TheDielsAlderReactionofAnthraceneandMaleicAnhydride LeadAuthor:SarahFerrans Reviewer:FrankRoberson Editor:ScottWatson Introduction: The Diels-Alder reaction is a famous and common organic cycloaddition reaction discovered by Otto Diels and Kurt Alder, for which the reaction is named. The reaction occurs between a conjugated diene and a substitute alkene, or dienophile, that results in a cyclohexene product. The reaction involves three pi bonds, two from the diene and one from the dienophile in a concerted reaction to form a six-membered ring. This reaction can sometimes be called a 4+2 cycloaddition because it involves four pi electrons in the diene and two pi electrons from the dienophile1. Being able to make a six-membered ring containing an alkene is useful in various types of synthesis reactions. In this experiment a Diels-Alder reaction between anthracene and maleic anhydride will result in 9,10-dihydroanthracene-9,10-α,β-succinic anhydride. The mechanism for this is illustrated below in Figure 1. Figure 1: Diels Alter mechanism for anthracene and maleic anhydride Table1.TableofReagents2 Name Anthracene Maleic anhydride Xylene 9,10dihydroanthracene9,10-α,β-succinic anhydride Molecular Weight (g/mol) 178.23 98.06 106.16 276.29 Density (g/mL) 1.25 1.48 0.86 N/A Melting Point (Degrees C) 215-217 51.6-53.6 12.2-14.2 262-264 Boiling Point (Degrees C) 340 202 138 N/A Experimental To begin, 0.3g of anthracene, 0.15g of maleic anhydride, and 3 mL of xylene were added to a 5 mL round bottom flask with a spin bar. A hot plate was set down for stirring, and on top of the hot plate a sand bath was prepared to actually heat the experiment. The round bottom flask was attached to a condenser tube that had a wet paper towel wrapped around it and was partially submerged in a sand bath. The experiment was heated at reflux for at least 30 minutes before being removed, allowed to cool fully to room temperature, and poured into a beaker for easier transfer later on. The beaker was then set in an ice bath for 10 minutes. The resulting solid was isolated by utilizing vacuum filtration with a Hirsch funnel, washing with an additional 1.5 mL of ice-cold xylene, and allowing the solid to dry fully. However, the main function of xylene in the experiment was to allow the solution to be heated to a higher temperature without boiling. This was possible because of xylene’s high boiling point. So, with the xylene in solution, it could be heated more which drove the reaction further toward the final product without actually evaporating it. Once the product was dry, its mass was recorded and a portion was taken to determine the melting point, and an IR spectrum was taken to determine if the experiment was successful in reaching the end product. Results The percent yield of 9,10-dihydroanthracene-9,10-α,β-succinic anhydride is calculated using the general template of the equation shown below. % Yield= Actual Yield ∗100 % Theoretical Yield The mass of the reactants used, over their respective molar mass will give the moles of product available for use in this experiment. In an ideal setting, one mole of anthracene and one mole of maleic anhydride would make one mole of 9,10-dihydroanthracene- 9,10-α,β-succinic anhydride when multiplied together. The mass of the anthracene used was 0.286 g. This over the molar mass of anthracene, 178.23, gives the moles of anthracene used, 0.00160. The mass of the maleic anhydride used was 0.152 g and the molar mass is 98.06, giving 0.00155 moles of maleic anhydride to use in this reaction, and making maleic anhydride the limiting reactant. Multiplying the moles of the limiting reactant against the molar mass of the product (276.29 g/mol), gives the theoretical yield of 0.428 g of 9,10-dihydroanthracene- 9,10-α,β-succinic anhydride. However, the weighed mass of the product was 0.103 g. 0.103 g ∗100 %=24.06 % 0.428 g A melting point range was taken, coming out to be 240-243 °C which is lower than % yield of product= expected, indicating there might be impurities in the product, as impurities do tend to lower the melting point of a product. This can happen because of the overall weakening of the product’s structure due to impurities, which can lower the stability of the product, effectively lowering the melting point. This theory is further supported by the IR table obtained in lab, which can be found below in figure 2. Figure 2: IR Spectrum of crystalized product Most of the values in this IR spectrum are in the fingerprint region and don’t reveal useful data. However, the small peak at 3048.44 is not the peak expected for the product, it is more consistent with the reactants. The areas of interest expected as a part of an IR spectrum for 9,10-dihydroanthracene- 9,10-α,β-succinic anhydride is an aromatic C-H stretch at around 3030 and alkyl C-H stretching at 2850-2950. Neither of these are present and indicate that errors were made in the experimental portion of the lab. Figure 3 Figure 3 describes the IR for the product 9,10-Dihydroanthracene- 9,10-α,β-succinic anhydride. This is similar to the IR that should have been obtained in the laboratory if not for experimental errors. Table 2 Peak range shape Conclusion 3100-2900 Weak Sharp C-H 1750 Strong Sharp C=O 1450 Strong sharp C=C Table 2 describes the significant peaks used to identify the product 9,10-Dihydroanthracene9,10-α,β-succinic anhydride in the IR seen in figure 3. Discussion The lower percent yield is the easiest of the errors to explain. The product crystalized more quickly than anticipated while in the 5 mL flask once it was removed from the heat, and much of it was lost before it could be transferred to the beaker and then dried in the Hirsch funnel. The glass was washed with extra xylene, but it was not until after the samples were weighed that heated xylene was used to successfully wash the glassware and remove the excess crystals. The IR spectrum revealed that the reaction did not proceed all the way through as anticipated, showing signs of reactants present in the sample rather than product. This is supported too by the melting point of 240-243°C; which is significantly lower than the expected melting point of 9,10-dihydroanthracene- 9,10-α,β-succinic anhydride at 262-264°C, indicating impurities or reactants leftover in the crystalized product. According to the IR obtained in lab, it is safe to assume that the leftover reactant was most likely maleic anhydride because of the similarities between the IR obtained and the IR of maleic acid around 3100cm^-1 range. A small stretch almost identical to the IR of maleic acid is visible in the same region on the experimentally obtained IR. All of these point to a somewhat incomplete reaction. Perhaps this was due to the long time for the sand bath to heat up, and the experiment did not proceed as it needed to due to a lack of heat, leaving some or all of the anthracene and malic anhydride unreacted. Additional time, more care setting up the experiment, and taking care to ensure heating and cooling baths are adequately prepared ahead of time could reduce such errors in the future. In addition, it is important to discuss what the actual IR spectrum for 9,10Dihydroanthracene- 9,10-α,β-succinic anhydride should have looked like if the procedure was carried out without error. In figure 3 above, the IR of 9,10-Dihydroanthracene- 9,10-α,β-succinic anhydride was included, showing the most important parts of the spectra that the experimenter should take note of. Further discussing this theoretical IR chart, it can be seen that there is C-H stretching evident in this region just above 3000cm^-1, or the aromatic region which is anything greater than 3000cm^-1 on an IR spectrum. C-H stretching can also be observed in the region just below 3000cm^-1 around 2900cm^-1. This stretching was not evident in the actual IR spectrum obtained experimentally for reasons previously discussed. Another important signal to look for is the C=C bending, present at around 1450cm^-1. This bending can be seen in the experimental IR obtained, but is not quite as pronounced as the experimenter should expect it to be. Finally, the experimenter should expect to see C=O stretching in their obtained IR. These stretches can be observed in the region of around 1750cm^-1. These stretches are somewhat present in the obtained IR, but are not reliable signals as they would likely be classified as bends rather than stretches where in the actual product IR these stretches are very deep and pronounced. As stated previously, the important stretches that would identify the product have been included in Figure 3 and Table 2 above. Conclusion The Diels-Alder reaction between anthracene and maleic anhydride, with a yield of 24.06% and a melting point of 240-243, did not result in the amount and quality of product that was expected initially. The reasons for this are likely due to experimenter error in the setup of the reaction. In the future, extra care should be taken to ensure a quality product in the lab, specifically when it comes to refluxing. After some additional research, we found that in order to get a higher product yield for more accurate IR and melting point readings, the experimenter should attempt to add more acetic acid to the reaction and reflux for a longer amount of time. So long as the temperature is stablized throughout the reflux, these techniques should result in pushing the reaction further toward the products, resulting in a higher yield and more accurate data. References 1. The Diels-Alder Reaction. https://www.masterorganicchemistry.com/2017/08/30/thediels-alder-reaction/ (accessed Jul 13, 2019). 2. Chem Spider. Database of Chemical Properties. www.chemspider.com (accessed July 12, 2019) 3. Spectra Base. Database of Chemical IR Spectra.https://spectrabase.com/spectrum/BrxNcecaJ5o?a=SPECTRUM_BrxNcecaJ5o (accessed July 17, 2019) AnswerstoPre-LabQuestions 1. NaphthalenewillnotparticipateinaDielsAlderreaction.Why? It does not take part in a typical Diels-Alder reaction because the pi-electrons of the diene are part of an aromatic ring. If cycloaddition were to occur the aromacity of the ring would be lost, which would require a great deal of heat energy to overcome. 2. PentaceneshownbelowwillundergoaDielsAlderreactionatonering.Whichring wouldyoupredicttoundergothereactionandbrieflyexplainyourchoice? The middle ring of pentacene would take place in the reaction because the loss of that one aromatic ring does not destabilize the structure. There will still be two fully formed ring structures attached to either side of the final molecule. 3. Which of the following dienes cannot take part in a Diels Alder reaction? Why? Both of the double bonds should be in one plane in order for a Diels Alder reaction to take place. This rotation cannot occur in the second molecule pictured as it is a trans-diene that cannot be rotated or converted to a cis conformation.