Synthesis of Tetraphenylcyclopentadienone. Becky Ortiz

Transcription

1 Synthesis of Tetraphenylcyclopentadienone Becky Ortiz Introduction An aldol reaction is a reaction in which aldehydes or ketones undergo a base- catalyzed carbonyl condensation reaction to form a beta- hydroxy- substituted, or aldol, product. Further condensation of this product resulting in the loss of water is known as an aldol condensation reaction. In basic conditions, enolate ions will expel a hydroxide group. 1 An example of an aldol condensation reaction is the synthesis of tetraphenylcyclopentadienone, TPCPD, from benzyl and 1,3- diphenylacetone. 2 TPCPD is a derivative of the cyclopentadienone, which is a highly reactive compound whose synthesis has not been very successful. 3 TPCPD itself has no biological significance, but it is a favored aldol condensation reaction example since it is a useful visualization of the reaction due to its dark purple color. 4 Figure 1. Synthesis of TPCPD 3 from benzyl 1 and 1,3- diphenylacetone 2

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3 In the first step in the formation of TPCPD, an intermolecular proton transfer, the oxygen of hydroxide from the base will accept a hydrogen bonded to one of the carbons bonded to the carbonyl of 1,3 diphenylacetone. The hydrogen will in turn donate its electrons to that carbon to create a partial negative. In a nucleophilic addition, one of the double bonds of the carbonyl in benzyl will break and the electrons will move to the oxygen to create a partial negative charge on the oxygen. The partial negative carbon of 1,3- diphenylacetone will donate electrons to form a bond to the carbon that is now single bonded to the oxygen. The partial positive oxygen will then accept a hydrogen from water in an intermolecular proton transfer and that hydrogen will donate its electrons to the oxygen of the water to create a partial negative oxygen in hydroxide. In an E2 elimination, the oxygen of hydroxide will accept the hydrogen bonded to the carbon of 1,3- diphenylacetone bonded to benzyl. That hydrogen will donate its electrons to form a double bond between the carbons bonding benzyl and 1,3- diphenylacetone. The bond of the alcohol will break and the electrons forming the bond will be accepted by the oxygen of the alcohol to form a partial negative charge on the oxygen. In an intermolecular proton transfer, the oxygen of the hydroxide will accept a hydrogen from the carbon bonded to the carbonyl of the 1,3- diphenylacetone portion. The electrons from the hydrogen will then be donated to the carbon to form a partial negative charge. The double bond of the carbonyl of benzyl will break and donate the electrons to the oxygen to form a partial negative charge. The partially negative carbon will then donate electrons to form a bond between itself and the carbon bonded to the partial negative oxygen in a nucleophilic addition. The partial negative oxygen will then accept a hydrogen from water and the hydrogen will in turn donate its electrons to the oxygen of the water, an intermolecular proton transfer. The last step, an E2 elimination, the oxygen of hydroxide will accept the hydrogen of the carbon bonded to the carbonyl which will donate its electrons to form a bond between the carbon and carbon of the alcohol group. The bond of the alcohol group will break

4 and the electrons of the bond will be accepted by that oxygen, creating a partial negative charge on the oxygen of the leaving hydroxide group. The purpose of this experiment is to synthesis TPCPD, monitor this aldol condensation reaction by TLC, purify by recrystallization, and analyze the product. TLC will be used to monitor the reaction progression towards production formation. Recrystallization will then be utilized to purify the product after determining the reaction is over using TLC. To measure the efficiency of the synthesis and purification techniques, percent yield of the crude TPCPD and percent recovery of the pure TPCPD were calculated. In the analysis of the product s purity and characterization, melting point range, infrared spectroscopy, and nuclear magnetic spectroscopy were used. Experimental In the synthesis of TPCPD, benzyl (0.432g, 2 mmoles) was reacted with 1,3- diphenylacetone (0.435 g, 2 mmoles) in 95% ethanol (5mL, ) for 40 minutes. A solution of potassium hydroxide (0.175g, ) and 95% ethanol (1.5 ml, ) was added to the reaction mixture during this time. The reaction was monitored by TLC (2:1 mixture of dichloromethane:hexanes). The dark purple crude TPCPD was isolated by vacuum filtration and washed with ice cold 95% ethanol (5 ml, ). The crude TPCPD was purified by recrystallization using a mixture of 95% ethanol (20 ml, ) and toluene (20 ml) as the solvent. The pure TPCPD crystals were then isolated by vacuum filtration. Rf values obtained from the reaction mixture monitored by TLC at time 0 Rf value Benzyl, spot Benzyl, spot Reaction mixture ,3- diphenylacetone, spot ,3- diphenylacetone, spot

5 Rf values obtained from the reaction mixture monitored by TLC at time 10 minutes Rf value Benzyl, spot Benzyl, spot Reaction mixture, spot Reaction mixture, spot Reaction mixture, spot Reaction mixture, spot ,3- dipheylacetone, spot ,3- diphenylacetone, spot Rf values obtained from the reaction mixture monitored by TLC at time 20 minutes Rf value Benzyl, spot Benzyl, spot Reaction mixture, spot Reaction mixture, spot Reaction mixture, spot Reaction mixture, spot ,3- dipheylacetone, spot ,3- diphenylacetone, spot Rf values obtained from the reaction mixture monitored by TLC at time 40 minutes Rf value Benzyl, spot Benzyl, spot Reaction mixture, spot Reaction mixture, spot Reaction mixture, spot Reaction mixture, spot ,3- dipheylacetone, spot ,3- diphenylacetone, spot

6 1 H NMR data of TPCPD using deuterated chloroform solvent on a 60 MHz NMR label Chemical shift ppm observed Chemical shift ppm theoretical multiplicity integral a d b m c m d d e m f m acetone s 1.38 IR data of TPCPD Functional group Absorption frequency observed (cm - 1 ) Absorption frequency theoretical (cm - 1 ) C- H, aromatic C=O C=C, aromatic , ,1440.9, , 1580, 1500,1450 Results and Discussion In the synthesis of crude TPCPD, the reaction was monitored by TLC. In the standard spots of all the TLC plates for benzyl and 1,3- diphenylacetone, there are two spots. This is most likely because both started to decompose. The second spot is most likely the benzyl and 1,3- diphenylacetone. Since benzyl has a higher ratio of electronegative atoms to carbons and hydrogens, it is the most polar and will have the lowest Rf value. TPCPD will have the highest Rf value as it is the least polar and has a much lower ratio of electronegative atoms to carbons and hydrogens due to only one carbonyl and four aromatic rings. 1,3- diphenylacetone will have an Rf value closer to benzyl, but slightly higher since it has one less carbonyl and a lower ratio than benzyl, but still a higher ratio compared to TPCPD. The reaction lane only has one large spot with a higher Rf value, 0.63, than the second spot of the benzyl lane, 0.53, and the 1,3- diphenylacetone, This indicates that the product could be already forming since the Rf value is close to those of the product spot Rf values of 0.63, 0.65, Since there are

7 numerous intermediates in the synthesis of TPCPD, it will be difficult to determine whether the starting material is present as a spot, or if it is an intermediate. The focus will be on the size and color intensity of the spots in the reaction lane. At 10 minutes, the reaction turns from a clear liquid to a dark purple liquid. On the TLC plate, all four spots were equal in intensity with the three first intermediate spots being of slightly larger size compared to the last spot, of the TPCPD spot. This indicates the reaction is moving toward product formation, but is primarily intermediates. At 20 minutes, the TPCPD spot has the darkest intensity and is slightly larger than the intermediate spots. This indicates the reaction is almost over. At 40 minutes, the reaction is over. All the spots are very faint, but the intermediate spots are slightly smaller than the last TPCPD spot. The faintness of the spots could be due to a light spotting of the lane. The reaction produce a dark purple powder solid. The crude TPCPD had a melting point range of C. Since the melting point range is slightly higher than its melting point range of C 5, this could be a result of heating the sample too fast. The percent yield was calculated to be 88.3%. Since this is slightly high, it is possible there is still some ethanol present from the wash to create a higher than expected percent yield. It is also possible synthesis techniques were properly carried out. After recrystallization of the crude TPCPD, dark purple crystals were formed. The melting point range was determined to be C which is the same as the crude melting point. This could be a result of toluene present after recrystallization. This is most likely the result since toluene has a high boiling point and as the sample began to melt, it appeared to be evaporating as it melted. The percent recovery of TPCPD was calculated to be 41.5%. This is a little low, especially compared to the percent yield, this indicates poor purification techniques. This could be due to loss of product that stuck to the flask, filter paper, or stirring rod during vacuum filtration.

8 The NMR spectrum (figure 3) obtained also helps to support the purity of the crystals. NMR displays the peaks of hydrogen groups present in the tested compound. Hydrogen groups unique to TPCPD will help identify the product. Any peaks that are not unique to TPCPD indicates an impurity and a potential error in the synthesis or purification of the product. The integral values will also indicate how many hydrogens are present in the peak. There are 6 distinct hydrogen groups in TPCPD. However, they are all aromatic hydrogens so their theoretical range is ppm. The integral value of the aromatic hydrogens was 19.45, which is close to the actual integral value of 20. The 20 aromatic hydrogens appeared in a range of ppm which is within its expected range. However, there was a singlet at ppm which is most likely acetone. While the NMR shows the product is pure from starting material or intermediates, there is an acetone impurity. This could be the result of acetone left over in the NMR tube or not having fully evaporated after cleaning glassware. The NMR can still support that TPCPD was properly purified. The IR spectrum (figure 4) also helps to determine the purity of the compound as it will absorb different types of bonds between functional groups at different wavelengths. In the observed IR spectrum, the C- H bond of the aromatic ring was absorbed at cm - 1 and was within the theoretical range of cm - 1. The C=O bond of the carbonyl was absorbed at cm - 1 and was within its theoretical range of cm - 1. The C=C of the aromatic ring was observed at a range of to cm - 1 and is close to its theoretical range of cm - 1. Since the absorption frequencies were fairly similar, this can support that TPCPD product is pure. Conclusion After analyzing the NMR scan and the IR scan, it can be concluded that the TPCPD is pure. Even though the NMR had an acetone peak, the TPCPD was still pure from starting material and other compounds used in the process of synthesis and recrystallization. It does indicate there was some error

9 in producing the sample for the NMR like not letting cleaned glassware evaporate the acetone completely. The melting point also indicates there were some poor techniques in determining the melting point, even though it was close to its melting range of C. 5 However, since the percent yield, 88.4%, was a little high, and percent recovery, 41.5%, was a little low, it indicates there were some errors during the synthesis and purification process. These errors include ethanol not being completely evaporated before weighing the crude TPCPD and not collecting all of the crystals after recrystallization. In order to improve this experiment, it is suggested that the techniques of recrystallization, determining melting point range, and filtration be practiced. Signs of a more complete synthesis would be a more fitted percent recovery, a higher percent yield, and a more accurate melting point range. References 1 McMurry, J. Fundamentals of Organic Chemistry: Seventh Edition. Brooks/Cole, Belmont, Bortiantynski, J. & Beiswenger, M. Chapter 11: Synthesis of Tetraphenylcyclopentadienone (TPCPD). Lab Guide for Chemsitry 203. Hayden McNeil, Plymouth, Zhi, L., Wu, J., Li, J., Stepputat, M., Kolb, U., & Mullen, K. (2005), Diels Alder Reactions of Tetraphenylcyclopentadienones in Nanochannels: Fabrication of Nanotubes from Hyperbranched Polyphenylenes. J. Chem. Education, 17 (12) Doi: /adma Fieser, L.F. & Fieser, M. Topics in Organic Chemistry. Reinhold Pub. Corp., New York, Tetraphenylcyclopentadienone, MSDS No Sigma Aldrich. 5 Dec. 2015