutes 1st)
12
Spectral Features Comparison
12
Work-up - Explanation of the product isolation and purification process –
Filtration, washing, isolation and drying, etc
12
PreLab Exercise 6 pts. each
12
TOTAL FOR PRELAB
100
Date Handed in:
Total Points:
General Comments:
3
Synthetic Experiment Final Report Grading Sheet
Name:
TA:
Date:
Final Report For Exp't #:19
Title: Mystery Product: The Reaction of Cinnamaldehyde and Acetone
Possible
Points
Name, Date, Experiment Title (abbreviated after 1st page) and every page
numbered
4
OBSERVATIONS and DATA - Overall organization, readability, completeness
8
Data: Weighing data, molecular weights, moles, density, volumes, Rf’s.
12
Missed
Points
Product analysis conditions i.e. UV-Vis solvent and wavelength range; NMR
solvent and field strength, IR sample and KBr weights, MS ionization mode.
Yield: Show % yield calculations with limiting reagent clearly stated.
Purity: Record melting points, color, or other indicators of purity.
RESULTS AND DISCUSSION – Overall organization, readability, completeness
Results; Achievement of goals
12
8
16
Product Analysis Data: Quality and Interpretation – Structure drawn on
Spectrum
Assignment and discussion of all UV/Vis peaks and l max to isomers based on
the extent of conjugation. If MS or IR, interpret major peaks.
24
See Lab Guide Chapter 3, Section 3.4 for guidelines in annotating spectra and
Ch 11 for help with interpretation.
POSTLAB QUESTIONS –8 pts. ea.
16
TOTAL POINTS
100
Date Handed in:
Total Points:
General Comments:
4