Ochem Lab - Diers-Alder PDF

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1 Diels-Alder Reaction Data/ Results:

Melting point range ( ℃ )

Sample

Isolated compound from reflux reaction

85.7 - 87.7 ℃

Final isolated compound after recrystallization procedure

90.02 - 92

℃

Literature value melting point for cis-4- cyclohexene-1,2-dicarboxylic anhydride: 102 ℃ Ref.: Tetrahydrophthalic anhydride. (n.d.). Retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/THPA

Literature value melting point for maleic anhydride: 52.8

℃

Ref.: Maleic anhydride. (n.d.). Retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/Maleic-anhydride Table 1 – Summary of observed melting points for final isolated samples of the compound, cis4- cyclohexene-1,2-dicarboxylic anhydride. Theoretical literature values for melting points of relevant compounds are provided for reference/comparison.

Calculated percent yield of recovered product: 1.311 g ×100 %=52.44 % yield 2.5 g Table 2 – Summary of the present and expected functional groups indicated by peaks shown in the infrared spectrum generated for the isolated compound (see Appendix A). Sample structure of cis-4- cyclohexene-1,2-dicarboxylic anhydride:

Ref.: Tetrahydrophthalic anhydride. (n.d.). Retrieved from

2 https://pubchem.ncbi.nlm.nih.gov/compound/THPA Functional Bonds Location (Wavenumber cm-1) Group sp2 C-H 3053.47 cm-1 3 sp C-H 2958.48 cm-1 Anhydride C=O 1257.75 - 1838.98 cm-1 C-O 1237.58 cm-1 Alkene C=C 1439.24 cm-1

Intensity Weak Moderate Strong, two bands Weak Weak

Figure 1 – Thin-layer chromatography with recrystallized compound and maleic anhydride using a solvent system of 95% ethyl acetate. The lower spots in Lanes A, B, and C have an Rf factor of 0.263 and confirm the presence of maleic anhydride in all three lanes. The top spots in Lanes A and B have an Rf factor of 0.921 and were identified as cis-4- cyclohexene-1,2-dicarboxylic anhydride.

Discussion: 1.)

A reflux reaction was conducting by heating maleic anhydride, xylene, and sulfolene, so

that ultimately, an isolated sample of cis-4- cyclohexene-1,2-dicarboxylic anhydride could be obtained. At the start of the reflux reaction, white steam was observed to be escaping from the top of the reflux condenser which indicated that the heat of the thermowell was initially too high. Furthermore, the identity of the white steam was determined to be excess sulfur dioxide (SO2) which is known to have a very low boiling point of

−¿ 4 ℃

(see Reagent Table, p.009).

3 Sulfur dioxide gas is also known to be an irritant therefore it was an important safety measure to keep the hoods closed at a safe level as to avoid inhaling the gas. After the reflux reaction, the remaining contents of the round-botton flask were described as a dark yellow, transparent liquid. 2.)

Sulfolene was used as an alternative reactant which forms excess 1,3-butadiene (g) and

sulfur dioxide (g) when heated to ~ 135 ℃ . A large excess of sulfolene was used to drive the equilibrium reaction towards the product to ensure that cis-4- cyclohexene-1,2-dicarboxylic anhydride would be formed. Given that the Diels-Alder reaction was reversable, according to La Chatlier’s principle, the concentration of reactants needed to be increased so that the formation of products, in this case, cis-4- cyclohexene-1,2-dicarboxylic anhydride, would be favored by equilibrium. 3.)

After the reflux distillation was completed, the isolated compound was filtered and

vacuum-dried for further analysis. The final product was described as a soft white crystalline powder with a pinkish hue; somewhat resembling Himalayan pink salt. The isolated compound had a melting point range of 85.7 - 87.7

℃

(see Table 1). The literature value for melting

point of pure that cis-4- cyclohexene-1,2-dicarboxylic anhydride is established as 102

℃

(Tetrahydrophthalic anhydride. (n.d.)). The experimentally obtained melting point for the isolated compound was much lower than it’s expected melting point which indicated that there were some impurities that remained in the sample after the reflux reaction. Therefore, a recrystallization procedure, was required to further purify the sample. After recrystallization of the isolated compound, a new melting point range of 90.02 - 92

℃ was obtained (see Table

1). Although the melting point after recrystallization was still lower than the expected theoretical melting point of 102

℃ , the additional purification process was successful in purifying the

isolated sample to some small degree. The impurities present in the final isolated compound were

4 hypothesized to be the result of inconsistent heating of the reflux reaction. At the beginning of the reflux, escaping steam indicated to researchers that the heat was too high, so it was lowered so as to avoid thermal loss of the solvent. However, in doing so, the heat from the thermowell had lowered too much and did not heat the solution to the adequate temperature of >103 ℃ (see Pre-lab p. 009). This meant that the sulfolene was unable to decompose and form 1,3butadiene, which should have reacted with all of the maleic anhydride. Therefore, it was likely that a small amount of unreacted maleic anhydride, which has a literature value melting point of 52.8 ℃ (see Table 1), remained in the isolated sample and had contributed to the lower observed melting point. This assessment of purity of the recrystallized/ isolated sample was confirmed by the data obtained by infrared spectroscopy (IR), thin-layer chromatography (TLC), and melting point analysis. Infrared spectrum of the isolated compound exhibited all of the peaks that were expected to be present in the final compound of cis-4- cyclohexene-1,2-dicarboxylic anhydride (see Table 2). A peak at 3053.47 cm-1 frequency wavenumber (see Appendix A), was indicative of an sp2 hybridized C-H bond, and another peak at 2958.48 cm-1 (see Appendix A), indicated the presence of an sp3 hybridized C-H bond (see Table 2). The presence of this sp3 hybridized C-H bond, provided by the IR spectrum data, supported the presence of cis-4- cyclohexene-1,2dicarboxylic anhydride in the final isolated product (see Appendix A). If only the compound maleic anhydride were present, IR spectra would not have shown a peak due to a sp3 hybridized C-H bond because it does not possess this bond. While the IR data supported the successful synthesis of the Diels-Alder reaction, the melting point analysis discussed earlier, in addition to the TLC characterization, implied the presence of some form impurity.

5 The TLC characterization of the isolated compound and maleic anhydride was developed using a solvent system of 95% ethyl acetate. This solvent system was chosen because ideally, the relatively more polar compound, maleic anhydride, would have had a lower retention factor (Rf), while the less polar, isolated compound, would have a higher Rf. This would allow for differentiation between the two compounds by comparing the size, color, and retention factors of spotted samples. Results from TLC gave qualitative evidence that there was a small amount of maleic anhydride in the final isolated sample. Lane A, which was spotted with the isolated compound alone, had two spots. One spot had an Rf of 0.921 and the other spot had an Rf of 0.263, and had traveled very little from the starting point (see Fig.1). While the lower spot was UV active, the higher spot was invisible under UV light and needed to be submerged in Iodine for further analysis. The presence of two spots in lane A confirmed an impurity since a pure sample would have had only one spot due to a single compound. Also, fact that lower spot could be seen under UV light indicated some form of impurity since cis-4- cyclohexene-1,2-dicarboxylic anhydride is known to be invisible under ultraviolet light. Lane C, which was only spotted with authentic maleic anhydride, gave an Rf of 0.263, which was consistent with the Rf of the lower spot in lane A (see Fig.1) and was also visible under UV light. The co-spot in lane B, was inoculated with both the isolated compound from lane A, and the authentic maleic anhydride from lane C. Lane B showed two spots. The spot that had traveled the highest up the TLC plate had an Rf of 0.921 and the lower spot that had traveled less, had an Rf of 0.263 (see Fig.1). The highest spots in Lane A and Lane B were described as yellow after having been stained with Iodine, while the lower spots in Lanes A, B, and C were unaffected by the iodine. Therefore, the presence of unreacted maleic anhydride remaining in the final isolated compound was confirmed based on

6 the three spots present in all three lanes: A, B, and C, with the exact same Rf of 0.263 (see Fig.1). 4.)

Given that the theoretical yield of final product was supposed to be 2.5 grams (see pre-

lab, p.009), the calculated percent recovery of isolated product was calculated as 52.44% (see Calculated Percent Yield). The biggest source of lost crude product was from transferring the isolated compound from the Erlenmeyer flask that it had crystallized in, to the vacuum filter in which the product was dried. Much of the sample was unable to be recovered from the E-flask by scraping the glass with a spatula. Therefore, the percent yield of isolated compound was largely attributed to loss of crude product during the vacuum-drying step.

7 References 1,2-Xylene; 1,3-xylene; 1,4-xylene. (n.d.). Retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/1_2-Xylene_-1_3-xylene_-1_4-xylene Characteristic IR Absorption Frequencies of Organic Functional Groups: IR-frequencies. (n.d.). Retrieved from http://www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRfrequencies.html Maleic anhydride. (n.d.). Retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/MaleicAnhydride Sulfolene. (n.d.). Retrieved from http://www.chemspider.com/Chemical-Structure.6253.html Tetrahydrophthalic anhydride. (n.d.). Retrieved from https://pubchem.ncbi.nlm.nih.gov/compound/THPA

8 APPENDIX

Appendix A – Infrared spectrum of isolated compound after reflux and recrystallization procedures.

Infrared spectrum of the final isolated compound confirmed the presence of cis-4- cyclohexene1,2-dicarboxylic anhydride in the final sample based on the peak located at 2958.48 cm-1 representing an sp3 hybridized C-H bond, only present in the compound of interest. However, IR data could not provide conclusive data that identified the impurity that caused the observed melting point to be lower than the theoretical melting point of 102 ℃ (Tetrahydrophthalic anhydride. (n.d.))....