Preparation and Crystallization of Aspirin

CHOONG SHING YI

CHOONG SHING YI

Preparation and Recrystallization of Aspirin
Preparation and Recrystallization of Aspirin

Date of the experiment: 9 May 2013
Date of submission: 23 May 2012
Date of the experiment: 9 May 2013
Date of submission: 23 May 2012

Synopsis

1. General Objective: To familiarize with the preparation of some simple organic compound and purify the compound by recrystallization.

Specific Objective:

The experiment is carried out to:

1.1 Conduct the synthesis of aspirin 1.2 Reinforce the skills of recrystallization 1.3 Reinforce the technique of melting point determination

2. Procedure:

2.1 Preparation of Aspirin 2.2 Recrystallization of Aspirin 2.3 1.4Melting Point Determination of Aspirin

3. Results:

3.1 Mass of dried, recrystallized aspirin: 1.62g 3.2 Percent yield: 51.75% yield 3.3 Temperature range: 138.2 – 140.0°C 3.4 Appearance: Fine needle like white crystal

Introduction

Aspirin or acetylsalicylic acid is acquired from salicylic acid, which is mild and nonnarcotic analgesic. It helps to remove the substance in the body that trigger pain, fever and inflammation.

The father of modern medicine was Hippocrates, who lived sometime between 460 B.C and 377 B.C. Hippocrates was left historical records of pain relief treatments, including the use of powder made from the bark and leaves of the willow tree to help heal headaches, pains and fever. By 1829, scientists discovered that it was the compound called salicin in willow plants, which gave you the pain relief.

The scientists extracted salicin successfully and pure salicylic acid was obtained. However, salicylic acid caused the upsetting of stomachs. Frederic Gerhardt, a French chemist solved this problem by neutralizing it with sodium salicylate and acetyl chloride, creating acetylsalicylic acid (Mary Bellis, inventors – about.com).

Figure 1.1 Willow Tree Figure 1.2 1899 Aspirin Bottle First Aspirin Bottle

Nowadays, Aspirin is still used to treat mild to moderate pain, inflammation and fever. However, side effects such as upset stomach, heartburn, drowsiness and headache may still occur. Also, overdosing may have symptoms like ringing the ears, headaches, nauseas, vomiting, dizziness, confusion, hallucination, rapid breathing, fever, seizure (convulsion) and coma. One is advised to seek medical treatment if symptoms like these occurred.

Theory

Alcohols can combine with carboxylic acids under acidic conditions to form functional groups called Esters.

There are some esters, which are solids because of their high molecular weights or other properties. As most of these esters are not soluble in water, they can be separated from the mixture by crystallization. This experiment deals with an ester of this sort, the substance commonly called Aspirin. Aspirin (acetylsalicylic acid) is the active component in headache pills and is one of the most effective, relatively non-toxic, painkillers.

Aspirin can be made by reacting salicylic acid with acetic anhydride:

The anhydride is split half form aspirin and the other half form acetic acid. The severe conditions (concentrated H2SO4 and heating) are carried out to speed the reaction. After that, the aspirin must be freed from the solvent (water), acetic acid, and unreacted acetic anhydride. Because the solubility of aspirin decreases with temperature, purification can be accomplished by cooling the reaction mixture, to initiate the formation of crystals.

To enhance crystallization, the sides and bottom of the flask are scratched to provide a microscopically rough surface where the molecules can adhere to and build up as crystals. The subsequent filtration of these crystals removes the acetic acid and any unreacted acetic anhydride. Any remaining contaminants are removed by recrystallization in a solvent in which aspirin is sparingly soluble.
As Aspirin (crystals) are separated through suction filtration, leaving impurities behind. This process should give a relatively clean product, whose purity can be determined by melting point analysis. A sharp melting point -when all the crystals melt over a 1 - 2 degrees temperature range signifies purity.

Procedure

1. Preparation of Aspirin

1.1 Approximate of 2.4 g of salicylic acid was weighed in a dry 100ml conical flask. The actual weight was recorded.

1.2 In the fume hood, 6ml of acetic anhydride was added to the salicylic acid in the flask.

1.3 3-4 drops of conc. Sulfuric acid was added to this mixture, swirled to mix.

1.4 The mixture was heated in the water bath for 10 to 15 minutes to complete the reaction.

1.5 The flask was removed from the water bath. About 1 ml of distilled water from dropper was added while it is still hot to decompose the excess acetic anhydride.

1.6 An additional 40 ml of cold water was added. A stirring rod was used to stir and rub the mixture to induce crystallization.

1.7 The crude product was collected by suction filtration and was washed with a little cold water.

2. Recrystallization of Aspirin

The crude product of aspiring prepared is relatively impure and may be purified by recrystallization. A solvent suitable for this recrystallization process is a mixture of ethanol and water.

2.1 The crude product was dissolved in approximately 5 mL of ethanol in a 100 mL conical flask and was being warmed on a hot plate.

2.2 Approximate of 30 mL of hot distilled water was added to the solution.

2.3 The solution was being warmed until the solid dissolved completely.

2.4 The solution was allowed to cool.

2.5 A clean, dry watch glass together with a filter paper was weighed and the weight was recorded.

2.6 The weighed filter paper was used to obtain the recrystallized product by suction filtration.

2.7 The crystals and filter paper was transferred onto the weighed watch glass. Then, the crystal was dried in the oven (100°C) for 15-20 minutes.

2.8 The crystals, filter paper and watch glass was placed in a desiccator for 5 -10 minutes.

2.9 The dried crystal was weighed together with the filter paper and watch glass. The weight was recorded. The weight of dried, recrystallized aspirin was calculated.

2.10 The expected yield of aspirin from the amount of salicylic acid that was used was calculated. The percentage yield of dried, recrystallized aspirin was also calculated.

3. Melting Point Determination of Aspirin

3.1 The melting point of aspirin was determined.

4. The aspirin waste was discarded in the ‘Aspirin Waste’ container.

Results and Calculation

Mass

Mass of salicylic acid (a) = 2.4g

Mass of filter paper & watch glass (b) = 37.23g

Mass of dried, recrystallized aspirin, filter paper & watch glass (c) = 38.85g

Mass of dried, recrystallized aspirin (d) = (c) – (b) = 38.85 -37.23 = 1.62g

Percent yield

Number of moles of salicylic acid used (e) = 0.0174mol
(mol wt of salicylic acid = 138)

Expected number of moles of aspirin (f) = 0.0174mol

Expected mass of aspirin (g) = 3.130g
(mol wt = 180)

Percent yield = (d)(g) x 100%

= 1.6203.130 x 100%

= 51.76%

Melting point

Temperature range 138.2 – 140.0 °C

Appearance

White, fine-needle like crystal.

Discussion

Preparation of Aspirin

The first part of the experiment was the preparation of Acetylsalicylic Acid (Aspirin). The first step in the synthesis is the mixing of salicylic acid and acetic anhydride as starting materials and addition of sulfuric acid as a catalyst and it promotes reaction to be carried out at lower temperature (Fleck, 1955). A white, milky mixture was then obtained.

Then the materials were heated in the water bath. This is so because heat will facilitate and smooth the progress of reaction between the starting materials. Heating the materials also prevents conversion of salicylic acid again and ensures that all of the starting materials would be consumed (Paula Bote, 2013).

Following with the additional of water. The additional of water was added to remove the excess acetic acid. Then 40 mL of cold water was added into the solution (acetylsalicylic acid is insoluble in cold water), and the solution was being stirred and rubbed to induce crystallization. The adding of cold water is to purify the solution, as there might still be salicylic acid, acetic anhydride, acetic acid product and sulfuric acid that did not react. Then, suction filtration was carried out to obtain the crystal.

Recrystallization of Aspirin

The second part of the experiment was recrystallization. 5 mL of ethanol was added to remove impurities that are still present in the crude sample. The crude sample was dissolved on hot distilled water, and warmed until the solid dissolves completely. While the solution is allowed to cool, crystals started to form. Then, suction filtration was carried out again to obtain the crystals and remove the impurities.

The crystals, together with the filter paper were transferred to the watch glass and being dried in the oven (100°C) for 15-20 minutes to evaporate the water. The crystals, filter paper and watch glass were placed in the desiccator for 5-10 minutes afterwards, allowing it to dry.

The greatest source of error during the procedure is when the reactants and products are transferred between containers multiple times. For example, procedure 1.7, while transferring the solution into the suction filter flask, there might still be some crystal remained in the conical flask. The lost of the crystals might affect the results of the experiment.

Another source of error may have occurred during the weighing of the crystals. Perhaps, the crystals were not weighed properly or it was still wet when the weighing session is carried out.

My partner and I were managed to produce a percent yield of 51.75% yield. The aspirin produced has a melting point that range of 138.2 to 140.0°C. Compared to the expected melting point of 140.0°C, also, taking into the account that the range of the melting point is very narrow, these results suggests that there are only a small amount of impurities such as water in the product. The dried, recrystallized aspirin that we managed to obtain is in fine needle like structure and white in color, which is the same as the expected color of aspirin.

Conclusion

The crystals obtained from the experiment has a percent yield of 51.75% and a melting point range from 138.2 to 140.0°C. In order to achieve a pure sample, we will need more time to carry out this experiment. As we did not have enough time to dry the crystal during this experiment, it might be the reason for the remaining of impurities in the crystals.

References

1. Mary Bellis. History of Aspirin [online]. Available from: http://inventors.about.com/library/inventors/blaspirin.htm
[Accessed 12 May 2013].

2. Trees: A Vital Resource To Be Treasured [online]. Available from: http://yellowmagpie.com/about-trees-a-vital-resource-to-be-treasured/
[Accessed 12 May 2013].

3. Aspirin [online]. Available from: http://www.drugs.com/aspirin.html
[Accessed 12 May 2013]

4. Aspirin Synthesis, HPLC Analysis [online]. Available from: http://www.columbia.edu/itc/chemistry/chem-c2507/navbar/Aspirin_2002.doc‎ [Accessed 12 May 2013]

5. Fleck, H.R. (1955). Synthetic drugs: A handbook for chemistry, physician and pharmacists. London: The Aberdeen University Press Ltd., pp. 28-30.

6. Paula Bote, Synthesis of Aspirin [online]. Available from: http://www.scribd.com/doc/131798608/CHEM-40-1-Exercise-11-Synthesis-of-Aspirin-Lab-Report [Accessed 12 May 2013].

7. Sample Report. Department of Chemistry. Los Angeles: Harbor College.
Available from: http://www.chem.earlessdog.com/
[Accessed 12 May 2013]