Wittig Reaction And Photoisomerization

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Wittig Reaction and Photoisomerization
Abstract:

For this laboratory experiment stilbene was produced through a Wittig reaction with benzyltriphenyl phosphonium and benzaldehyde producing a form of stilbene (Figure 1). This reaction favored a crude Z-Stilbene crystal product over its E counterpart. When Z-Stilbene underwent photoisomerization with iodine for 1 hour it reconfigured almost exclusively into its more stable counterpart E-Stilbene. The reaction produced very low yield of 6.3% due to the nature of the reaction and the speed at which iodine reacts. The purity of E-Stilbene could have been increased by allowing the reaction to perform longer and to use a faster reactant such as Bromine. Introduction:

In this experiment a Wittig reaction is performed to help synthesize our product. Wittig reactions are useful because they provide an easy way to convert ketones and aldehydes into a double bond while attaching a substituent1. A Wittig reaction can undergo two different mechanisms depending on the conditions in which the reaction is performed. One of these mechanisms (Figure 2) starts with the Wittig reagent, also known as an ylide, attacking as a nucleophile1. This causes a lone pair on oxygen to act as a nucleophile as well attacking the phosphorus producing an oxaphosphetane intermediate1. Due to the steric hindrance of a 4-membered ring the intermediate breaks apart causing triphenylphosphine oxide to leave1.

In order to produce stilbene solely in its Trans configuration a photoisomerization reaction must occur with iodine and stilbene. An isomerization reaction occurs when a light is shined upon iodine causing it to break apart and become a free radical (Figure 3)1. It then forms a new bond with one electron in a double bond causing a free electron to be on the nearby carbon1. This allows the molecule to twist into its more stable configuration before having the iodine leave and recreating the double bond1. The reaction does not use up iodine in the process.

Discussion:
First, benzyltriphenyl phosphonium was reacted with benzaldehyde in dichloromethane. It was refluxed and a solution of 50% sodium hydroxide was added and allowed to stir for 30 minutes. During this time the solution turned yellow and then grey. The solution was cooled to room temperature causing it to turn back to yellow and was transferred to a seperatory funnel. It was then washed and extracted with water followed by sodium bisulfate. After washes it was washed with water until a neutral pH was observed. During this time the solution unexpectedly turned purple likely due to unclean glassware. Sodium sulfate was added to dry the remaining solution. The leftover solution was then evaporated with dichloromethane and washed again with sodium chloride. It was then roto-vapped and produced very little white crystal product. It was then unsuccessfully recrystallized from ethanol producing almost no product and could not be weighed.

For the isomerization the crude product was added with iodine under a 120 watt light and allowed to irradiate with stirring for 1 hour. When Iodine was added the solution turned a deep red but as the reaction went on some unwanted reactant caused the reaction to turn white and consume the iodine. The experiment continued on by washing with bi-sulfate followed by sodium chloride before being roto-vapped. When NMR was taken it was found the product was not synthesized, thus all NMR data used is from the data provided. The percent yield of crude product was found by student Devon Wolf to be 6.3% and Wolf’s data will be used instead. The limiting reagent was benzyltriphenyl phosphonium and the percent yield was very low due to the nature of the reactions and workup in this experiment. If more benzyltriphenyl phosphonium was added it or purer reactants were used it could have increased the yield of the reaction. The crude product Z-Stilbene was completely favored 100/0 to E-Stilbene as an integration...
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