Comprehensive Investigation and Successful Execution of Diels-Alder Reaction in Aqueous Medium: Spectral Analysis and Purity Confirmation

This experiment highlights the successful execution of a Diels-Alder reaction conducted in an aqueous medium. The reaction involved the reflux of anthracene-9-methanol and N-ethylmaleimide in water, resulting in a Diels-Alder product obtained with a satisfactory yield of 78.5%. The product exhibited a melting point range of 184.8º to 186.3ºC.

Thorough analysis employing extensive 1H and 13C NMR spectroscopy provided conclusive evidence of the presence of a product with multiple chiral centers, along with the identification of a cyclohexene species. These findings strongly support the accomplishment of the Diels-Alder reaction under the specified conditions.

The observed narrow melting range and the indication of purity in the spectra further bolster the argument for the successful formation of the Diels-Alder product. This evidence not only confirms the reaction's success but also underscores the reliability of the obtained product. Additional details and information pertaining to the experimental procedure or characterization techniques could be included for a more comprehensive account of the research.

This laboratory experiment serves as an illustrative example of the Diels-Alder reaction conducted in an aqueous environment.

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The Diels-Alder reaction is a concerted process involving a diene and a dienophile, resulting in the formation of cyclohexene. This reaction is versatile, accommodating molecules with diverse substituents, provided there is a diene with high electron density and a dienophile with low electron density. The reaction entails the breaking of π bonds in the starting materials and the formation of two σ bonds and one π bond.1 Crucially, the Diels-Alder reaction maintains the stereochemistry of both starting materials in the final product, often leading to chirality in the resulting molecule, with inequivalent carbons and hydrogens at the core.

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The specific Diels-Alder reaction under investigation involves anthracene-9-methanol and N-ethylmaleimide, producing a new chirality center that renders the central carbons and protons inequivalent.1 Figure 1 illustrates the concerted mechanism of a general Diels-Alder reaction.

Noteworthy aspects of this experiment include its impeccable atom economy and the use of environmentally friendly reagents. Every atom in the starting materials contributes to the final product, generating no byproducts. Water and methanol serve as the only reagents alongside the starting materials, and their use is safe, with no emission of harmful gases during heating. The favorable nature of bond formation in the Diels-Alder reaction allows it to proceed under mild conditions.

The product's purity will be assessed through 1H and 13C NMR analysis and melting point determination. Anticipated outcomes include characteristic peaks in the spectra, indicative of the cyclohexene formation and preserved stereochemistry, rendering the central carbons and protons inequivalent. The correct number of protons and carbons within the aromatic regions of both spectra is expected, reflecting the breaking of π bonds characteristic of the Diels-Alder equation. Comprehensive predictions and assignments of these peaks, based on additive substituent parameters, are detailed on pages 8-9 of this report.2,3

While the melting point of this product may not be extensively documented in the literature, the observed melting range serves as a useful indicator of relative purity. Additional information on the experimental procedure, specific reaction conditions, or potential applications of the Diels-Alder reaction in organic synthesis could further enhance the depth of this introduction.

In the conducted experimental procedure, precision was upheld as 0.0650 g (3.1210-4 mol) of anthracene-9-methanol, 25 mL (1.38 mol) of water, and 0.1162 g (9.310-4 mol) of N-ethylmaleimide were meticulously measured and combined in a 100 mL round-bottom flask. The ensuing mixture underwent reflux with continuous stirring over a 30-minute duration. Post-reflux, the flask was carefully removed from heat, allowed to cool to room temperature, and subsequently placed on ice to encourage crystallization. Once crystals formed, a meticulous collection was facilitated through suction filtration, accompanied by thorough washing with water and ethanol. Following this process, the crystals were pressed between filter paper to eliminate excess moisture, and subsequent drying on the suction filtration setup was carried out for fifteen minutes. The resulting product was then weighed, yielding 0.0817 g, reflecting a commendable 78.5% yield.

Further characterization of the product included 1H and 13C NMR analyses, providing valuable insights into its structural composition. Additionally, the melting point of the product was determined to be within the range of 184.8º - 186.3ºC.

13C NMR (CDCl3, 75 MHz, Diels-Alder Product): The observed chemical shifts included 177.022, 176.865, 142.327, 142.251, 139.484, 139.141, 127.075, 127.044, 126.872, 126.785, 125.525, 124.293, 123.400, 122.679, 60.822, 49.618, 47.943, 46.428, 45.940, 33.408, and 12.256 ppm.

1H NMR (CDCl3, 300 MHz, 400 MHz, Diels-Alder Product): For a detailed analysis of the proton NMR spectrum, please refer to the attached peak assignments on page 8.

To augment the experimental details, additional information on the equipment used, reaction conditions, or any deviations from the standard procedure would be valuable. Furthermore, discussing the significance of specific NMR peaks in relation to the reaction mechanism and potential implications of the observed melting point range would enhance the overall understanding of the experiment.

Within the discussion, the assignment of labels to the observed protons and carbons was carried out according to the scheme outlined in the report. Predicted ppm values for each carbon and type of proton were determined utilizing additive parameters for substituents as referenced in Mohrig, providing a basis for comparison with the actual values detailed on pages 8-9 of this report.

The 1H NMR spectra revealed a significant peak at 7.609 ppm, assigned to the 'a' proton on the aromatic ring. Although the predicted value was 7.14 ppm, the observed downfield shift is attributed to deshielding effects from the nearby OH group. The peak exhibited J3 of 7.6 Hz, indicating the presence of the 'b' protons and resulting in the observed doublet splitting. The agreement between predicted integration and splitting and the experimental data adds credibility to the results.

The 'b' proton displayed a peak at 7.267 ppm, slightly deviating from the predicted value of 7.08 ppm due to its proximity to the OH group. Triplet splitting, induced by both the 'a' and 'c' protons, was observed, forming a triplet with an integration of one. Overlapping with the peak corresponding to CDCl3, the solvent used, influenced the appearance of this peak, highlighting the necessity of careful spectral interpretation.

Moving on, the 'c' proton presented a peak at approximately 7.147 ppm, with a minor deviation from the predicted 7.08 ppm. While appearing to exhibit triplet-like splitting, the overlap with the 'f' protons complicated the determination of the coupling constant. As the only proton of its kind, the 'c' proton's peak displayed an integration of one.

The 'd' proton exhibited a peak at 7.195 ppm, a subtle variation from the predicted 7.14 ppm. Characterized by J3 and J4 coupling, typical of aromatic rings, it appeared as a doublet of doublets split by 'c' and 'i' protons, with coupling constants of 1.4 Hz and 3.87 Hz. Due to the conserved stereochemistry of Diels-Alder and the chirality center of the product, the 'd' proton was unique, resulting in an integration of one.

The 'e' proton displayed a similar pattern with a peak at 7.23 ppm, slightly downfield from the predicted 7.14 ppm. Its peak was split by 'f' and 'i' protons, forming a doublet of doublets. As the only proton of its type, its integration was one.

The 'f' proton was also unique with an integration of one, though its peak overlapped with that of 'c'. This non-equivalence led to coincidental overlap, making the exact location of individual peaks approximate, around 7.147 ppm. The predicted triplet for each proton resulted in an overlapping peak resembling a multiplet, a logical outcome given the similarity in splitting and location relative to other substituents.

The 'g' proton appeared as a multiplet peak at 7.314 ppm, deviating from the predicted 7.08 ppm. The multiplet likely arose from fine splitting by adjacent protons, contributing to the integration of one as 'g' had no equivalent protons.

Finally, the 'h' proton exhibited a peak at 7.39 ppm, considerably downfield from the predicted 7.14 ppm due to proximity to the carbonyl groups. Showing integration of one and appearing as a doublet of doublets as predicted, 'h' experienced J3 splitting of 7.2 Hz from the 'g' proton and J4 splitting of 1.2 Hz from the 'f' proton. The three-dimensional proximity to other substituents further contributed to the observed downfield shift.

To further enrich the discussion, considerations about the potential impact of these chemical shifts on the overall molecular structure or any implications for the success of the Diels-Alder reaction could be explored. Additionally, a brief exploration of the significance of the coupling constants and their role in elucidating molecular conformation would contribute to a more comprehensive analysis. Furthermore, discussing how the observed NMR peaks support the formation of the Diels-Alder product, especially considering the chirality center, would enhance the overall understanding of the experimental results.

Expanding upon the discussion, the 'Hk' protons' doublet of doublets appearance at 5.157 ppm with integration one is notably further downfield than predicted. This shift is attributed to the pronounced hydrogen bonding with the OH group, confining the 'k' proton in close proximity to both the OH and the adjacent carbonyl. The observed J3 coupling of 11.55 Hz from the 'j' proton and J4 of 5.157 Hz contributes to the complexity of the peak. Following this, the 'j' proton's doublet of doublets at 4.99 ppm with integration one is also downfield of the predicted value due to its proximity to the OH group. The coupling constants of 5.55 Hz and 5.925 Hz further characterize this peak. Subsequently, the 'Hm' proton at 4.764 ppm, a doublet with integration one, displays a considerable shift from the predicted 2.5 ppm due to its proximity to the OH group, and it exhibits J3 coupling of 3 Hz. The 'i' proton, predicted at 4.3 ppm, appears upfield at 3.329 ppm, attributed to shielding effects from the surrounding aromatic rings. Its doublet with J3 coupling of 4.05 Hz is due to coupling with 'Hp'. The 'p' proton at 3.266 ppm, a doublet of doublets with integration one, is shifted downfield due to proximity to the carbonyl group, and it displays coupling due to both 'Hm' and 'Hi' of 4.2 Hz and 3.3 Hz. The 'Hl' proton appears as a triplet with J3 coupling of 6.45 Hz at 2.773 ppm due to the effects of 'Hj' and 'Hk,' fitting well within the predicted range for an alcohol proton. The remaining protons in alkane groups, 'Hn' and 'Ho,' exhibit a quartet-triplet pattern indicative of CH3CH2. The 'Hn' peak at 3.166 ppm, close to the predicted value of 3.3 ppm, integrates to 2, coupling with 'Ho' protons with constants of 7.7275 Hz and 7.05 Hz. 'Ho' at 0.419 ppm, slightly upfield of the predicted value of 0.9 ppm, integrates to three and displays J3 coupling with 'n' protons with a constant of 6.6 Hz.

In the 1H NMR spectrum, the absence of additional aromatic protons and the presence of all expected protons in the correct chemical shift regions provide compelling evidence for the successful formation of the product. The presence of a peak indicating water at 1.566 ppm suggests some impurity, but overall, the compound appears pure relative to the starting material.

Turning to the 13C NMR, predicted values based on additive parameters showed similarity, especially for aromatic carbons. The proximity of functional groups and their effects on shielding contributed to some variance in chemical shifts. The aromatic carbons' pairs displayed shifts in line with expectations, affirming the three-dimensional structure. Alkane region peaks further supported the successful formation of the product.

While many predicted values were similar, specific functional groups, distance to substituents, and unique interactions influenced the observed 13C NMR peaks. Despite the proximity to functional groups, the alkane region showed a convincing number of peaks, strengthening the evidence for successful product formation.

In the absence of conclusive published data on the compound's melting point, the determined range of 184.8º - 186.3ºC is a valuable indicator of purity due to its narrowness. Further exploration into potential literature on similar compounds or additional characterization techniques could contribute to a more comprehensive understanding of the synthesized product. Additionally, discussion on the potential impact of impurities, if any, on the overall interpretation of the NMR spectra and the importance of the observed melting point range in confirming the purity of the product would enhance the discussion.

This experimental investigation successfully showcased the execution of the Diels-Alder reaction within a water-based environment. The observed product spectra demonstrated values that align with the anticipated outcomes, affirming the desired chirality and inequivalence of both carbon and hydrogen components. The presence of protons and carbons in the alkane regions of the spectra provided concrete evidence of the triumph in product formation. Consistency in the number and chemical shift of protons and carbons within the aromatic region, when compared to predicted values, reinforced the precision of the executed reaction.

Despite the limited information on the melting point of the specific product in available literature, the identified narrow melting range from the synthesized product serves as a robust indicator of its high purity. The amalgamation of spectral analysis and melting point determination strongly supports the assertion that the product was successfully generated with a commendable level of purity.

In future explorations, delving into the Diels-Alder reaction with N-methylmaleimide could prove insightful, offering a demonstration of the pivotal role played by electron-withdrawing groups on the dienophile and their influence on reaction rates. Expanding the scope of the study to encompass additional reactions or investigating variations in reaction conditions could contribute to a more nuanced understanding of the fundamental principles governing Diels-Alder reactions. Moreover, exploring potential applications or derivatives of the synthesized product could pave the way for further research avenues and practical implications in the realm of organic chemistry.