Williamson Synthesis

Williamson Synthesis - University of Kentucky W illiamson...

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Unformatted text preview: University of Kentucky W illiamson Synthesis of propylp-totylether Terpsichore Lindeman Drawer U33A March 2011 Ma9 Fields University of Kentucky – Department of Chemistry University of Kentucky - Department of Chemistry Table of Contents Williamson Synthesis of Ar-O-P Purpose Theory Results and Discussion Waste Disposal Conclusion Appendix A IR SPECTRUM-attached Bibliography 1 1 1 4 6 6 7 7 8 Name of report i University of Kentucky - Department of Chemistry W illiamson Synthesis of Ar-O-P Purpose Understanding the mechanisms of a reaction between a phenoxide and an alkyl halide when synthesizing ethers and yielding an appreciable percentage of propyl -p-totylether for that purpose, while further understanding the role of phase transfer catalysts that assist in such reactions was also observed. Theory Synthesizing ether is most commonly achieved using the Williamson synthesis. The Williamson synthesis method is basically an SN2 reaction of aryl-oxides (phenoxides) and alkyl halides. This reaction is by nucleophilic substitution, which is a method of yielding about various functional groups. This method of ether synthesis may yield both symmetrical and non-symmetrical (asymmetrical – i.e. dialkyl ether, aryl alkyl ether) ethers. Though it should be noted that indeed aryl halides have low reactivity to nucleophilic substitution and thus are not preferred. F igure 1 Williamson Synthesis of Ar-O-P Tore Maras-Lindeman #8241 1 University of Kentucky - Department of Chemistry During this experiment p-Cresol (4-methylphenol) and propyl iodide were utilized to yield propyl-p-tolylether (Ar-O-Pr) via the Williamson synthesis method. F igure 2 The addition of NaOH as the base p-Cresol, which is highly acidic, is transformed into a phenoxide ion. The hydroxyl group of the NaOH is the recipient of the donated proton of the p-Cresol thus, as depicted is responsible for the formation of water. Furthermore, the phenoxide ion is in the solution and a salt is formed though it is hydrophilic, thus it is soluble in water so dissociation is expected and observed. Previously mentioned a negative charge on the oxygen atom is adjacent to the proton that is donated, though, it is mentionable to note its delocalization throughout the ring structure. This in turn allows us to observe the negative charge is due to the electron pair straggling by the donated hydrogen that in turn due to its delocalization(Steven F. Pedersen 2010) is deemed to be resonance stabilization. Williamson Synthesis of Ar-O-P Tore Maras-Lindeman #8241 2 University of Kentucky - Department of Chemistry p-Cresol in itself is highly acidic, because conjugate base is very weak. Hence, this ability of distributing charge over a great volume, which in essence is delocalization, allows it to result in resonance stabilization. Aforementioned, Williamson synthesis, is an SN2 reaction, bimolecular nucleophilic substitution. This nucleophilic attack is only able to occur if the substances are both in the same solvent. Though, since in this case p-cresol and propyl iodide are not able to do this “alone”. This is because the manifested phenoxide is indeed water-soluble and propyl iodide is not. Hence as previously noted, the electron rich phenoxide is the one that acts as the nucleophile by attacking the carbon of the propyl iodide from the back, which in essence ushers the halide out. In turn, as in all SN2 reactions, t(Maruoka 2008)he iodine, which is the leaving group, associates with the sodium ion. Though, these two components cannot be in the same solvent as they create two immiscible layers, thus a phase transfer catalyst is necessary to assist in our reaction to occur. A phase transfer catalyst known as PTC is a catalyst that assists a substance to migrate from one phase into another phase in which a reaction may be observed. PTC’s are used to allow us to obtain higher conversion, faster reactions and most of all allow reactants that are soluble in aqueous phases but insoluble in organic phases to react with one another. How this is actually achieved by PTCs is still under speculation. There are many mechanisms describing how the PTC behaves.(Maruoka 2008) A method previously used in experiments throughout this course is column chromatography. This is a method by which impurities and water are extracted from our sample to assist in yielding a purified product. In this case, silicon dioxide and sodium sulfate were used. Once a purified product was obtained, it Williamson Synthesis of Ar-O-P Tore Maras-Lindeman #8241 3 University of Kentucky - Department of Chemistry was reheated to further assist in remove impurities. Once the boiling ceased weight of the product was recorded, and an IR spectrum was generated(Mayo 2010 ). Results and Discussion Figure 3 Component/compound 80 mL p-Cresol 1.32 mL NaOH 0.748 mL propyl iodide 92mg Tetrabutylammonium bromide Propyl-p-totylether (Ar-O-P) 0.8250 g 1.3920 g 1.1233 g 0.0090 g 0.9365 g Mass Table 1 Evidently there is less isopropyl iodide, hence this would be the limiting reagent. Thus, using the moles(Smith 2009) calculated by our limiting reagent we are able to calculate the theoretical mass yield of Ar-O-P. Williamson Synthesis of Ar-O-P Tore Maras-Lindeman #8241 4 University of Kentucky - Department of Chemistry In turn the calculations of the percent yield of product is calculated as follows: The percent yield attained is not very sizable but humble. This would be a consequence of product loss during boiling with the methylene chloride due to the uncontrollable instability of heat produced, and maintained by our heating element. Though, during the column chromatography, a small portion of the product had overspilled out of the pipette during transfer and that indeed could have been the cause of these results. Examination of IR Spectra Group C-O-C Sp3 Sp2 O-H C-H C-H 1048.6 cm-1, 1068.0 cm-1 2876.23 cm-1 Wavelength 2963.76, 3100 cm-1 ~3400 cm-1 Table 2 The IR spectrum overall was apprehensively satisfactory. The Sp2 C-H indications were “weak” and in general the results were noisy, even though the IR spectrum coincided(Steven F. Pedersen 2010) with Ar-O-P. It should be mentioned that a slight trace of OH at about 3400 cm-1 (in bold) was indicated on the IR spectrum, though, the peak itself was not significant enough to indicate precise peak, hence, indicating that some p=cresol must have(Hart 2006) been left over in our sample unreacted. Williamson Synthesis of Ar-O-P Tore Maras-Lindeman #8241 5 University of Kentucky - Department of Chemistry Waste Disposal Waste Container Acid-Base container Halogenated Waste Container Non-halogenated Waste container Solid Waste Container Glass Box NaoH Propyl iodide, methylene chloride Chemical/item Ehter, p-cresol Tetrabutylammonium bromide, solid sodium sulfate, silicon dioxide, boiling chips pipettes Table 3 Conclusion This experiment allowed exploration and observation of a reaction of a phenoxide and alkyl halide via the method Williamson synthesis of an ether. Understanding what a PTC, phase transfer catalyst is and how it is necessary in this reaction was examined and questioned. As I found the concept of PTC intriguing, I did further research only to discover that PTC is quite ambiguous in its exact method and that there are 3 leading “mechanisms” of how PTC actually assists two immiscible solvents each housing a reagent unable without its presence to react. Once, again using sodium sulfate to extract water and impurities was used as in other experiments executed throughout this course prior to this, known as column chromatography, and silicon dioxide was introduced to this method as an additional “purifying” agent. Williamson Synthesis of Ar-O-P Tore Maras-Lindeman #8241 6 University of Kentucky - Department of Chemistry Appendix A IR SPECTRUM-attached Williamson Synthesis of Ar-O-P Tore Maras-Lindeman #8241 7 University of Kentucky - Department of Chemistry Bibliography Hart, C., Hart, Hadad (2006). Laboratory Manual for Hart/Craine/Hart/Hadad Organic Chemistry: A Short Course, Brooks Cole. Maruoka, T. H. a. K. (2008). Asymmetric Phase Transfer Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA. Mayo, P., Forbes (2010 ). Microscale Organic Laboratory, John Wiley & Sons Inc. Smith, J. G. (2009). Organic Chemistry, McGraw-Hill Science/Engineering/Math. Steven F. Pedersen, A. M. M. (2010). Understanding the Principles of Organic Chemistry: A Laboratory Course, Brooks Cole. Williamson Synthesis of Ar-O-P Tore Maras-Lindeman #8241 8 ...
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