BRANDEIS UNIVERSITY
CHEMISTRY 29A
PRELAB WRITEUP AND EXPERIMENTAL REPORT SHEET
NAME Richard Fan______________ T.A.'S NAME
David Gay_______________________________
DATE 7/27/06_______EXPERIMENT# ___6___TITLE ___Wittig Reagent___________________________
PURPOSE Synthesizing the ketone trans-9-(2-Phenylethenyl)anthracene using a Wittig reaction of an aldehyde
9-Anthraldehyde and the Wittig reagent_
PRELIMINARY WRITEUP:
Materials:
0.2g Benzyltriphenylphosphonium chloride (btppCl)
0.115g 9-Anthraldehyde
0.6mL dichloromethane (CH2Cl2)
1.5mL dichloromethane
1.0mL dichloromethane
0.6mL 50% NaOH
1.5mL water
Na2SO4, 3-propanol, septum cap, reaction tubes, stir bar,
pipettes, Hirsch funnel, ice
EXPERIMENTAL PROCEDURES*
(Include in preliminary writeup.)
-In reaction tube, mix 0.2g btppCl, 0.115g 9anthraldehyde, 0.6mL CH2Cl2. Add stir bar to tube.
-While stirring, add 0.26mL 50% NaOH dropwise.
Proceed to stir for 30 minutes.
-After removing from stir plate, add 1.5mL water and
1.5mL CH2Cl2.
-Cap tube and shake well.
-Remove the organic layer (bottom) and place in new tube.
-Wash the aqueous layer with 1mL CH2Cl2, combine this
wash with the removed organic layer.
-Dry the organic layer by forcing through pipette packed
with cotton and Na2SO4.
-Aspirate the result, and recrystallize the solid crude
product in 3-propanol, then place on ice.
-Dry the final product on a Hirsch funnel, then obtain the
final weight and final melting point.
%yield = experimental/theoretical × 100%
theoretical yield = weight9-anthraldehyde/MW9-anthraldehyde ×
MWfinal product
9-Anthraldehyde
MW 206.2438
trans-9-(2-Phenylethenyl)anthracene
MW 280.37
MP 131º-132ºC
MODIFICATIONS AND OBSERVATIONS*
(Obtained from the lab experience)
The original mixture created before the addition of NaOH was brown
and cloudy in appearance. After the addition of NaOH and 30 minutes
of stirring, the mixture had two thick layers and a very thin darker
layer in between. The bottom organic layer was extracted, as well as a
solvent layer that was then used to wash the aqueous layer. These
combined layers were then dried with Na2SO4 using the two-pipette
method. The result was a brightly yellow colored liquid. This solution
was then aspirated to remove the reaction solvent (CH2Cl2). This
process required a relatively long period of time, and a small portion
of moisture could not be removed. The resulting solid crude product
was recrystallized in 3-propanol while retaining a small amount of
moisture. After drying using a Hirsch funnel, the final product weight
of the result (trans-9-(2-Phenylethenyl)anthracene) was 0.0416g.
While no crude weight was obtained, the crude melting point was 89º108ºC, while that of the final product was 100º-112ºC.
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RESULTS, DISCUSSION AND CONCLUSIONS*
This experiment involved the synthesis of a Wittig reagent in-situ and reacting that with an aldehyde to create an alkenic structure.
In the first preparatory mixture, btppCl, 9-anthraldehyde, and CH2Cl2 are used. The btppCl is the reagent used to create the Wittig
reagent, 9-anthraldehyde will react with the Wittig reagent once it is created, and CH 2Cl2 is used as the solvent. As NaOH is slowly
added (with fast stirring), a proton and a chlorine are removed from btppCl to create two resonance forms that constitute the Wittig
reagent. One of the resonance forms has a negatively charged carbon (after deprotonation) that then reacts with a proton on 9anthraldehyde to form the trans-9-(2-Phenylethenyl)anthracene product. Ph3P from the reagent and oxygen from 9-anthraldehyde
are removed and react to form the by-product triphenylphosphine oxide, which is later trapped in the 3-propanol solvent used to
recrystallize and removed in the Hirsch funnel drying process. The amount of expected final yield should be dependent on the
amount of 9-anthraldehyde that was used (0.115g), which is 0.0005576mols (0.115/206.3438). A 0.000557mol result should yield
0.1563g of trans-9-(2-Phenylethenyl)anthracene (0.000557 × 280.37). The actual final yield was only 0.0416g, which was a 26.6%
yield. There was only a small amount of crude solid material obtained after aspirating off the solvent, suggesting that the low yield
was probably primarily due to an incomplete reaction as opposed to product loss during aspiration or recrystallization. Two possible
reasons for the low yield are that the Wittig reagent was not sufficiently stabilized during the production and reaction stages. If this
were the case, possibly not enough reagent was created, or some of the reagent reverted to other compounds before reacting with 9anthraldehyde. Another possible reason for the low yield is that the product was not completely in the organic layer that was
removed. The addition of the solvent CH2Cl2 to the aqueous layer showed the necessity of removing any organic compound that
may have remained in the reaction tube even after the organic layer had been removed. It may be possible that product was lost to
the discarded layer. While the yield was relatively low, the purity of the solid final product was not very good either, as shown by
the experimental melting point of the product. The expected melting for the compound was 131º-132ºC but the experimental value
for the final product was only 100º-112ºC, which is much lower and wider in range than the expected. The melting point of the
crude product was even lower and wider in range at 89º-108ºC. This difference in melting points shows that the recrystallization
process was important in purifying the product (improved the melting point), but not completely. The main by-product that was
removed during this process was triphenylphosphine oxide, which may have been the main contributor to the difference between the
crude and final melting points. Perhaps not all of this by-product was removed during recrystallization, and that additional
recrystallization may prove useful.
ANSWERS TO END-OF-CHAPTER QUESTIONS*
1.
2. Strong stirring is necessary in this experiment to ensure that sufficient reagent is created in-situ, especially
since the btppCl is dehydrohalogenated with the weaker base NaOH instead of a stronger base like
phenyllithium in dry ether. Also, the reagent ylide is unstable so it is better to help drive the reaction with 9anthraldehyde with stirring as the reagent is created.
3. In a simple Wittig reagent, there is no benzene bonded to the primary carbon position as is the case in the
reagent created in this experiment. This additional benzene ring helps to stabilize the reagent by making
deprotonation more difficult. Because of this, a very strong base such as n-butyllithium is not necessary.
4. The structure of the unknown compound Z (C6H12) is likely (CH3)2C=CHCH2CH3.
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