The Wittig Reaction Lab Report

The Wittig Reaction Experiment 5 Introduction: Georg Wittig discovered a method for creating alkenes from ketones and aldehydes. He did this by using phosphonium ylides, which are neutral molecules with positive and negative charges on neighboring atoms. He went on to win a Noble Prize in Chemistry for this Wittig synthesis.1 The Wittig synthesis converts a carbon-oxygen double bond into a carbon-carbon double bond. The driving force of this reaction is the formation of phosphine oxide product. This reaction will also form an alkene as a product. This reaction has two main stages. The first stage is the forming of the phosphonium ylide, and the second stage is the reaction of the phosphonium ylide with the carbonyl group of an aldehyde or ketone.1 One downside of this reaction is the possibility of steric hindrance. Products will be more abundant with aldehydes, due to less steric hindrance. Products are lower with ketones due to more steric hindrance. An upside to the Wittig reaction is the synthetic value it has. This reaction can take place under regular conditions due to the fact that location of the carbon-carbon bond is pre-determined.1 In this experiment, a Wittig reaction was performed. The mechanism for this reaction is shown in figure 1. In this reaction, an aldehyde, trans-cinnamaldehyde, is turned into an alkene, 1,4-diphenyl-1,3-butadiene. The alkene product has three possible isomers which are (E,E)-1,4-diphenyl-1,3-butadiene, (E,Z)-1,4-diphenyl-1,3butadiene, (Z,Z)-1,4-diphenyl-1,3-butadiene. Another product from this reaction is also a phosphine oxide product, triphenylphosphine oxide. All chemicals used in this reaction are shown in table 1. Figure 1: Figure shows the mechanism for the formation of (E,E)-1,4-diphenyl-1,3-butadiene, (E,Z)-1,4-diphenyl-1,3-butadiene, (Z,Z)-1,4-diphenyl-1,3-butadiene, and triphenylphosphine oxide, from the reagents benzyltriphenylphosphonium chloride and trans-cinnamaldehyde. Table 1: Table of Reagents2 Compound Molecular Weight (g/mol) Boiling Point (°C) Melting Point (°C) Density (g/cm3) (E,E)-1,4-diphenyl-1,3-butadiene 206.282 350.0 153.0 0.99 (E,Z)-1,4-diphenyl-1,3-butadiene 206.282 135.0 88.0 0.99 benzyltriphenylphosphonium chloride 388.875 181.0 337.0 1.18 ethanol 46.068 78.2 -114.1 0.79 hydrochloric acid 36.458 -85.1 -114.2 1.64 petroleum ether 86.181 42.0-62.0 -73.0 0.65 dichloromethane 84.927 40.0 -95.1 1.33 sodium hydroxide 39.997 1388.0 323.0 2.10 sodium sulfate 142.041 1429.0 884.0 2.66 trans-cinnamaldehyde 132.162 246.0 -7.5 1.05 triphenylphosphine oxide 278.29 360.0 158.0 1.21 water 18.015 100.0 0.0 1.00 Experimental: For this experiment, 0.125 mL of trans-cinnamaldehyde, 1.0 mL of dichloromethane, and 0.38 g of benzyltriphenylphosphonium chloride were put into a 5 mL round bottom flask. A spin bar was placed in the flask, and the flask was placed on a spin plate. While the mixture was stirring, 0.5 mL of sodium hydroxide was added. This mixture was allowed to rapidly stir for 30 minutes. The mixture turned from a light yellow color to a white-yellow color. Next, the mixture was transferred to a conical tube, and 2.0 mL of dichloromethane and 1.5 mL of water was added. The top aqueous layer was removed and put into a waste container. Then, anhydrous sodium sulfate was added, and the mixture was shaken. The dichloromethane was then removed from the solid anhydrous sodium sulfate and put into a pre-weighed vial. A silica TLC plate was obtained, along with a 50 mL beaker with 3 mL of petroleum ether. The TLC plate was spotted with three drops of the dichloromethane mixture. The plate was then placed in the beaker with the petroleum ether. After five minutes, the plate was removed. The plate was analyzed under a UV light, and Rf values were calculated for each of the 3 spots. Next, the dichloromethane solution was placed in a 40°C bath to allow the dichloromethane to evaporate. After 20 minutes, clear solid had formed on the side of the vial, and there was an oily liquid remaining. The vial was placed in an ice bath, and a solid immediately formed. The solid was washed with 3 mL of ethanol and placed in a suction filtration apparatus to dry the solid. The solid appeared to be a shiny white crystal. The solid was then placed back into the pre-weighed vial, and weighed to determine the weight of the product. Then, some product was put into a capillary tube, and a melting point was taken. Next, a small amount of product was dissolved in dichloromethane. Another silica TLC plate was obtained, and 3 drops of the dichloromethane mixture were placed on the plate. The plate as placed in a 50 mL beaker with 3 mL of petroleum ether. After 5 minutes, the plate was removed. The plate was analyzed under a UV light, and Rf values were calculated for each of the 3 spots. Results: In this experiment, a Wittig reaction was used to react trans-cinnamaldehyde with benzyltriphenylphosphonium chloride to produce 1,4-diphenyl-1,3-butadiene. TLC plates were constructed for both the reaction mixture and the product mixture. These plates are shown in figure 2. These plates were then analyzed by calculating the Rf values of each of the 3 spots. The formula for calculation the Rf value and a sample calculation are shown as equation 1. The Rf value average for the reaction mixture was 0.33. The Rf value average for the product mixture was 0.26. All of the Rf values for both plates, along with the average of each 3 dots, are shown in table 2. Next, percent yield was calculated for the product. First, the limiting reagent was determined to be cinnamaldehyde. This calculation is shown as equation 2. Then, the actual yield was calculated. This is shown as equation 3. Finally, the percent yield was calculated, which was 14.7%. This is shown as equation 4. Next, the melting point was obtained which was a range from 151.2°C to 157.3°C. Table 3 gives a summary of all of the data collected for the product. Next, proton NMR spectroscopy data was obtained from a source.3 The NMR spectroscopy graph was analyzed. First, the coupling constants for the two major groups of peaks were calculated. The two coupling constants for the major peaks were 14.82 Hz and 14.73 Hz. This calculation is shown as equation 5. Then, both (E,E)-1,4-diphenyl-1,3-butadiene and (E,Z)-1,4-diphenyl-1,3-butadiene were compared to the proton NMR spectroscopy graph, and the protons were labeled accordingly. This data is shown in figure 3. The peaks on the proton NMR graph was labeled to the corresponding protons in the two isomers. This is shown in figure 4. A summary of the proton NMR data is shown in table 4. solvent front: 2.8 cm solvent front: 3.0 cm #1 #2 #3 #1A #2A 0.8 cm 0.9 cm #3A 1.1 cm 0.9 cm 0.8 cm 0.6 cm Figure 2: Figure 2 illustrates the two TLC plates used in this reaction. The first TLC plate represents the reaction mixture, and the second TLC plate represents the product mixture. The spots, solvent front, and distance traveled for each spot are labeled. Equation 1: Equation 1 shows the Rf equation and a sample calculation. Table 2: Rf Values Spot Label Reaction Mixture Rf Value 1 0.39 2 0.32 3 0.29 Average: Product Mixture 0.33 1A 0.27 2A 0.30 3A 0.20 Average: Equation 2: Equation 2 shows the two calculations to determine the limiting reagent. Cinnamaldehyde is the limiting reagent, and 0.000946 mol is the theoretical yield. Equation 3: Equation 3 shows the calculation for the actual yield. Equation 4: Equation 4 shows the formula and the calculation for the percent yield. 0.26 Table 3: Data for 1,4-diphenyl-1,3-butadiene Percent Yield (%) Melting Point (°C) 14.7 151.2 - 157.3 Reaction Mixture Rf Value Product Mixture Rf Value 0.33 Equation 5: Equation 5 shows the conversion from parts per million to hertz for the two coupling peaks. Figure 3: Figure 3 shows both (E,E)-1,4-diphenyl-1,3-butadiene and (E,Z)-1,4-diphenyl-1,3butadiene with the differentiating hydrogens labeled Figure 4: Figure 4 shows the proton NMR graph for both (E,E)-1,4-diphenyl-1,3-butadiene and (E,Z)-1,4-diphenyl-1,3-butadiene. The peaks are labeled to match the hydrogens of each compound. These figures can be found in figure 3. 0.26 Table 4: Proton NMR for 1,4-diphenyl-1,3-butadiene Label E-E Isomer: E-Z Isomer: Multiplicity Integration Chemical Shift (ppm) Coupling Constant (Hz) A 2 2 6.95 - 7.03 14.82 B 2 2 6.65 - 6.75 14.73 C 2 2 6.95 - 7.05 — D 2 2 6.63 - 6.78 — Discussion: Benzyltriphenylphosphonium chloride and trans-cinnamaldehyde were reacted to form 1,4-diphenyl-1,3-butadiene. The product obtained with this reaction was a crystal white color. Pure 1,4-diphenyl-1,3-butadiene is a white-yellow crystal, so the data obtained agrees with this.4 Two TLC plates were composed during the course of this experiment. The stationary phase was a silica gel plate, and the mobile phase was non-polar petroleum ether. Because the mobile phase was non-polar, non-polar substances should move further up the plate than polar substances.5 The first plate composed was for the reaction mixture. The reaction mixture had an Rf value of 0.33. This indicates that it was more non-polar than the product mixture, which had an Rf value of 0.26. This confirms that the products were more polar than the reactants, thus helping confirm the purity of the products. Next, the percent yield for 1,4-diphenyl-1,3-butadiene was 14.7%. Several errors could have occurred to give this percent yield. During the purification of the product, some product could have been lost by evaporation, suction filtration, or by remaining on the glassware used throughout the course of the experiment. The percent yield could also be inaccurate. The product could possibly contain some solvent, dichloromethane, that did not evaporate.6 This would cause a higher and inaccurate percent yield. Next, a melting point was taken on the product. The melting point obtained was a range from 151.2°C to 157.3°C. The melting point of pure (E,E)-1,4-diphenyl-1,3-butadiene is 153.0°C. The melting point of pure (E,Z)-1,4-diphenyl-1,3-butadiene is 88.0°C. Because the melting point obtained is closer to the melting point of (E,E)-1,4-diphenyl-1,3-butadiene concludes that the product contained a larger percentage of the trans-trans isomer. The broadened melting point range also infers that the product contained could have contained the cis-trans isomer and impurities.7 These impurities could be dichloromethane, water, or unreacted reactants. Next, the proton NMR spectrum for the product was analyzed. The cis-trans isomer is expected have multiplets between 6.2 to 6.5 ppm, 6.7 to 6.9 ppm, and 7.1 to 7.5 ppm. The trans-trans isomer is expected to have multiplets between 6.6 to 7.0 ppm and 7.2 to 7.5 ppm.8 In the spectrum obtained, the trans-trans isomer had multiplets from 6.95 to 7.03 pm and 6.65 to 6.75 ppm. These two peaks appeared to be doublet of doublets. This happens when the coupling constants are unequal and a doublet is split again into another doublet. This agrees with the NMR spectrum obtained. The two coupling constants calculated for these peaks were 14.82 Hz and 14.73 Hz. Because both of these coupling constants are above 10 Hz, this indicates that the peaks come from a trans isomer.8 This confirms that the peaks were from (E,E)-1,4-diphenyl-1,3-butadiene. Next, the cis-trans isomer had minor multiplets from 6.95 to 7.05 ppm and 6.63 to 6.78 ppm. The proton NMR spectrum confirmed that (E,E)-1,4-diphenyl-1,3-butadiene was the major product. It confirms this because the peaks formed from this isomer are major compared to the peaks formed from the cis-trans isomer. Conclusion: During this experiment, a Wittig reaction was done with the reactants benzyltriphenylphosphonium chloride and trans-cinnamaldehyde. This reaction can form three possible products which are (E,E)-1,4-diphenyl-1,3-butadiene, (E,Z)-1,4-diphenyl-1,3-butadiene, and (Z,Z)-1,4-diphenyl-1,3-butadiene. It is inferred that the cis-cis isomer never forms due to its extreme instability from the steric hindrance of the two phenyl groups.9 In conclusion, (E,E)-1,4-diphenyl-1,3-butadiene was determine to be the major product. This isomer was the suspected major product because there is less steric hindrance between the phenyl groups in the trans-trans isomer than the cis-trans isomer. Due to less steric hindrance, this makes it more stable, thus making it the major product.9 Several methods confirmed this. The melting point of the product indicated that the trans-trans isomer was the major product. Proton NMR spectroscopy also confirmed this. The major peaks and their coupling constants indicated a trans product. More analysis tests could be done on the product, such as gas chromatography. Gas chromatography could be used to separate and confirm the major and minor products of this reaction. Ways to improve this reaction could be to dry the product for a longer period of time to ensure all solvents evaporated. Another way would be to stir the initial mixture for a longer period of time to insure that all of the reactants have time to react. 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)-1,4-diphenyl-1,3-butadiene isomer is not formed. It is not formed because the reactant is not an un-stabilized ylide. The reactant is a stabilized ylide because it has a double bond that connects the alpha carbon to the beta carbon. This bond allows electrons to be stabilized, which increases stability due to resonance. This isomer also has steric hindrance due to the two phenyl groups being close in proximity and on the same plane. 2. Why is the E,E isomer the thermodynamically most stable isomer? ! The (E,E)-1,4-diphenyl-1,3-butadiene isomer is more stable because it has less steric hindrance between the two phenyl groups than the other two isomers. 3. Your cinnamaldehyde was contaminated with a large amount of cinnamic acid. The Wittig reaction did not yield any product. Why? The Wittig reagent will only react with the carbonyl carbon on aldehydes and ketones. Because cinnamic acid contains a carboxylic acid, it will not react, thus no product will be formed. If more cinnamaldehyde was present, a reaction would have possibly occurred because it contains an aldehyde, which a Wittig reagent can react with. !