The synthesis of Acetaminophen is based on the amine group of p-aminophenol being acetylated by acetic anhydride to form an amide functional group. Acetaminophen is isolated as a crude solid which is then recrystallized to purify the product. Using melting point determination the recovered products purity is identified and can be compared to the theoretical value.
In this experiment 3.0g of p-aminophenol along with approximately 10ml of deionized water will be heated in a water bath regulated at 65-75. After heating for 5 minutes 4.0mL of pure acetic anhydride will be added to dissolve the p-aminophenol. Following this there will be continuous heating in the regulated water bath to ensure all p-aminophenol dissolves. Gradual cooling will lower the mixtures temperature to room temperature followed by further cooling in an ice-water bath. The mixture will begin to precipitate out a solution.
Using vacuum filtration the crude product is collected and with a sample from the crude solid we can determine a melting point. With the remaining solid we undergo recrystallization by adding deionized water into an Erlenmeyer flask and repeat the heating process. Once the crude solid dissolves the mixture is cooled again to room temperature and then in an ice-water bath. Again with vacuum filtration the solid can be extracted and another melting point can be determined of the purified sample. Measuring the mass of the collected sample will provide an approximation of the product yield.
In this experiment we will be synthesizing Acetaminophen and comparing the melting point of the crude solid and purified solid against a theoretical value obtained from the CRC Handbook. This will enable us to determine the purity of the crude solid.
The experimental procedure used for this experiment was outlined in the CHEM123L lab manual, Experiment #1. All steps were followed without deviation. (Stathopulos, 2015)
The initial observation is of the solid p-aminophenol, white fine-grained powder, and with the addition of water forms a cloudy white solution. When dissolution occurs following the addition of acetic acid a clearer mixture can be observed. Stirring and heating forms a gelatinous precipitate and you can feel the solution thicken whilst stirring. The precipitated solution undergoes vacuum filtration where a white solid like substance remains. During the recrystallization process, the white powder is again dissolved in water according to the mass obtained and until a clear solution is attained, the solution is heated. The crystallization process is slow however crystal solids can be seen during precipitation. The filtered end product is a fine grained powder again and a melting temperature of 165 can be observed.
*CRC Handbook of Chemistry and Physics
Results and Calculations
The theoretical value for the melting point of Acetaminophen in accordance to the CRC Handbook of Chemistry and Physics provides a value of 170 whereas the value we observed was 165. The pure value is relatively close to the theoretical value and this helps identify the product as acetaminophen.
However, the yield for this experiment was significantly less than the theoretical value with percent yield of 27.39%. Although all experimental procedures were followed without deviation, the time constrained on the experiment could be a possible source of error. The hot-plate warmed the water bath to 65 over a significantly long time resulting in less time to observe the reaction. In order to complete the lab within the time allowed, the heating and cooling stages were compromised resulting in less time for precipitation and recrystallization. This may have resulted in an incomplete reaction forcing the yield to significantly decrease.
Another source of error could have occurred in the transfer of samples from one vessel to another. A more thorough practice of quantitative transfer of samples could be conducted to increase the yield and prevent the loss of sample during transfer. The crude solid having a significantly lower melting point also indicates a high amount of impurities in the sample that could also have resulted in the low yield observed at the end of the experiment.
In terms of our purpose of identifying the product as acetaminophen, the melting point determination encourages our confidence in supporting the theory. The crude product obtained a value of 94 in the MelTemp apparatus indicating a high amount of impurities however; the final pure product is similar to the theoretical melting point.
What could happen if 5 mL of water was added for every 1g of crude product?
What could happen if 15 mL of water was added for every 1g of crude product?
Cold solvent was used to aid in the transfer of recrystallized product. What could happen if room temperature solvent was used?
The recrystallized product has been undisturbed and remained in an ice-water bath to encourage the precipitation process. Adding room temperature water will reverse the process and initiate dissolution of the product. This will reduce the yield obtained.
What is the purpose of scratching the inner wall of the round bottom flask? The precipitation process is encouraged when the inner walls of the round bottom are scratched.
The purpose of the experiment was to identify the melting temperature of pure acetaminophen after undergoing synthesis. The experimental melting point of 165 is very close to the theoretical value of 170, which supports the identification of the acetaminophen. The crude solid had a significantly lower melting temperature with an experimental value of 94.
As the percent yield was 27.39% the low crude solid temperature could support the high impurity level in the sample. During the recrystallization the impure sample could have been extracted resulting in a low yield. Also the time limitation could have reduced the amount of precipitation during the experiment.
The experiment allowed us to compare theoretical and experimental values of acetaminophen and determine the product was acetaminophen. In terms of identifying the product, the MelTemp assisted in determining the melting point which helped verify the product. However, improving the technique and surpassing time limitation would have provided a more accurate and higher percent yield experiment.
Stathopulos, Sue. CHEM123L Laboratory Manual. Winter 2015 ed. Waterloo: University of Waterloo, Department of Chemistry, 2014. Print.
David R. Lide, ed., CRC Handbook of Chemistry and Physics, Internet Version 2005, , CRC Press, Boca Raton, FL, 2005.