The resulting white crystals are then mixed with a small amount of aqueous methylated spirit (1:1 volume) and heated to be dissolved completely. It is then left to recrystalise slowly using an ice bath. The solution and crystals are then filtered in a Buchner flask and the remaining crystals in the filter paper is removed and heated in a vacuum oven to be dried at 40° C.
The resulting crystals (Phenacetin drug):
1. Are weighed to calculated the percentage yield
2. A sample is taken and put in an Infrared spectrum to determine the structure of the product
3. A sample is taken using a capillary tube, to find the melting point using a device
Results:
Yield:
Ethyl Iodide: Mr = 155.97 m = 5.6
Moles
Para-acetamidophenol: Mr = 151 w = 3.80 g
Moles
0.025 x 179.22 = 4.4805 Theoretical yield
≅ 75% percentage yield
Melting point:
Discussion:
Yield:
As shown above the calculation shows that 74.86% of yield is obtained in the experiment. this result shows that the experiment is fairly precise. There have been a few inefficiencies that have been the cause of the remaining 25.14% error. One of the main sources of the error in the experiment is the human error. This may be:
- spilling
- wrong method of cooling to recrystalise
- not dissolving well
- missing crystals when removing from the filter
-
Weighing inefficiently etc...
One of the reasons of causing errors whilst recrystalising is losing the product either in the filter paper or not cooling the solution efficiently to obtain the maximum amount of yield.
Other than that, some of the yield may have been lost during the process if transferring crystals and solution into the filter. It is very difficult to be sure that all of the crystals are transferred even if it is washed into the filter with cold water.
Melting point:
The obtained melting point of Phenacetin from the experiment (135°C) falls exactly in the middle of the literature value range (134° C- 136° C). This may show that, however the product yield was reduced to 74.86%, but the resulting crystals are fairly pure and there is not much impurities to cause an error in melting point.
This might have been the result of weighing ingredient accurately to avoid excess of reagents and etc...
Infrared spectra:
As shown on the graph the main three groups are found on the graph. As found on the
Type of action:
The overall reaction is called the Williamson Synthesis. It’s formation of ether by reaction of an alkyl halide with the conjugate base of an alcohol. This reaction is a SN2 substitution reaction which only happens when a primary alcohol is the nucleophile. This substitution reaction involves Para-acetamidophenol nucleophile, attacking the electrophilic carbon atom of a primary alkyl halide which is Ethyl Iodide in this case and as a result, the reaction produces an ether which is a drug called Phenacetin and a halide ion. This mechanism is shown below:
Reference: Two methods for the synthesis of Phenacetin, by chemical Education Resources, prepared by Manion, University of Central Arkansas (1999), Palmyra, Pennsylvania 17078.
Conclusion:
In summary, the percentage yield obtained was 75% which was a reasonable amount considering the limitations of the experiment. The melting point range complies with the literature value which determines the purity of the obtained Phenacetin.