Synthesis of trans-Stilbene via Witting Reaction in Organic Chemistry

This assignment details an organic chemistry experiment focused on the synthesis of trans-Stilbene using the Wittig reaction. The objective is to prepare trans-Stilbene, with the introduction explaining the reaction mechanism, including the formation of betaine and oxaphosphetane intermediates. ...Read More

Running head: ORGANIC CHEMISTRY
ORGANIC CHEMISTRY
[Author Name(s), First M. Last, Omit Titles and Degrees]
[Institutional Affiliation(s)]

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ORGANIC CHEMISTRY
Title: Witting Reaction-the Preparation of trans-Stilibene
Objectives
The objective of the experiment is to:
 prepare trans-Stilibene
Introduction
This experiment aims at preparing trans-Stilibene by the use of a witting reaction. In the witting
reaction, the first step involves the combination of the carbonyl and ylide to form an intermediate
known as betaine. The next stage involves carbonyl approaching the ylide carbon at an angle of
90⁰ with respect to the C-P bond (Ahluwalia, 2010). An oxygen atom is able to form a new
covalent bond as soon as the betaine is formed. The oxygen atom forms a covalent bond with a
phosphorous atom to form an oxaphosphetane ring.
Upon the formation of the oxaphosphetane, the substitutes change to be cis. This is influenced by
the chosen path of the carbonyl as the betaine is formed. What this means is that the final alkene
product that is favored will be formed and thus it will be the Z isomer (Dana W. Mayo, 2010).
The four main steps that are used in the synthesis of trans-stilbene using benzaldehyde are
similar and use identical reagents despite the different conditions for the reactions. The equation
below shows the reaction mechanism for the synthesis of trans-cinnamic acid through the
Knoevenagel condensation of benzaldehyde with malonic acid.

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ORGANIC CHEMISTRY
Below is the reaction mechanism for the synthesis of 1-bromo—2-phenylethene through the E1
decarboxylative elimination of 2,3-dibromo-3-phenylpropionic acid (Dewick, 2011).
Materials
 4.6 g of benzyl chloride
 6.6 g triphenylphosphine
 10 ml petroleum ether
 10 ml sodium hydroxide
 Anhydrous magnesium sulphate
Methodology
 Place 4.3 g of benzyl chloride and 6.6 g triphenylphosphine in a 100 ml conical flask

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 Close the flask with petri dish and heat it on a hot plate until the mixture solidifies (about
10 minutes)
 Scratch the solid form the flask and dissolve it with 50 ml petroleum ether until all the
unreacted starting materials dissolve in the solution
 Filter by vacuum filtration and dry the product in the air
 Add 10 ml of sodium hydroxide (50%) carefully into a mixture of 2.03 g, 20 mmol
benzaldehyde, 7.86 g, 20 mmol benzyltriphenylphosphonium chloride and 30 ml
dichloromethane. Stir the mixture
 Leave the exothermic reaction to occur for 10 minutes
 Separate the organic phase
 Wash with 30 ml water
 Dry with anhydrous magnesium sulphate
 Filter by gravity filtration and evaporate the solvent on a steam bath
 Recrystallize the product by adding 5 ml ethanol before heating on a steam bath
 Cool the solution and filter the crystals formed via vacuum filtration
Record the weight and melting point
Obtain an IR spectrum of the product

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Results
Fig.1: Weight and Melting Point Results
Fig. 2: IR Results

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Fig. 3: NRM Results
Discussion
The kinetic study of the witting reaction for the synthesis of stilbene is conducted through the
reaction between benzaldehyde and benzyltriphenylphosphonium chloride present in the NaOH/-
CH2C12 two-phase system (Stefan Bräse, 2011). The reaction rate is influenced by the
agitating speed, the concentration of the C6H5CHO, NaOH and BTPPC. The rate of the reaction
is also influenced by the temperature of the reactants. The rate of the reaction tends to be under
the control of diffusion when the agitating speed is lower than 200 rpm (Dana W. Mayo,
2010). At an agitating speed of more than 600 rpm, the two phases of the reaction are found to
mix uniformly forming a homogenous layer. Even at temperature of 0⁰, the rate of reaction id
very high and it goes on completion within a short while. The rate of reaction is directly
proportional to the concentration of BTPPC and NaOH as well as asymptotically on the
concentration of C6H5CHO

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Conclusion
The experiment investigated the synthesis of trans-Stilibene using the witting reaction. Both the
two phases of the reaction progressed to yield the required results at each of the reaction phases.
The results obtained confirmed the synthesis trans-Stilibene as illustrated in the IR,H NMR
results and the results on the weight and the temperature of the final products. The yield of trans-
Stilibene was 97% showing that the experiment supported the theoretical findings on the study
area.

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References
Ahluwalia, V. K. (2010). Intermediates for Organic Synthesis. Kansas: I. K. International Pvt
Ltd.
Dana W. Mayo, R. M. (2010). Microscale Organic Laboratory: with Multistep and Multiscale
Syntheses. New York: John Wiley & Sons.
Dewick, P. M. (2011). Medicinal Natural Products: A Biosynthetic Approach. New York: John
Wiley & Sons.
Stefan Bräse, K. B. (2011). Organic Azides: Syntheses and Applications. London: John Wiley &
Sons.