The purpose of the Wittig reaction is to make a carbon-carbon double bond and have more carbons in the products than the reactants. One part of the carbons comes from the alkyl halide and the other one from an aldehyde or ketone. The reaction must start with Ph3P (Triphenyl phosphine), and via a Sn2 reaction, the electron from the Triphenylphospine attacks the carbon on benzyl chloride. The chlorine leaves, making the alkyle halide nucleophilic.
The Benzyltriphenylphosphonium chloride is water soluble. Typically butyl Lithium is used, which is a strong base; however, in this lab, NaOH is used. The OH combines with the H and leaves as water. Wittig reaction is used in the formation of alkenes. To yield alkenes, it involves the combination of an aldehyde with phosphonium salt. The resulting arrangement of the final product depends on the reactivity phosphonium salt.
Georg Wittig discovered the Wittig reaction in 1954 and the reaction was named after him. In 1974 he won a Nobel Prize in the chemistry field. Later on, his development was examined and reviewed by Eisch. Georg’s discovery helped Eisch develop and contribute in the development of inorganic chemistry.
The Wittig Reaction is mainly used in the formation of alkenes. It involves the reactivity of carbonyl and Wittig reagent. Other reactions during Wittig process are meant to eliminate and separate alcohols or alkyl halides. According to Bestmann (1983), the Wittig reaction comprises of carbon units that are bond together to yield an alkene double bond.
Elimination reactions normally form mixtures of both substituted and less substituted structural isomers. The Wittig reaction is more similar to aldol condensation, which involves the reaction between carbonyl phosphorus ylide. This condensation of carbonyl yields enones, which are alkenes with a carbonyl attached. However, Wittig reactions seem to be more general compared to aldol condensation in the fact that, carbonyl compounds are not attached to any carbonyl. Phenyletheneis used as the colorizer for the chemiluminscence experiment.
The key step of the mechanism of the ylide reaction is the nucleophilic addition of theylide to the electrophilic carbonyl group, forming a 4-membered ring that dissociates into the product molecules. The stereoselectivity of the reaction is predicated on the stability of triphenylphosphoniumylide, which determines which of two ring intermediates form. Because ylides contain adjacent positive and negative charges, R groups that can better stabilize the adjacent negative charge produce more stable ylides.
General Balanced Chemical Equation
The Wittig reaction undergoes a simple reaction between 2-nitrobenzaldehyde and methyltriphenylphosphoranylidene) acetate to yield methyl (2E)-3-(2-nitrophenyl) acrylate and triphenylphosphine oxide as the final product. This reaction works best in a silica gel matrix to facilitate complete separation of colors (chromatography). A study done by Williamson depicts that the trans alkene is formed over alkene because the ylide residue comprises of an ester family (Williamson, 2010).
Primary Mechanism of the Wittig Reaction
The first phase of the reaction combines product (1) benzaldehydewith product (2) methyl triphenylphosphoranylidene) and the reaction between the products yields product (5) which undergoes a new ring transformation to form product (3) methyl (2E)-3-(2-nitrophenyl) acrylate and product (4) triphenylphosphine oxide.
Summary on the Reaction
The Wittig reaction is normally applied when forming alkenes. The reaction involves the formation of a double bond between the original aldehyde and ketone. Ylides are joined by an alpha bond. The Ylides despite being neutral, they also have positive and negative centers on adjacent atoms. The ylide is prepared following the two-step process discussed above.
2 g ofBenzyltriphenylethenylphosphonium Chloride was measured, weighed, and placed in a reaction tube. The exact amount measured was 205 g. Then 115 g 9-Anthraldehyde was measured and placed in the same reaction tube. The amount of 9-Athraldehyde added was 1146 g. A magnetic stir bar was obtained and placed to the side for use later 7 mL dichloromethane was used as an organic solvent. It was added to the reaction tube, and the magnetic stir bar was added to the reaction tube after the organic solvent was added. A hot plate was plugged in, and the product in the reaction tube was placed in a beaker onto the plate. Without heat and only stirring on the highest setting considering the product was not splashing. The stirring was to be vigorous to thoroughly mix the product, which led to a high yield of product. 3 mL of 50% NaOH was put into another reaction tube.
A glass pipette was used to add NaOH to the reaction tube with the reagent while it continued to stir. It was added drop by drop until the 3mL was gone. The product was mixed for 30 minutes on the highest stir setting without any splashing. 2 additional reaction tubes were obtained, and 1.5 mL dichloromethane was added to one tube while 1.5 mL deionized water was added to the other. A glass pipette was used to transfer the liquid from a reaction tube to a test tube. The reaction tube was rinsed with dichloromethane then transferred to the test tube. 1.5 mL deionized water was added to the reaction tube to rinse it then transferred to the test tube. The tube had two layers present, one of which was a light yellow layer, and a bottom that was a dark brown organic layer. The organic layer was extracted and transferred to a new tube. Approximately 10% anhydrous sodium sulfate was added to the reaction tube. A Hirsch funnel was connected to the vacuum tube. The filter paper was used and covered completely with anhydrous sodium sulfate.
A 200 mL beaker was filled 80% full with tap water and brought to a boil. The Hirsch funnel was turned on half way, and the organic layer was transferred to the filter. 1 mL dichloromethane was added to the test tube. The vacuum was turned on all the way to aid in completely evaporating the dichloromethane. Once evaporated, the apparatus was disconnected. 3 mL of 1-propanol was added to a flask. It was then held in the boiling water until it dissolved after which it was removed to get rid of the side product Ph3P=O. The filter flask was then placed into the ice bath where the sides were scraped with a glass rod to initiate crystallization. In order to separate the crystals, another Hirsch funnel was obtained and assembled. The filter paper was placed into the new funnel. It was optional to rinse with a small amount of 1-propanol; however, this step was not done. The funnel was used to dry the product after which the product was pat with filter paper to attempt to dry it more. Then it was weighed, and the final weight was .0354 g. A sample was collected into a capillary. A melt station was obtained to determine the melting point and turned to 150 degrees Celsius. The product began to melt at 128 degrees Celsius. The entire product was melted by 129.8 degrees. The known melting point is 130 degrees Celsius.
Results and Calculations
Substance Amount used Molecular weights
Trans -9-(2-Phenylethenyl)anthracene .0345 g 280.4 g/mole
9-Anthraldehyde .1146 g 206 g/mole
BenzyltriphenylethenylphosphoniumChloride .205 g 388.88 g/mole
% yield = moles product/ moles limiting reagent * 100%
.0345 g Trans-9-(2-Phenylethenyl) anthracene * 1mole/280.4 g= .000123 moles
.205 g Benzyltriphenylethenylphosphonium Chloride * 1mole/388.88 g= .000527 moles
.1146 g 9-Anthraldehyde * 1mole/206 g= .000556
Due to the 1:1 ratio, the limited reagent is determined to be Benzyltriphenylethenylphosphonium Chloride. To calculate the yield:
Moles of product .000123 moles/.000527=.239 *100= 23.9%
Discussion and Conclusion
The yield of the experiment was recorded at 239 *100= 23.9%. Evidently, the yield was relatively lower than expected, making the reaction procedures and conditions used to be uneconomical and insufficient for commercial production of Trans -9-(2-Phenylethenyl)anthracene. The resulting product yielded at the end of the experiment was a white powdery crytal. The melting point of the white crystals was found to be ranging from 130-132 degrees centigrade. After extraction and purification, the final product obtained yielded 23.9%, and the melting point now decreased from 132 C to 126 C.
From the observations made, we can draw some reasons why there was fluctuation in the melting point. During extraction and purification, there are chances that moisture and solvent present in the crystals were not fully eliminated and therefore leading to variations in the final melting point. From this experiment, it is not possible to form the E and Z isomers. It is only achieved when planning a synthesis reaction involving Wittig reaction. However, chromatography is achieved using Wittig reaction.
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