Sample Lab Report - The Wittig Reaction

The Wittig Reaction Introduction The Wittig reaction is a commonly used reaction in order to create large molecules from smaller ones via the production of alkene molecules. It was originally discovered by Georg Wittig through his use of organophosphorus compounds called ylides1. Ylides are the resonance stabilized intermediates in this reaction that allow the attachment of the two smaller molecules to form the larger molecules with the creation of a double bond1. Ylides are formed in two steps and are used to produce the Wittig reagent. The first step is the formation of a phosphonium salt by an SN2 reaction in which a nucleophilic phosphorus compound reacts with an alkyl halide1. The second step is the removal of the acidic proton from the carbon attached to the phosphorus by a strong base1. This generates the phorphorus ylide known as the Wittig reagent. This molecule is a resonance stabilized carbanion2. The mechanism for the Wittig reaction is shown in Figure 1 below. Figure 1: The mechanism shown below shows the formation of an alkene using the Wittig reaction. In this experiment, the Wittig reaction will be used to synthesize 1,4-diphenyl-1,3butadiene. This molecule has two stereocenters allowing more than one product (i.e. stereoisomers) to form3. The potential isomers formed in this reaction are (E, E)-1,4diphenyl-1,3-butadiene and (E, Z)-1,4-diphenyl-1,3-butadiene1. The ylide will be formed using benzyltriphenylphosphonium chloride with sodium hydroxide. Then, the ylide will be reacted with trans-cinnamaldehyde and dichloromethane to form the desired product. These reactions are summarized in Figure 2. Thin layer chromatography (TLC) will also be used to check the purity of the product throughout the experiment. Figure 2: The reactions below show the formation of the ylide that will be used in the experiment in order to form the final product. Formation of the Ylide: Table 1: The table below shows the molecular weight, boiling point, melting point, and the density of all the reagents used in the experiment. Name Molecular Weight (g/ mol) Boiling Point (°C) Melting Point (°C) Density (g/ cm3) trans-cinnamaldehyde 132.20 248 -7.5 1.05 Dichloromethane 84.93 39.6 -96.7 1.33 Benzyltriphenyphosphonium Chloride 388.87 Not available 337 1.18 Sodium Hydroxide 39.99 1,388 315 2.13 Water 18.020 100 0 1.00 Sodium Sulfate 142.04 1,429 884 2.66 Petroleum Ether 87.20 42-62 -73 0.65 Ethanol 43.06 78.4 -114 0.789 (E, E)-1,4-diphenyl-1,3butadiene 206.28 350 153 n/a (E, Z)-1,4-diphenyl-1,3butadiene 206.28 n/a 88 n/a Experimental To begin this experiment 1 mL trans-cinnamaldehyde, 1 mL dichloromethane, and 0.41 g of benzyltriphenylphosphonium chloride were added to a 5 mL round bottom flask. A stir bar was placed in the flask and then 0.5 mL of sodium hydroxide was added while the mixture was stirring on the stir plate. An air condenser was then added to the top of the flask and the reaction sat at room temperature and was stirred for 40 minutes. Once the stirring was complete, the mixture was transferred to a conical tube. The reaction flask was washed with 2 mL of dichloromethane followed by 1.5 mL of water. The conical tube was then shaken gently in order to separate the organic layer from the aqueous layer. Once the layers were distinct, the aqueous layer was removed and discarded and the organic layer was added into a test tube with anhydrous Na2SO4 in order to remove the remaining water. The organic solution formed was then compared to trans-cinnamaldehyde using TLC. The TLC plate was prepared and one drop of the organic layer was added to the base line along with the trans-cinnamaldehyde. The TLC plate was placed in a vial with the petroleum ether solvent and capped. Once the solvent had reached the top of the plate, the plate was removed from the solvent and checked with UV light in order to find how far the compounds traveled up the plate. These values were recorded and the Rf values were calculated. The remainder of the organic solution was then placed in a warm water bath in order to evaporate the dichloromethane. Once the dichloromethane was removed, a yellow solid formed in the glassware. The glassware was washed with cold (from an ice bath) ethanol and the solid was collected via vacuum filtration. The product was allowed to dry and the weight and melting point were determined. A small amount of the product was dissolved in dichloromethane and TLC was repeated with one drop of transcinnamaldehyde and one drop of the dissolved product. The Rf values were once again collected and compared to the first values. Finally, the remaining product was dissolved in chloroform and submitted for NMR analysis. All the glassware was then cleaned and returned to the drawer. Results The following figures, graphs, and tables are representative of the data that was collected during this experiment. The melting point for the product was determined to be 123.7°C. The weight of the organic layer was 1.931 g and the weight of the final product after drying was 0.87 g. Figure 3: The two drawings below represent the TLC plates that were obtained in the process of this experiment. TLC plate 1 is for the organic layer and tanscinnamaldehyde and TLC plate 2 is for the product (dissolved in dichloromethane) and trans-cinnamaldehyde. 4.3 cm 4.5 cm 2.0 cm 1.5 cm 0.5 cm 0.5 cm TLC Plate 1 TLC Plate 2 Figure 4: The equation below how the Rf values were calculated during the experiment. The Rf values obtained from experiments are summarized as well. Rf TLC Plate 1 Rf Values (1) trans-cinnamaldehyde Rf (2) organic layer Rf TLC Plate 2 Rf Values (1) trans-cinnamaldehyde Rf (2) product Rf Graph 1: The graph below is the graph obtained from performing proton NMR analysis on the product. Table 2: The table below shows the chemical shift, integration, multiplicity, and the corresponding label assigned to each peak based on these values. Chemical Shift (ppm) Integration Multiplicity Label Smallest Value- 6.6 1 Doublet A 6.8 1 Doublet B 6.94 1 Doublet C Largest Value- 6.97 1 Doublet D Discussion Based on the data presented it is possible to determine that (E,E)-1,4diphenyl-1,3-butadiene was the isomer that was formed. Even though the melting point that was observed was lower than the literature value for this isomer, it is likely that it was still formed. The lower melting point could be due to impurities in the solution or the reaction not reaching completion and therefore allowing intermediates to be found in the product. The weight of the product decreased as it dried because water and dichloromethane were removed from it in the drying process. The TLC tests were done to determine purity throughout the experiment. Since the first TLC plate’s organic product traveled further up the plate (2 cm) than the second product did (1.5 cm), the product became more polar throughout the experiment. This helps explain why the melting point was lower than expected and accounts for the presence of impurities. However, the product was still nonpolar in comparison to the polar starting substance. This was the expected the result (i.e. that the product would travel higher on the plate than the trans-cinnamaldehyde). Alkenes are less polar than the trans-cinnamaldehyde so that is why they appear higher on the TLC plate since they have a higher affinity for the mobile phase (petroleum ether). It is clear to see that impurities were found in the final product. Both of the isomer verification techniques (TLC, and melting point) show the presence of impurities. It is likely that these impurities were due to unclean glassware leftover from a previous staining experiment. The product also seemed to pick up a slightly differently color when vacuum filtered and allowed to dry. This, along with other experimental error, explains the presence of impurities in the product. Conclusion In conclusion, the major product of this reaction was the (E, E) isomer of 1,4diphenyl-1,3-butadiene. Even though the product was impure, the data still provided enough evidence that this isomer was the major product of the reaction. An improvement to this experiment could be to use plane polarized light in order to further determine which isomer was present in the reaction. Certain enantiomers will rotate the light in specific directions and allow for additional information to help identify the product. Questions 1. There is an additional isomer (mp =70°C) that has not been shown in this experiment. Draw a structure of this isomer. Why isn’t it formed? ! The Z,Z isomer is the isomer that is not formed. This product is not formed because the first stereocenter will always be E due to the stereochemistry of the reactants. The alkene carbon furthest from the benzene ring would have to react with the ylide, but the ylide only attacks the carbonyl group. 2. Why is the E,E isomer the thermodynamically most stable isomer? The E, E isomer has the least steric hindrance. This allows a faster reaction to occur and for a more stable product to be formed. 3. Your cinnamaldehyde was contaminated with a large amount of cinnamic acid. The Wittig reaction did not yield any product. Why? The Wittig reaction only reactions with the carbonyl groups on ketones or aldehydes. The cinnamic acid is a carboxylic acid and does not react with the Wittig reagent. Therefore, no product would be yielded.