Inorganic and Organic Chemistry – Preparation and Recrystallisation of Aspirin

The purpose of this experiment was for students to prepare some simple organic compound and to purify the compound by recrystallisation.

This experiment allowed the students to conduct the synthesis of aspirin, acquire the skills of recrystallisation using normal filtration and reinforce the technique of the melting point determination.

The method used to synthesize aspirin was esterification by reacting salicylic acid with acetic anhydride with the presence of dehydrating agent, conc. sulphuric acid, to speed up the reaction. Having obtained the crude aspirin which was still impure, recrystallisation was conducted to remove the impurities.

The aspirin harvested at the end of the experiment was 1.19g with the percentage yield of 37.8%. The melting point observed from the product ranged from 133°C to 137°C.

Introduction

Acetylsalicylic acid or commonly known as aspirin is an organic compound belong to ester group with condensed structural formula of CH3COOC6H4COOH.

(2) (2)

C6H4(OCOCH3)COOH 3D model of chemical structure of aspirin

Aspirin derived from salicylic acid is categorized as nonsteroidal anti-inflammatory drugs or NSAIDs. It is commonly used to reduce minor pain and aches (analgesic), lower fever (antipyretic), thin the blood (anticoagulation), reduce the inflammation such as rheumatism and arthritis (anti-inflammatory), enhances the elimination of uric acid (uricosuric) and reduce the risk of stroke, heart attack and other heart problems in long-term low-doses.

The related compound of aspirin from willow bark (Salix alba) has been used to relieve pain and reduce fever since hundred of years a go. The Chinese used the willow bark as a remedy in 500 BC while the record that willow bark could ease aches and pains and lower fever was written by Hippocrates, a Greek physician in 400 BC.

However, it was only until 1763 when the Reverend Edward Stone discovered the effectiveness of willow bark in reduce fever

In 1828, a French pharmacist, Henri Leroux isolated salicin, the active extract of the bark into the crystalline form and Raffaele Piria, an Italian chemist separated the salicin in its pure state. It was named salicylic acid (SA), as it was acidic.

In 1939, German researchers also isolated the compound from meadowsweet flowers. Despite being effective, it caused digestive problems and even death.

In 1897, Felix Hoffmann, an employee of Friedrich Bayer & Co in Germany synthesized a stable form of salicyclic acid namely aspirin which relieved his father’s rheumatism.

Bayer patented aspirin on 6 March 1899.

Theory

One of the reactions used to synthesized aspirin is esterification whereby an acid reacts with an alcohol to produce an ester with the presence of an acid catalyst commonly conc. sulphuric acid.

(3)

Esterification

Some esters are solids which mostly are insoluble in water as they have a high molecular weight or other properties. Crystallization is one of the method used to separate esters from the mixture.

The chemical equation

(4)

The symbolic equation

In this reaction, the -OH group from salicylic acid is substituted with -COOH group from acetic anhydride to form ester while the carboxylic acid group of salicylic acid remains unchanged. This reaction is slow because of the pure acetic anhydride involved; therefore, a catalyst, namely concentrated sulphuric acid is added to speed up the reaction.

Synthesis of aspirin involves two steps, which are isolation and recrystallisation.

Having filtered, cold water was used to wash the crude aspirin to isolate the most of the impurities leaving the crude product and pure solvent on the filter paper.

Then, the crude aspirin is purified by recrystallisation. The most important component in recrystallisation is the solvent used as it has to be able to dissolve the aspirin near its boiling point so that the crystals can be form during the cooling leaving the impurities dissolved. Sometimes, the best recrystallisation solvent is a mixture of two miscible solvents, one which dissolves the compound readily, the other which does not (5).

In this experiment a mixed solvent of ethanol and water was used as aspirin is very soluble in ethanol but quite insoluble in water (5).

Procedure

a) Preparation of Aspirin

i. 2.4 grams of salicylic acid was placed in a dry 100 ml conical flask.

ii. In the fumehood, 6 ml of acetic anhydride was added.

Safety Note: Acetic anhydride is a strong irritant and it is corrosive and volatile. Contact with skin should be avoided and the vapour should not be breathed

iii. Three or four drops of conc. sulphuric acid was added to the mixture and was swirled to mix

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

v. The flask was removed from water bath. While it is hot, about 1 ml of distilled water was cautiously added from a dropper to decompose the excess acetic anhydride.

vi. An additional of 40 ml of cold water was added to the mixture. The mixture was stirred and rubbed with stirring rod if necessary to induce crystallization.

vii. The product was collected by suction filtration and is washed with a little of cold water.

viii. The crude aspirin was recrystallised with ethanol/water.

b) Recrystallisation of Aspirin

The crude product aspirin prepared was relatively impure and might be purified by recrystallisation. A solvent convenient for this crystallization process was a mixture of ethanol and water.

i. The crude product was dissolved in approximately 5 ml of ethanol in a 100 ml conical flask. It is warmed on a hot plate if necessary

ii. Approximately 30 ml of hot water was added to the solution.

iii. If a solid separates out at this point, the solution is warmed until solid dissolves completely.

iv. The solution was cooled.

v. The crystals was filtered using suction filtration and is dried in the oven (100 °C) for 15-20 minutes.

vi. The recrystallised product was cooled in a desiccator for 5-10 minutes and was weighted.

vii. The theoretical yield of aspirin expected from 2.4 grams of salicylic acid was calculated. The percentage yield for the reaction was calculated by comparing the actual yield with the theoretical yield.

viii. The melting point of aspirin was determined by melting point meter.

ix. The aspirin waste was discarded into the container labeled ” Aspirin Waste “.

Result and Calculation

PREPARATION AND RECRYSTALLIATION OF ASPIRIN

Weight of salicylic acid 2.42 g

Weight of dry recrystallised aspirin 1.19 g

Percent yield

Mass of salicylic acid

Moles of salicylic acid used =

(mol wt of salicylic acid = 138) mol wt of salicylic acid

2.42 g = 138 = 0.0175 mol

Theoretical number of moles of aspirin = 0.0175 mol

From the stoichiometry of the equation above,1 mol of salicylic acid reacts with 1 mol of acetic anhydride to produce 1 mol of aspirin. Since 0.0175 mol of salicylic acid is used in this experiment, therefore the theoretical number of moles of aspirin should be 0.0175 mol as well.

Theoretical weight of aspirin = number of moles ´ the mol wt of aspirin

(mol wt = 180) = 0.0175 ´ 180 = 3.15 g

weight of aspirin obtained

Percent yield = ´ 100

theoretical weight of aspirin

1.19

= ´ 100

3.15

= 37.8 % yield

Melting point

Temperature range 133 – 135______ °C

Appearance

The aspirin crystals are white and shiny needles.

Discussion

Compound TheoreticalYield (g) Actual Yield(g) Percent Yield

Acetylsalicylicacid 2.42g 1.19g 37.8%

Table 1. The result obtained after recrystallisation of acetylsalicylic acid derived from salicylic acid.

Compound Observed m.p.(°C) Literature m.p.(°C)

AcetylsalicylicAcid 133-135 140

Table 2. The observed melting point of acetylsalicylic acid compared to its theoretical melting point.

Table 1 shows the amount of acetylsalicylic synthesized compared to the amount of salicylic used. Theoretically, the ratio of the number of moles of the acetylsalicylic acid and the salicylic acid is 1:1 according to the equation below.

Therefore, the weight of aspirin produced should be 3.15g.

Theoretical weight of aspirin = number of moles ´ the mol wt of aspirin

(mol wt = 180)

= 0.0175 ´ 180

= 3.15 g

According to Table 1, the acetylsalicylic produced was only 1.19g instead of 3.15g and the percentage yield was only 37.8%, which was quite low as the expected percentage yield obtained should be over 60%. This proved that many aspirin crystals formed were lost during the preparation and recrystallisation.

The possible causes during preparation and recrystallisation of aspirin could be firstly, after adding 1 ml of distilled water, the excess acetic anhydride was not decomposed completely which would affect the forming of aspirin crystals.

Then after adding additional of 40 ml of cool water and the flask was cooled down in an ice bath, the time used to cool the flask might not be long enough as resulting in the incomplete forming of crystals.

In addition, the ice bath used might not be cold enough therefore; the recrystallisation was probably not at the maximum rate.

After that during filtration, some of the crystals formed might redissolve and be filtered as a lot of cold water was poured in to clear the flask and the Büchner funnel. The chance of redissolving would be greater as the large amount of water was used.

Subsequently some of the crude product of suction filtration was left both in the surface of Büchner funnel and in spatula upon transferring the crystals to watch glass.

Last but not least, upon transferring the crystals out of the desiccator, the crystals might absorb moisture from the surrounding resulting in the error of the weight of the aspirin produced.

Table 2 shows the range of melting point of the acetylsalicylic acid observed compared to its theoretical melting point. A pure substance has one sharp melting point or liquefies within a narrow range. Whereas an impure substance melts over a range of temperature as the impurities cause the lowering and broadening of the temperature (1). The impure substances usually have lower melting point than the pure substance.

The range of melting point acetylsalicylic acid was 133-135°C while the pure acetylsalicylic has melting point of 140°C. This showed that there was some impurities of the crude product possibly were water, ethanoic anhydride, unreacted salicylic acid which appeared as white crystals mixed with the acetylsalicylic crystals as it was not soluble in the cold water, 2-hydroxybenzoic acid and polymerized 2-hydrobenzoic acid. The 2-hydrobenzoic acid could be the result of incomplete reaction or the hydrolysis of the product during its isolation (1).

Conclusion

The experiment’s result was not a satisfactory as the weight of acetylsalicylic obtained was only 1.19g from 2.42g of salicyclic acid used with the percentage yield of 37.8% which was quite low than the expected result due to lost of acetylsalicylic crystals. Whereas the lower range of melting point from 133-135 °C indicated that the product still contained impurities.

References:

1. Lewis, David, 1998.Aspirin: a curriculum resource for post-16 chemistry courses.1998.London: The Royal Society of Chemistry, Education Division.

2. Aspirin. [online].available from http://www.answers.com/aspirin [Accessed on 18 June 2006].

3. Schneider R.F., 2004.Synthesis of Aspirin. available from http://www.sinc.sunysb.edu/Class/che134/susb/susb028.pdf [Accessed on 19 June 2006].

4. The Chemical Heritage Foundation, 2001.Making Aspirin [online]. Available from: http://www.chemheritage.org/EducationalServices/pharm/tg/asp/asp31.htm [Accessed on 19 June 2006].

5. Experiment 26A: SYNTHESIS AND PURIFICATION OF ESTERS [online]. available from: www.chemistry.usna.edu/manual/Ex26A.pdf [Accessed on 23 June 2006].

6. Aspirin Synthesis. [online]. Available from: http://homepage.smc.edu/gallogly_ethan/files/Aspirin%20Synthesis.pdf

[Accessed on 23 June 2006].

7. Synthesis of Aspirin [online]. Available from: http://academic.bowdoin.edu/courses/f03/chem225/laboratory/images/aspirin.pdf [Accessed on 23 June 2006].