Lab Report - Synthesis Of 1,4-Diphenyl-1,3-Butadiene

Synthesis of 1,4-Diphenyl-1,3-butadiene By the Wittig Reaction Introduction The Wittig reaction is often used to synthesize larger molecules from smaller ones by connecting the smaller molecules together with carbon-carbon double bonds. Georg Wittig, who the reaction is named after, developed this reaction by observing reactions involving carbanions and ylides which is a class of organophosphorus compounds.1 The phosphorus ylide is highly charged, but stabilized by resonance between the phosphorene form and ylide form. The Wittig reagent reacts readily with aldehydes and ketones and can synthesize mono, di, and trisubstituted alkenes.2 Alkene products are a mixture of E and Z stereochemistry, but the stability or instability of the ylide can be E selective or Z selective respectively.3 In this experiment, benzyltriphenylphosphonium chloride with an addition of sodium hydroxide is used to generate the ylide. The ylide is then reacted with cinnamaldehyde to synthesize either (E,E) or (E,Z) isomers of 1,4-diphenyl-1,3-butadiene (C16H14). Thin layer chromatography analysis is used to test the purity of a compound and see how many different compounds are in a mixture.4 Table 1. Table of Reagents Compound Molecular Melting Point Boiling Point Weight (g/mol) (¼C) (¼C) Density (g/ cm3) Cinnamaldehyde 132.16 -7.5 248 1.05 CH2Cl2 84.93 -97.7 39.6 1.33 Benzyltriphenylphosphium Chloride 388.87 323-332 - - NaOH 40 318 1388 2.13 Water 18.02 0 100 1.00 Na2SO4 142.04 844 1429 2.66 Petroleum Ether 82.2 < -73 42-62 0.65 (E,E) 1,4-Diphenyl-1,3butadiene (C16H14) 206.28 150 350 - Triphenyl phosphine oxide 278.29 156 360 1.21 (E,Z) 1,4-Diphenyl-1,3butadiene (C16H14) 206.28 88 350 - Ethanol 46.07 -114 78.37 0.79 ! Figure 1. The general reaction for the synthesis of 1,4-Diphenyl-1,3-butadiene. Figure 2. The general reaction to create the resonance stabilized ylide. Figure 3. The mechanism for the synthesis of 1,4-diphenyl-1,3-butadiene. Experimental In a 5 ml round bottom flask, 0.13 ml of cinnamaldehyde was combined with 1.0 ml CH2Cl2 and 0.394 grams of benzyltriphenylphosphonium chloride. A spin bar was added and while it rapidly stirred, 0.50 ml of 50% aqueous NaOH was added. A long air condenser was attached to the 5 ml round bottom flask and a cold, wet paper towel was wrapped around the condenser to reflux the mixture. The solution was stirred vigorously at room temperature for 30 minutes. It was noted that the mixture turned more yellowish in hue as time progressed, but was translucent white after 20 minutes. The mixture was then transferred to a conical tube and the reaction flask was washed with 2.0 ml of CH2Cl2 followed by 1.5 ml of water. The mixture in the reaction flask was then used to wash the conical tube. The conical tube was gently mixed. The aqueous layer was removed with a pipet and discarded. The organic layer was pipetted into a test tube and a small amount of anhydrous Na2SO4 was added. The dry CH2Cl2 solution was pipetted into a pre- weighed sample vial and analyzed by TLC with silica gel plates and petroleum ether. The Rf values were calculated. The CH2Cl2 solution was evaporated in the hood using a warm water bath set around 40¼C. A semi-solid white residue was observed. It was placed in an ice bath to further solidify and the solid was washed with cold 60% aqueous ethanol. The solid was collected via vacuum filtration and allowed to dry. The mass of product and melting point was determined. A small amount of the solid was dissolved in CH2Cl2 and analyzed by TLC with silica gel plates and petroleum ether. The Rf values were calculated. Finally, an NMR sample was created by dissolving a small amount of the solid and analyzed. Results Figure 4 shows the silica gel plates that the reaction mixture, solid, and filtrate were sampled on. The reaction mixture travelled 1.8 cm and the solvent travelled 6.5 cm. The solid dissolved in CH2Cl2 travelled 0.7 cm and the solvent travelled 5.7 cm. The filtrate separated into three dots and from the origin up, the distances travelled were 0.6, 1.0, and 1.5 cm. The solvent for the filtrate also travelled 5.7 cm. Table 2 quantitatively lists the distances travelled by the samples and solvents with their respective Rf values. Equation 1 is used to calculate the Rf values of each sample. Figure 5 displays the NMR spectrum for 1,4-diphenyl-1,3-butadiene that was sourced from a group in section A8. There are 3 peaks that are more downfield and 2 peaks that are more upfield. Equation 2 is used to calculate the NMR coupling constant, which is 4 Hertz. The observed melting point of C16H14 was 152 Ð 156¼C as listed in Table 3. The literature melting point of the (E, E) isomer of C16H14 is 150¼C and of the (E, Z) isomer of C16H14 is 88 ¼C. Equation 3 is used to calculate the limiting reagent in the synthesis of 1,4-diphenyl-1,3butadiene. Cinnamaldehyde theoretically produces 1.03 x 10-3 mole of C16H14 and BTTPC, the precursor to forming the ylide, can theoretically produce 1.01 x 10-3 mole of C16H14 and that makes it the limiting reagent. Equation 4 is used to calculate the theoretical yield of C16H14 which is 2.09 x 10-1 grams of C16H14. Equation 5 is used to calculate percent yield. The experimental yield, which was 0.33g of C16H14 , is divided by the theoretical yield calculated by Equation 4 and this number is multiplied by a 100. The percent yield of C16H14 was 16%. Figure 4. The chromatograms of the reaction mixture, solid collected, and filtrate. Table 2. Thin Layer Chromatography Data Mixture Distance from Origin (cm) Distance Travelled by Solvent (cm) Rf Value Reaction Mixture 1.8 6.5 0.28 Solid Dissolved in CH2Cl22 0.7 5.7 0.12 Filtrate (Dot 1) 0.6 5.7 0.11 Filtrate (Dot 2) 1.0 5.7 0.18 Filtrate (Dot 3) 1.5 5.7 0.26 Rf Value Equation 1. Figure 5. The NMR spectrum of 1,4-diphenyl-1,3-butadiene. NMR Coupling Constant Equation 2. (Integration of Peak A Ð Integration of Peak B) x 400 Hertz = Hz (1.01-1.00) x 400 = 4 Hertz Table 3. Experimental & Literature Melting Point of 1,4-diphenyl-1,3-butadiene Melting Point of Experimental C16H14 (¼C) Literature Melting Point of (E, E) Isomer of C16H14 (¼C) Literature Melting Point of (E, Z) Isomer of C16H14 (¼C) 152 Ð 156 150 88 Limiting Reagent = 1.03 x 10-3 mole of 1,4-diphenyl-1,3-butadiene Equation 3. = 1.01 x 10-3 mole of 1,4-diphenyl-1,3-butadiene Theoretical Yield Equation 4. = 2.09 x 10-1 grams of 1,4-diphenyl-1,3-butadiene Percent Yield Equation 5. Discussion Figure 4 displays the chromatograms of the reaction mixture, the solid C16H14 dissolved in CH2Cl2, and the filtrate. The reaction mixture had an Rf value of 0.28 and while the sample did not separate, it should have separated into at least 4 spots as seen in the second chromatogram containing the product and the filtrate. This experimental error could have been caused by having placed a large amount of the sample on the origin, and the quantity of the sample would have made it difficult to separate.4 However, as Figure 4 illustrates in the first TLC chromatogram, it is still visible that the reaction mixture had components that were nonpolar and at the top of the spot outline, and some components that were polar and at the bottom of the spot outline.4 The solid C16H14 had an Rf value of 0.12 indicating that it was very polar and was probably one of the components located at the bottom outline of the spot in the first chromatogram. The filtrate had 3 spots with decreasing polarity and increasing Rf values of 0.11, 0.18, and 0.26 respectively as listed in Table 2. These components were probably found in the center and near the top of the outline of the spot in the first chromatogram. The NMR spectrum for 1,4-diphenyl-1,3-butadiene is shown in Figure 5. The E,E isomer is said to produce peaks around 6.6 to 7.0 ppm, 7.2 to 7.5 ppm. The E, Z isomer is said to produce peaks around 6.2 to 6.5 ppm and 7.1 to 7.5 ppm.5 The peaks around 7.2 to 7.5 ppm were ignored as both isomers have peaks within this range. However, the upfield peaks around 6.6, rather than 6.2, are important because these peaks indicate that the trans isomer is the major isomer. Equation 2 calculated the NMR coupling constant to be 4 Hertz. If the coupling constant was greater than 10 Hertz, than it indicates the trans isomer is in greater quantity. If the coupling constant is less than 10 Hertz, the dominant isomer.1 This is inconsistent with the rest of the data because the coupling constant suggests that the cis isomer is the dominant isomer. This experimental error could have been caused by the fact that the NMR was done on a different source of 1,4-diphenyl-1,3-butadiene, rather than the original solid used in the rest of the techniques. Table 3 listed the experimental and literature melting points of C16H14 in terms of isomers. The observed melting point of C16H14 was 152 Ð 156¼C. The literature melting point of the (E, E) isomer of C16H14 is 150¼C and of the (E, Z) isomer is 88 ¼C. The results indicate that the major product was the trans isomer of C16H14, because the melting point range is close to the melting point of the (E, E) isomer. However, because the melting point range is broad and higher than the range of the (E, E) isomer, the C16H14 produced does contain impurities.1 The experimental error of introducing impurities probably occurred between the frequent transferring of the mixture from one apparatus to another. Equation 3 was used to calculate the limiting reagent in the synthesis of 1,4diphenyl-1,3-butadiene. Cinnamaldehyde theoretically produces 1.03 x 10-3 mole of C16H14 and BTTPC, the precursor to forming the ylide, can theoretically produce 1.01 x 10-3 mole of C16H14 and that makes it the limiting reagent. Equation 5 was used to calculate percent yield. The experimental yield, which was 0.33 grams of C16H14, is divided by the theoretical yield calculated by Equation 4, which is 2.09 x 10-1 grams of C16H14 and multiplied by a 100. The percent yield of C16H14 was 16%. This is an acceptable yield considering it was a microscale experiment and the reaction was not heated. Heat floods the reaction with energy, and because the reaction was not flooded with heat, the more stable isomer is predominant which is the trans isomer. Conclusion The Wittig reaction is often used to attach molecules to create a product via a specific ylide. The stability or instability of the ylide determines if the reaction is E-selective or Zselective and in this experiment, the stable ylide produced E-isomers. The dominant isomer or major product was (E, E) 1,4-diphenyl-1,3-butadiene. The chromatograms confirmed the purity of the compound when the solid sample separated into one spot and the filtrate separated into 3 spots. The Rf values were inapplicable due to the reaction mixture chromatogram failing to separate. The NMR spectrum confirmed that the major product was the trans isomer based on the peaks that were present. The coupling constant of 4 Hz was inconclusive because it indicated that the cis isomer was the dominant isomer. However, the melting point of 152-156¼C of the solid C16H14 confirmed the trans isomer to be dominant, since the literature melting point of the trans isomer is 150 ¼C. Although the melting point indicated the dominant isomer was trans, the broad range indicated impurities were still present in the solid sample. The limiting reagent was the ylide and the percent yield was 16%. This is an acceptable amount considering stable ylides are slow to react and the reaction wasnÕt flooded with energy. Questions 1. There is an additional isomer (mp = 70¼C) that has not been shown in this experiment. Draw a structure of this isomer. Why isnÕt it formed? This is the (Z,Z) isomer. This isomer is not formed because stabilized ylides have electron withdrawing group that cause conjugation and this stability is what causes stabilized ylides to be E-selective. Unstabilized ylides are Z-selective. 2. Why is the E,E isomer the thermodynamically most stable isomer? The E,Z isomer forces the large substituents to be closer together and on the same side, causing steric hindrance. In contract the E,E isomer does lays out the large substituents away from one another and has much less steric hindrance. The decrease in steric hindrance is what makes the E,E isomer the more thermodynamically most stable isomer. 3. Your cinnamaldehyde was contaminated with a large amount of cinnamic acid. The Wittig reaction did not yield any product. Why? The cinnamic acid is a carboxylic acid, and the Wittig reaction only occurs with carbonyl groups like ketones and aldehydes. Additionally, the highly charged ylide will attack the acidic hydrogen on the carboxylic acid and then no reagent will be left to attack the carbonyl compound.