IV. DATA
Table 1. Sample description of salicylic acid, acetic anhydride and phosphoric acid SAMPLE
OBSERVATIONS
salicylic acid
white crystalline powder
acetic anhydride
solid white powder-like granules
phosphoric acid
clear colorless liquid with pungent odor
Table 2. Observations on the synthesis of Aspirin form Salicylic Acid. Observations After addition of acetic anhydride
White solution with white solids
After addition of phosphoric acid
White solution with white solids
After swirling and hot water bath
Colorless solution with white solids
After addition of three 1mL water
Cloudy solution (white)
Addition of 40mL cold water
Formation of white solids and clear solution
After placing on ice bath
Formation of white solids and clear solution
Crystals of filtration
White crystals
Filtrate of filtration
Clear solution
After washing with water
White crystals residue with clear filtrate
Table 3. Calculation of Percentage Yield Parameters weight of salicylic acid (g)
1.00
volume of acetic anhydride (mL)
6.00
volume of phosphoric acid (mL)
3 drops
weight of watch glass + weight of filter paper + product (g)
45.14
weight of empty watch glass (g)
43.64
weight of dry filter paper (g)
0.24
weight of crude product (g)
1.26
% yield of crude product:
%
Table 4. Observations of recrystallization of crude aspirin SAMPLE
OBSERVATIONS
Mixture of crude sample and solvent
clear solution
Suction filtration
a.) Filtrate
clear solution
b.) residue
white powder-like solid
Air-dried crystals
white long crystals
Table 5. Recovery data on recrystallization of aspirin Parameter Weight of container + sample (g)
Crude sample
Recrystallized sample
45.14
44.68
Weight of container (g)
43.64
43.64
Weight of filter paper (g)
0.24
0.24
Weight of sample (g) % recovery (B/A x 100):
1.26
(A)
0.80
(B)
%
Table 6. Melting point data on aspirin Parameter
Crude sample
Purified sample
Temp. when melting starts (°C)
124 °C
125 °C
Temp. when melting complete (°C)
127 °C
126 °C
Melting point range
130 °C
127 °C
Melting point of pure sample (literature value):
°C
Table 7. Solubility of starting materials and products in water SUBSTANCES
OBSERVATIONS
+/-
acetic anhydride
soluble
+
salicylic acid
insoluble
-
synthesized aspirin
insoluble
-
commercial aspirin
insoluble
-
Table 7. Ferric chloride test Description of ferric chloride: yellow-orange liquid solution SUBSTANCES acetic anhydride
OBSERVATIONS
+/-
underwent a vigorous reaction; hot; yellow-
-
orange liquid
salicylic acid
formation of black-violet complex
+
aspirin
formation of black-violet complex
+
Table 8. Acidic potassium permanganate test Description of potassium permanganate: deep purple-colored solution SUBSTANCES
OBSERVATIONS
+/-
acetic anhydride
Purple solution
-
Dark orange to black liquid with solid substance
+
salicylic acid
on the upper part
aspirin
Black solution
-
OBSERVATIONS
+/-
yellow-orange liquid with white precipitate at the
-
Table 9. Starch test Description of I2/KI: brown-black solution SUBSTANCES synthesized aspirin
commercial aspirin
bottom yellow-orange liquid with black precipitate at the bottom
+
VI. Results and Discussion Aspirin can be made by using a process called esterification. Esterification occurs when a carboxylic acid and an alcohol combine in a reaction to produce an ester. This reaction can be used to synthesize aspirin from salicylic acid. In the lab, the carboxylic acid alcohol mixture is heated in the presence of a catalyst. During the reaction process, a molecule of water splits off and the remaining carboxylic acid and alcohol fragments become attached producing an ester. The reaction that is used for the synthesis is shown below. In this reaction, an excess of acetic anhydride (C4H6O3) is added to a measured mass of salicylic acid (C7H6O3). The mixture is heated to form the acetylsalicylic acid (C9H8O4) and acetic acid (C2H4O2).
Based on Table 2, salicylic acid, a white crystalline powder, was used to synthesize aspirin. Phosphoric acid, a clear colorless liquid with pungent odor, was used as a catalyst or the substance which makes the reaction rate faster. Acetic anhydride was used as a solvent rather than the acetic acid because it contains a better leaving group, the acetoxy group (-O2CCH3), than the hydroxyl group (OH-) of the latter. Upon the addition of acetic anhydride and 85% of phosphoric acid in the salicylic acid, there was a formation of white solids in the cloudy solution. Aside from being a catalyst, phosphoric acid also donates H which binds to the reaction complex. H is generated by the end of the reaction. The following reaction shows the nucleophilic acyl substitution in the esterification of aspirin in which an acid-base reaction takes place. This allows the resulting intermediate product to be susceptible to nucleophilic attacks.
Salicylic attacks the carbon where positively charged oxygen is attached to via the oxygen from its hydroxyl group.
Rearrangement of positive charge occurs since the intermediate formed is not stable.
A reaction via elimination proceeds due to further rearrangement that leads the acetic acid to leave the intermediate.
Lastly, the resulting reaction intermediate donates a proton to dihydrogen phosphate hence, forming acetylsalicylic acid or aspirin.
The solution was then subjected to hot water bath to maximize solubility. Furthermore, heat enhances the reaction since it is an exothermic reaction, which ultimately favors the formation of products. As the reaction proceeds, the solid salicylic acid disappears and the acetylsalicylic acid product remains dissolved in the hot solution. Once the entire solid has disappeared, the reaction is complete. At this point, the excess acetic anhydride must be hydrolyzed to acetic acid. Acetic anhydride is very reactive toward water, so the hydrolysis has been done slowly – water was added dropwise. The excess acetic acid will be quenched with the addition of water. The aspirin product is not very soluble in water so the aspirin product will precipitate when water is added.
Ice cold water is then added and the flask is placed in an ice bath to lower the solubility and precipitate the acetylsalicylic acid product.
The aspirin crystals are collected on a Buchner funnel and washed with additional ice cold distilled water. Ice cold distilled water was used instead of room temperature distilled water because aspirin is insoluble in cold water and to avoid dissolving any of the aspirin products. The acetic acid and phosphoric acid are water soluble, in any temperature water, and can be removed by washing the aspirin with the chilled water. Salicylic acid is only slightly soluble in water and any unreacted salicylic acid cannot be removed completely in the washing process. The first set of crystals will be considered as the crude product. Using this, percentage yield was determined – 96.92%. The computed crude product yield is 1.26 g which is lower than the theoretical yield, 1.30 g. This error could be due to weighing of product when it is still wet. However, instead of having a higher yield due to presence of water, the crude is actually lower which means there were already errors from the start of the experiment. This incident might be a result of presence of impurities in the compound used, improper conducting of techniques and human errors – being reckless or not cautious. Once the final product (aspirin crystals) is purified it was allowed to dry. It was then weighed and tested for purity. Final purification is accomplished through recrystallization. The crude product was dissolved in water and subjected to heat; obtaining a supersaturated solution. Then, the solution was left to cool at room temperature before putting in a cold bath. Finally, unwanted substances mainly impurities were eliminated using cold water via suction filtration. Hot gravity filtration is not recommended since esters hydrolyze in boiling water under acidic conditions. And if we use hot water, no crystal/s will be extracted since high temperature will inhibit the crystals to form. Afterwards, final product was weighed and percent recovery was also computed – 63.49%. The mass of the recrystallized sample (0.8 g) is relatively higher than the crude product (1.26 g). The following factors may have influenced the result product: (a) amount of water used to dissolve the crude product, (b) failure to eliminate impurities/unwanted substances; and (c) weighing of wet recrystallized sample. These factors all contribute to low purity of the product. Melting point is a physical property inherent to a substance’s identity. The purer the substance, the smaller the melting range is. The broader range of an impure sample results from the creation of a solution upon melting. A mixture’s melting point is a colligative property which means it is dependent on the ratio of the chemicals present. For this experiment, we can assume that the acetylsalicylic acid is the solvent and the impurities as the solute. The more impurities present, the lower the melting point. The melting point range of pure aspirin is 138-140 °C and the melting point range of the salicylic acid starting material is 158-161 °C. Based on table 6, the melting point range of crude sample is 150-157 degrees Celsius while the recrystallized sample ranges from 151 to 154 degrees Celsius. If the product is not pure, it will acquire lower melting point relative to the literature value. The melting point range of the recrystallized sample that we have obtained is much higher than the literature value thus; the final product is not aspirin. If we compare the melting point ranges, we can say that the product is salicylic acid. Characterization tests such as solubility in water, FeCl3 test, KMnO4 test and Iodine test were conducted to differentiate the product form the starting materials. Due to the presence of large
non-polar group, the starting materials and the synthesized product as well as the commercial are all insoluble. However, acetic anhydride must give a positive result and react with water as shown in the reaction below.
When FeCl3 was used to test the recrystallized product’s purity, the product formed a purple complex which means it is not pure since Iron (III) ion reacts with phenols. Salicylic acid contains phenols while acetylsalicylic acid does not. .
FeCl3
Cl-
Fe
2H+
2Cl-
A test using Potassium permanganate will give a brown precipitate as a positive result. It is used to detect the presence of salicylic acid since it reacts with the former to produce a quinone and MnO2 (brown precipitate). As observed in table 8, final product gave a positive result hence, it is not a pure aspirin and it contains salicylic acid.
Lastly, commercial aspirin and synthesized aspirin were differentiated using I2/KI test which detects the presence of starch in the sample. Commercial aspirin reacts with the reagent since starch serves as binder for its tablet. Positive reaction will form a blue intense complex as shown below. Based on table 9, the synthesized aspirin gave a negative result. I- + I2 + starch
(starch, I-, I2)
VII. SUMMARY AND CONCLUSION Organic synthesis is a process of converting a starting material to a certain product through various reactions and observations. To understand this, an experiment was conducted – synthesis of Aspirin. Synthesis of aspirin was produced using esterification of salicylic acid in
acetic anhydride in the presence of an acid catalyst, which is 85% phosphoric acid and heat by means of nucleophilic acyl substitution. After heating, water was added to hydrolyze excess acetic anhydride into acetic acid. Cold bath and suction filtration proceeded to extract crystals that will serve as crude product. Percentage yield was determined and the value computed was 96.92%. This means that the mass of crude product (1.26 g) is lower than the theoretical yield (1.30 g). The crude product was used to synthesize aspirin through the use of the same procedure and set up. The recrystallized sample has relatively lower weight (0.8 g) than the crude sample giving a 63.49 % recovery. Melting point of both samples was determined to identify them and to classify their purity. Results showed that the crude sample has 150-157 degrees Celsius range of melting point while the recrystallized sample has 151-154 degrees Celsius range. The literature value of a pure aspirin is 135 degree Celsius and if the sample has lower melting point, then it contains impurities. However, our samples have relatively higher ranges thus; the final product was not aspirin and identified to be salicylic acid. It is so since the melting point of salicylic acid ranges from 158-161 degrees Celsius. This proves that certain errors were committed during the experiment. To characterize the products, different tests were also conducted. When solubility in water was conducted, result showed that the starting materials and the products were insoluble however acetic anhydride gave a positive result. Ferric chloride test which gives a positive result of forming purple complex was also done. Since Iron (III) reacts with phenols which can be found in salicylic acid, the recrystallized product gave a positive result. Potassium permanganate test was also used for the presence of salicylic acid. The product showed a positive result which is forming a brown precipitate and a quinone. The last test is for the commercial aspirin and the synthesized aspirin. Iodoform test was used to detect the presence of starch which gives a blue intense complex as a positive result. Since starch is used as a binder for a tablet, the commercial aspirin reacted. Based on the result, we can conclude that we fail to synthesize aspirin. This is mainly due to weighing method, time of drying and uncompleted filtration.
VIII. REFERENCES
Baum S., Bowen W., Poulter S., Laboratory Exercises in Organic & Biological Chemistry, 2nd Ed. Esterification. Macmillan Publishing Co.:New York [2] Bettelheim F., Brown W., March L., Introduction to Organic and Biochemmistry. 5th Ed. From Willow Bark to Aspirin-And Beyond. pg 214. Thomson Brooks/Cole: Singapore th [3] Beran J., Lab Manual for Principles of General Chemistry, 7 Ed. Experiment 28, pg 323. th [4] McMurry, J., Organic Chemistry, 7 Ed. Esterification. pg 317-318, 323. [1]
[5]
Rowland et al., Organic Chemistry Laboratory Manual: Susquehanna University. Thomson Learning: Ohio
IX. RECOMMENDATIONS Future experimenters could take steps to better the yield by running the reaction for longer than 15 minutes to encourage more product formation, or by more carefully rinsing the flask when transferring crystals. Also, some product may have been lost when transferring thus; I’d suggest being more cautious since the sample was transferred to different medium many times. Cleanliness of the glass wares must also be enhanced to avoid impurities and unwanted substances. We should add a lot more time on the experiment especially on the drying and filtration part. The drying of the filter paper and the melting point determination is not done properly because of the insufficient time allotted.
