Murphy M P, Bond E. M. March 29th, 2016. Arizona State University.
Abstract:
The aim of this experiment was to successively synthesize a pure sample of aspirin through the chemical synthesis of salicylic acid and acetic anhydride. This was performed by way of crystallization and recrystallization. These techniques were chosen in order to first isolate the product produced in the chemical reaction, and then purify it to the furthest extent. Upon crystallization of the isolated product it was found to obtain a mass of 0.197 g and experience melting between 158-159 ͦ C. Also the percent yield for the isolated product when compared to the starting reactant was found to be 59%. The pure recrystallized product was found to obtain a mass of 0.085 g and experience melting between 158-159 ͦ C. When examining the percent yield between the pure sample and the starting salicylic acid it was found to be a bleak 26%. Also the IR spectra for this pure sample displayed a specific bond vibration for an alcohol group present at 3231.01 .Although the results for this experiment support the hypothesis that very little, if any salicylic acid and acetic anhydride were successfully able to produce aspirin, this experiment does provide the reader with a better understanding of chemical synthesis and how chemical products can be isolated and then purified using two useful techniques.
Introduction:
The aim of this experiment was to synthesize a pure sample acetylsalicylic acid, which is commonly known as aspirin. This was achieved by performing chemical synthesis, which breaks and forms the bonds of simple molecules in order to manufacture more complex chemical structures. Chemical synthesis has proven to be vital for field of chemistry because it allows researchers to create chemical structures that are not readily available in nature, and in large quantities.
In this experiment salicylic acid (reactant) was combined with acetic anhydride (reagent), and phosphoric acid (catalyst), with the end goal of producing acetylsalicylic acid. This reaction was chosen because it is capable of breaking the alcohol bond of the reactant and one the acetyl bonds of the reagent. Also this reaction causes the acetyl bond to form with the reactant to form aspirin, and the alcohol group to bond with the other remaining acetyl group of the acetic anhydride. It is important to note that all organic chemical reactions have an activation energy which must be overcome in order to break a bond. This is why the catalyst, phosphoric acid, was also added to solution. In addition to a catalyst, the solution was also heated to further increase the reaction rate. Upon completion of this reaction the solution was cooled in order to isolate the product. This method, which is commonly known as crystallization, was used because as the temperature of a solution decreases so does its solubility constant. This concept is relevant because only pure products will appear as crystals, while all of its impurities will remain in solution. Once the isolated product was obtained, it was then purified using recrystallization in order to remove any side-products, by products, unreacted reagents, or starting materials. This technique was implemented by adding hot ethanol in order to break the numerous hydrogen bonds present in the benzene ring. This product was then characterized with the use of infrared spectroscopy and melting point measurements in order to see if the synthesis of aspirin was successful.
The goal for this experiment is to successfully synthesize aspirin by performing a chemical reaction between salicylic acid and acetic anhydride. The synthesized product for this reaction is expected to have a melting point near 136 degrees Celsius, and lack a bond vibration that is specific to an alcohol group. Also percent yields will be taking in this experiment in order to evaluate the effectiveness of crystallization and recrystallization. Upon the compleition of this experiment one should have a better understanding of chemical synthesis, why it is important to organic chemistry, and the different techniques that can be used to isolate and purify a product obtained from a chemical reaction.
Experimental:
For this experiment 0.255 g of salicylic acid was obtained and a placed into a test tube with 0.5 mL of acetic anhydride. From this point five drops of 85% phosphoric acid were added to solution to act as a catalyst for the reaction. This solution was then placed into a heat bath of 80 degrees Celsius for thirty minutes. It is important to note that the test tube was shook occasionally to ensure that the reaction occurred to the fullest extent. Concluding the thirty minute heat bath, 1.5 mL of D.I. water was added to the solution and allowed to cool to room temperate. Once at room temperature the solution was submerged in an ice bath for 15 minutes so that crystallization of the crude product could from. Following the 15 minute ice bath the crystals which had formed in the test tube were removed and placed on to a Hirsch funnel. This was done in order to dry the crude sample, which was still wet from solution. Once the sample appeared to be dry, it was weighed using an electronic scale and analyzed by measuring its melting point. The melting point was measured by obtaining a small sample via capillary tube. The capillary tube was then placed inside the Digi melt and analyzed. Measures of sample weight and melting point were recorded for later use. Also a percent yield was computed using the moles of the crude sample and the starting reactant.
The remaining crude sample was then transferred into a Craig tube. Once in the Craig tube, a small sample of ethanol was boiled and added to solution. It is crucial that ethanol is added dropwise, and that no more is added once the solid product is dissolved. Adding more ethanol that necessary could be destructive to the purification of the sample. From this point the solution in the Craig tube was placed into an Erlenmeyer flask and allowed to cool to room temperature for 10 minutes. Once at room temperature the Erlenmeyer flask with the Craig tube inside was submerged in an ice bath for further purification. Concluding the 15 min ice bath, the Craig tube was centrifuged at 1600 rpm. This was performed in order to separate aspirin from the ethanol solution. Once separated, the pure aspirin was placed on to weigh paper so it could be fully dried. It is important to adequately let the aspirin sample dry because excess water will produce inaccurate results. Once fully dried, the sample was weighed using and electronic scale and analyzed by measuring its melting point. The melting point was taken in the same fashion as the pervious trial. Measures of sample weight and melting point were then recorded for later use. Also a percent yield was computed using the moles of the pure sample and the starting reactant. This value is a direct indicator of how effective the experiment was in synthesizing acetylsalicylic acid. In addition to a percent yield, an IR spectrum was taken of the pure sample by crushing the crystals in a fine powder. This powder was then placed onto the ATR crystal of the spectrometer and the pressure arm was set to 80. A scan was then run to analyze the bond frequencies present in the pure sample of acetylsalicylic acid.
Results:
Table 1: Chemicals in experiment and their associated mass and melting point values.
| Chemical(s) | Weight (g) | Melting Point ( ͦ C) |
| Starting salicylic acid | 0.255 | N.A |
| Isolated crude product | 0.197 | 158-159 |
| Isolated pure product | 0.085 | 158-159 |
Calculations:
- Conversion of mass to moles for chemicals.
0.255 g salicylic acid x = 1.85 x
moles salicylic acid
0.197 g crude product x = 1.09 x
moles crude product
0.085 g isolated pure product = 4.72 x
moles isolated pure product
- Percent Yield
× 100% = 59.0%
• The percent yield of the crude sample and starting reactant
× 100% = 43.3 %
• The percent yield of pure product and the crude product.
× 100% = 26.0 %
• The percent yield of pure sample and starting reactant.
Figure 1: An infrared spectrum of the pure sample of acetylsalicylic acid.
Discussion:
This experiment was performed in order to create acetylsalicylic acid through chemical synthesis. The chemical reaction that occurs when salicylic acid is added to acetic anhydride in the presence of a catalyst is depicted below.
In this particular reaction the alcohol group is removed from the reactant, while one the acetyl groups is removed from the reagent. Through the process of crystallization and recrystallization two new bonds are formed in order to generate acetylsalicylic acid and a byproduct. By viewing the reaction above it is easy to see that the acetyl group removed from the reagent is added to form aspirin, while the alcohol group removed from the reactant is added to from a by-product.
This chemical reaction was performed by taking a sample of salicylic acid (0.255 g) and acetic anhydride (0.5 mL), which were then combined in solution. This reaction also used the presence of a catalyst, phosphoric acid (5 drops), and heat to increase the reaction rate (heat bath 80 ͦ C). Upon the completion of this reaction the product was then isolated through the process of crystallization. This technique was of great importance because it allowed the crude product to crystallize, and leave its impurities in solution. Once the crude sample had crystallized it was filtered through a Hirsch funnel to properly dry. After the crude sample had been dried it was reported to have a mass of 0.197 g and a melting point between 158-159 ͦ C. These results were concerning because not only was the percent yield found to be 59% when comparing the crude sample to the starting salicylic acid, but also the melting point matched that of salicylic acid. It would have been ideal for the melting point to fall somewhere around 135 ͦ C, because that is the reported melting point of aspirin. Following this step the crude sample was purified through recrystallization. This was accomplished by adding boiling ethanol to the crystal sample in order to break the many hydrogen bonds associated with the benzene ring. From this point the solution was cooled and centrifuged in order to separate the pure aspirin sample from the ethanol. Once separated, the pure sample was allowed to dry it and the mass and melting point were reported to be 0.085 g and 158-159 ͦ C respectively. Once again these results were problematic because not only was the percent yield found to be 26% when comparing the pure product to the starting salicylic acid, but also the melting point matched that of salicylic acid. Also upon review of the IR spectra for aspirin, it was apparent that an alcohol bond vibration was visible at 3231.01. The presence of this bond vibration raised suspicions because this bond type is not present in pure aspirin.
From the results obtained it would appear that very little, if any of the salicylic acid and acetic anhydride used in this experiment was successively synthesized to form acetylsalicylic acid. Upon review it could be argued that this chemical reaction did not successfully occur in the first process of crystallization. However, upon the completion of this experiment one should have a better understanding of what chemical synthesis is and how important it is to the field of chemistry. Also this experiment provides a detailed explanation of how chemicals can be isolated and then purified through the process of crystallization and recrystallization.
Conclusion:
The aim of this experiment was to generate a more complex chemical structure, acetylsalicylic acid, by way of chemical synthesis. This accomplished by isolating a crude sample of aspirin through crystallization, and then purifying it by way or recrystallization. These techniques were then evaluated on their effectiveness by the examination of their melting points, IR spectra, and percent yield.
Upon review of the data produced in this experiment it was confirmed that salicylic was not synthesized to form acetylsalicylic acid. This hypothesis is supported because the melting point measured for the pure sample was related to that of salicylic acid, and the IR spectra produced showed a bond vibration characteristic of an alcohol group. In addition the percent yield of the pure sample of aspirin was found to be a mere 26%.
References:
1) Arizona State University (2016). Organic Chemistry, Laboratory Manual and Techniques. Tempe, AZ: Dr. Brian Woodrum