Synthesis of Propyl p-Tolyl Ether: Experimental Analysis and Waste Management

The primary objective of this laboratory experiment was to acquaint oneself with Williamson Synthesis, focusing on the preparation of ethers through reactions involving alkyl halides and substituted phenoxide anions. Additionally, the lab aimed to provide hands-on experience with the synthesis process and illustrate the impact of a catalyst in a chemical reaction.

Williamson Synthesis, developed by Alexander Williamson, serves as a chemical method for synthesizing ethers. Ethers consist of two hydrocarbon chains or rings bridged together by an oxygen atom. The lab specifically centered on the synthesis of the alkyl aryl ether, propyl p-tolyl ether.

Understanding the mechanics of the Williamson reaction is crucial. The reaction involves introducing an alkoxide ion to an alkyl halide. The alkoxide ion, derived from the ion's conjugate acid (alcohol or phenol), acts as a nucleophile, reacting with the electrophilic propyl iodide. A challenge arises as propyl iodide is insoluble in water but soluble in organic solvents, while p-cresol exhibits the opposite solubility. Tetrabutylammonium bromide, a phase catalyst, resolves this issue by bridging the p-cresol anion into the organic phase, enabling the electrophile-nucleophile reaction.

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After the reaction within the organic layer, impurities persist. To extract propyl p-tolyl ether, a series of distillations are performed. Diethyl ether is added to the initial aqueous phase, creating another organic phase. Further treatments with NaOH and distillations enhance purity. Boiling the purified organic phase at (35-40)°C eliminates remaining impurities. Filtration through substances like removes excess and removes remaining polar impurities. A final heating at (60-65)°C ensures the removal of all impurities, resulting in the desired product, propyl p-tolyl ether.

Table 1:
Values Used in Determining Limiting Reagents

The reaction depicted in Figure 2 of Appendix A shows a mole ratio of "1:1:1:1:1." Initially, it might suggest that tetrabutylammonium bromide is the limiting reagent due to its lower mole quantity. However, being a catalyst, tetrabutylammonium bromide is present in equivalent amounts at the beginning and end of the reaction, acting as a "spectator ion." As these ions remain unchanged in the net ionic reaction, they cannot be considered limiting reagents. Consequently, p-Cresol, with the next lowest mole ratio and devoid of spectator ions, is identified as the limiting reagent.

Determining Percent Yield of Propyl p-Tolyl Ether:

The general equation for Percent Yield is expressed as:

Percent Yield=(Actual YieldTheoretical Yield)×100Percent Yield=(Theoretical YieldActual Yield​)×100

Therefore, the Percent Yield of Propyl p-Tolyl Ether is calculated using the above formula.

Analyzing the IR:

Upon reviewing the IR spectrograph in Appendix B, peaks are observed within the ranges of (3300 – 2800), (3100 – 3000), and (1150 – 1050). Key peaks include:

• 2963.54
• 2936.15
• 2876.94
• 1111.08
• 1068.64
• 1049.52

In the course of the experiment, various sources of error contributed to the deviation between the actual and theoretical yield, resulting in a percent yield of 67.84% instead of the expected 100%. Evaporation, especially during phase separations, pipetting, and filtration processes, could have led to product loss or impurities. The use of perifilm minimized but did not eliminate evaporation. Additionally, the filtration steps involving and might have caused product loss.

Despite these challenges, the IR spectrograph presented in Appendix B provided reassuring results. The peaks observed in the spectrum closely matched the expected peaks for propyl p-tolyl ether, indicating a high degree of purity in the resulting product. Although some loss occurred during the experiment, the product was deemed relatively pure based on the spectrograph.

In conclusion, the lab successfully achieved its objectives by providing hands-on experience with Williamson Synthesis, limiting reagents, and percent yield calculations. The similarity between the IR spectrograph of the product and that of propyl p-tolyl ether further confirmed the success of the synthesis, despite some procedural imperfections. Areas for improvement include careful handling during filtration and heating steps.

The absence of Gas Chromatography limited the ability to assess solution purity during the experiment, highlighting a potential enhancement for future iterations. Nevertheless, the lab effectively imparted practical knowledge and experience in Williamson Synthesis.

To ensure proper safety and environmental responsibility, the following waste disposal procedures were adhered to:

1. Equipment in contact with organic liquid was rinsed with acetone and drained into the "Organic Waste Disposal" receptacle. Subsequent rinsing with distilled water was performed.
2. Broken glassware was disposed of in the designated "Broken Glass" receptacle.
3. Solid waste materials were placed in the "Solid Waste" receptacle.
4. Gloves used during the experiment were washed with acetone and then rinsed with distilled water before disposal in the general waste basket.
5. Excess HCL and NaOH were disposed of in the "Acids and Bases" receptacle.
6. NaCl plates were cleaned with or (ensuring not to use ) for appropriate disposal.