E-Stilbene Lab Report
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 …show more content…
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 of 10.73 was recorded at 6.6 ppm for Z-Stilbene but no peak was recorded at 7.1 ppm for Z-Stilbene. When Stilbene undergoes photo isomerization it favors almost exclusively E-Stilbene as an integration of 15.14 at 7.1 ppm was observed compared to an integration of 0.89 at 6.6 ppm. This ratio of 94/6 almost completely favors the E product as it is more stable due to its conjugation. In order for proper conjugation a molecule must be flat which Z-Stilbene does not possess due to steric hindrance of its benzene rings being so close together. It causes the molecule to twist. E-Stilbene on the other hand does not have to twist due to hindrance and thus has full conjugation. The recrystallized E-Stilbene product is mostly pure as its NMR matches the literature value for E-Stilbene except for the small peak at 6.6 ppm due to a small amount of leftover Z-Stilbene2. This reaction could be improved by using Bromine instead of Iodine because Iodine is very slow and it is possible that more product could …show more content…
have been formed if Bromine was used. Some sources of error include not properly rinsing out seperatory funnels after each wash causing unwanted reactants to be left behind.
Conclusion:
While the individual experiment performed was a failure the data provided by others provides an interesting look into the selectivity of this reaction. It seems that the crude Wittig reaction predominantly favors Z-Stilbene while the recrystallization after photo isomerization favors a mostly pure E-Stilbene with some leftover Z. This is due to the stabilization provided by conjugation of the E-Stilbene molecule. Overall, the reaction is slow and provides very small yields and could be improved upon by using Bromine.
Experimental:
Reagents used in this experiment were: Sodium Hydroxide, Water, Benzyltriphenyl Phosphonium, Benzaldehyde, Dichloromethane, Sodium Bisulfate, Sodium Sulfate, Iodine, Ethanol, Sodium Chloride, and Ethanol. All reagents used were not further modified unless stated. Glass-ware and materials used: 50 ml round-bottom flask, seperatory funnel, glass pipettes, magnetic stirring rod, magnetic stirrer, glass beakers, 120 watt lightbulb with clamp, and a reflux condenser. All glassware used was obtained from the approved glass-ware kits. NMR was analyzed with a Nanalysis NMReady 60. Mixture was evaporated with an IKA HB 10 basic.
Wittig Reaction: A 10 ml solution of 50% sodium hydroxide and water solution was prepared. Benzyltriphenyl phosphonium (3.7 g) was placed into a 50 ml round-bottom flask with benzaldehyde (1.08 g) with a small amount of iodine and dichloromethane (10 ml). It was allowed to reflux with stirring while the 50% sodium hydroxide solution (5 ml) was added drop wise. Solution was stirred for 30 minutes. Solution was then cooled to room temperature and transferred to seperatory funnel. The round-bottom flask was washed with dichloromethane and added to the seperatory funnel to retrieve leftover product. The layers were separated and the aqueous layer was removed. The organic layer with product was washed with water (10 ml) followed by sodium bisulfate (15 ml). It was then washed with water until pH returned to neutral. The organic layer was then transferred to a dry flask and sodium sulfate was added until clumping stopped. 0.5 ml of product was transferred to a flask and evaporated with dichloromethane. It was then decanted into a seperatory funnel and washed with sodium bisulfate. The solution was then washed with sodium chloride (5 ml) and separated. The solution was rotovapped and recrystallized with ethanol producing a small yield of white crystals. Z-Stilbene: (0.239 g, 6.3%).
Isomerization:
Iodine (75 mg) was added to a dichloromethane solution containing Z-Stilbene in a round-bottom flask fitted with a reflux condenser near a 120 watt lightbulb.
It was then irradiated with stirring for 1 hour. The resulting solution was washed with sodium bisulfate (5 ml) in a seperatory funnel and shaken until de-colorization. The organic layer was washed with sodium chloride (5 ml) and transferred to a flask where sodium sulfate was added for evaporation. The remaining product was roto-vapped to produce crystals. The remaining product was then recrystallized in hot ethanol (12 ml) and cooled to room temperature. E-Stilbene: NMR: 6.1 ppm (0.89, 7.1 ppm
(15.14).
Works Cited & Acknowledgements
Gilbert, John C., Stephen F. Martin, and Royston M. Roberts. Experimental Organic Chemistry: A Miniscale and Microscale Approach. 5th ed. Fort Worth: Saunders College Pub., 1998. Print.
Klein, David R. "Aldehydes and Ketones" Organic Chemistry. 1st ed. Hoboken, NJ: John Wiley, 2012. 915-970. Print.
-Devon Wolf for data needed for percent yield (moles of benzyltriphenyl phosphonium, benzaldehyde, and crude product).
-Whoever made the NMR data provided to us (Gabe?).
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 …show more content…
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 of 10.73 was recorded at 6.6 ppm for Z-Stilbene but no peak was recorded at 7.1 ppm for Z-Stilbene. When Stilbene undergoes photo isomerization it favors almost exclusively E-Stilbene as an integration of 15.14 at 7.1 ppm was observed compared to an integration of 0.89 at 6.6 ppm. This ratio of 94/6 almost completely favors the E product as it is more stable due to its conjugation. In order for proper conjugation a molecule must be flat which Z-Stilbene does not possess due to steric hindrance of its benzene rings being so close together. It causes the molecule to twist. E-Stilbene on the other hand does not have to twist due to hindrance and thus has full conjugation. The recrystallized E-Stilbene product is mostly pure as its NMR matches the literature value for E-Stilbene except for the small peak at 6.6 ppm due to a small amount of leftover Z-Stilbene2. This reaction could be improved by using Bromine instead of Iodine because Iodine is very slow and it is possible that more product could …show more content…
have been formed if Bromine was used. Some sources of error include not properly rinsing out seperatory funnels after each wash causing unwanted reactants to be left behind.
Conclusion:
While the individual experiment performed was a failure the data provided by others provides an interesting look into the selectivity of this reaction. It seems that the crude Wittig reaction predominantly favors Z-Stilbene while the recrystallization after photo isomerization favors a mostly pure E-Stilbene with some leftover Z. This is due to the stabilization provided by conjugation of the E-Stilbene molecule. Overall, the reaction is slow and provides very small yields and could be improved upon by using Bromine.
Experimental:
Reagents used in this experiment were: Sodium Hydroxide, Water, Benzyltriphenyl Phosphonium, Benzaldehyde, Dichloromethane, Sodium Bisulfate, Sodium Sulfate, Iodine, Ethanol, Sodium Chloride, and Ethanol. All reagents used were not further modified unless stated. Glass-ware and materials used: 50 ml round-bottom flask, seperatory funnel, glass pipettes, magnetic stirring rod, magnetic stirrer, glass beakers, 120 watt lightbulb with clamp, and a reflux condenser. All glassware used was obtained from the approved glass-ware kits. NMR was analyzed with a Nanalysis NMReady 60. Mixture was evaporated with an IKA HB 10 basic.
Wittig Reaction: A 10 ml solution of 50% sodium hydroxide and water solution was prepared. Benzyltriphenyl phosphonium (3.7 g) was placed into a 50 ml round-bottom flask with benzaldehyde (1.08 g) with a small amount of iodine and dichloromethane (10 ml). It was allowed to reflux with stirring while the 50% sodium hydroxide solution (5 ml) was added drop wise. Solution was stirred for 30 minutes. Solution was then cooled to room temperature and transferred to seperatory funnel. The round-bottom flask was washed with dichloromethane and added to the seperatory funnel to retrieve leftover product. The layers were separated and the aqueous layer was removed. The organic layer with product was washed with water (10 ml) followed by sodium bisulfate (15 ml). It was then washed with water until pH returned to neutral. The organic layer was then transferred to a dry flask and sodium sulfate was added until clumping stopped. 0.5 ml of product was transferred to a flask and evaporated with dichloromethane. It was then decanted into a seperatory funnel and washed with sodium bisulfate. The solution was then washed with sodium chloride (5 ml) and separated. The solution was rotovapped and recrystallized with ethanol producing a small yield of white crystals. Z-Stilbene: (0.239 g, 6.3%).
Isomerization:
Iodine (75 mg) was added to a dichloromethane solution containing Z-Stilbene in a round-bottom flask fitted with a reflux condenser near a 120 watt lightbulb.
It was then irradiated with stirring for 1 hour. The resulting solution was washed with sodium bisulfate (5 ml) in a seperatory funnel and shaken until de-colorization. The organic layer was washed with sodium chloride (5 ml) and transferred to a flask where sodium sulfate was added for evaporation. The remaining product was roto-vapped to produce crystals. The remaining product was then recrystallized in hot ethanol (12 ml) and cooled to room temperature. E-Stilbene: NMR: 6.1 ppm (0.89, 7.1 ppm
(15.14).
Works Cited & Acknowledgements
Gilbert, John C., Stephen F. Martin, and Royston M. Roberts. Experimental Organic Chemistry: A Miniscale and Microscale Approach. 5th ed. Fort Worth: Saunders College Pub., 1998. Print.
Klein, David R. "Aldehydes and Ketones" Organic Chemistry. 1st ed. Hoboken, NJ: John Wiley, 2012. 915-970. Print.
-Devon Wolf for data needed for percent yield (moles of benzyltriphenyl phosphonium, benzaldehyde, and crude product).
-Whoever made the NMR data provided to us (Gabe?).