Preparation and Recrystallisation of Aspirin

1.Synopsis

The objective of this experiment is to learn to re-crystallise aspirin and to prepare aspirin.

To find out the purity of aspirin obtain by recrystallisation of prepared aspirin from reaction of salicylic acid with acetic anhydride. Purity can be determined by identify the melting point of aspirin. In this experiment, we get a range of 135.8°C – 136.7°C melting point of the synthesized aspirin. Comparing with the theoretical melting point, 140°C, we can conclude that the purity we obtain was high.

Percentage yield was calculated out to find the experimental amount of aspirin formed by esterification of reactants. To get percentage yield, mass of salicylic acid is used to calculate out the theoretical mass of aspirin. While mass of aspirin obtain practically is found out by doing several steps of experiment – preparation and recrystallisation. Aspirin was first prepared by reacting salicylic acid with excess acetic anhydride. It was then filtered off to get crude product aspirin (relatively impure) and dissolved in ethanol for recrystallisation. Pure aspirin was weighed after filtering and drying. With theoretical and practical value, percentage yield can be calculated. From this experiment, yield found to be 46.6%; this was indeed a fair result, according to Vogel’s Textbook of Practical Organic Chemistry.

2.IntroductionAspirin, also known as acetylsalicylic acid, is a salicylate drug, often use as active component in pills that relieve pains (analgesic), reduce swelling (anti-inflammatory) and reduce fever (antipyretic). When ingested, aspirin remains intact in the acidic stomach, but in the basic medium of the upper intestinal tract, it hydrolyzes forming the salicylate and acetate ions. It even helps reduce the tendency of blood clot formation in people at high risk for developing blood clots, and as a means of preventing heart attacks, strokes, diabetic retinopathy and colon cancer. Aspirin also has a growing reputation as an anticancer drug possibly by its action in promoting apoptosis (cell suicide) in cancer cells. There is no doubt that it reduces the risk of demential caused by repetitive small episodes of brain damage by thrombosis. However, if high amount of aspirin is consume, undesirable side effect like gastrointestinal ulcers, stomach bleeding, and tinnitus, may occur. It also causes risk of having Reye’s syndrome in children and adolescents.

The history of aspirin began on June 2, 1763, when Edward Stone, a clergyman, read a paper “An Account of the Success of the Bark of the Willow in the Cure of Agues.” By ague, Stone was referring to what we now call malaria. What his extract of willow bark actually did was to reduce the feverish symptoms of the disease.

Almost a century later, scientists discovered that it was the compound called salicin in willow plants which gave us the pain relief.

By 1829, Henri Leroux, French chemist had improved the extraction procedure to obtain about 30g from 1.5kg of bark. In 1838, Raffaele Piria, an Italian chemist, split salicin into a sugar and an aromatic component (salicylaldehyde) and converted the latter, by hydrolysis and oxidation, to an acid of crystallised colourless needles, which he named salicylic acid. Henri Leroux had extracted salicin, in crystalline form for the first time, and Raffaele Piria succeeded in obtaining the salicylic acid in its pure state.

However, salicylic acid was tough on stomachs and a means of ‘buffering’ the compound was needed. The first person to do so was a chemist, Charles Frederic Gerhardt, in 1853, which he neutralized salicylic acid by buffering it with acetyl chloride with sodium salicylate to produce acetylsalicylic acid. But he did not pursue it further.

Then in 1893, Felix Hofmann, a German chemist for the German firm Bayer began investigating acetylsalicylic acid as a less-irritating replacement for standard common salicylate medicines. This compound had the medicinal properties of salicylic acid, an extract of willow bark, without the unpleasant taste or the high degree of irritation of the mucous membranes lining the mouth, gullet, and stomach. By 1899, Bayer patent this drug Aspirin; a name derived from a- for acetyl, and the root -spir, from the Latin name for the meadowsweet plant, spirea and was selling it around the world.

3.Theory1.What is the ester functional group?A functional group is an atom or a group of atoms that gives a molecule its characteristic properties. Aspirin is both an organic ester and an organic acid.

Hence, it contain both Ester and Carboxylic Acids functional groups with the carboxyl group,-COOH and ester group, -COO. A carboxyl group consists of a carbon atom joined to an oxygen atom by a double bond and to a hydroxyl group, OH, by a single bond. While an ester group consist of a carbon atom double bond with an oxygen atom and single bond with another oxygen atom. Carboxylic Acids functional group gives a characteristic vinegar smell and ester functional group give sweet, fruity smell.

2.What is esterification? What are the different ways?Esterification is a reaction involve nucleophilic acyl substitution where the carbonyl(C=O) compound is used as an electrophile and is attacked by a nucleophilic alcohol (-OH) to form ester group (-COO).

In this experiment, the hydroxyl group (-OH) on the benzene ring in salicylic acid reacts with carbonyl group in acetic anhydride to form an ester functional group. Thus, the formation of acetylsalicylic acid (aspirin) is referred to as an esterification reaction.

There are several ways to make ester besides the one we did in this experiment; one was by reacting an acyl halide (CO-Cl) with an alcoholE.g. CH3COCl + C2H5OH → CH3COOC2H5 + HClAnother way was by reacting carboxylic acid with alcohol.

E.g. CH3COOH + C2H5OH → CH3COOC2H5 + H2OLastly, by reacting an alkali carboxylate and an alkyl halide.

E.g. CH3COO- Na+ + C2H5Br → CH3COOC2H5 + Na+ + Br-3.Why was acetic anhydride used instead of acetic acid to reach with salicylic acid?Acetic acid is used as it is cheap and forms a by-product, acetic acid, which is noncorrosive and can be recovered to make more acetic anhydride. Making acetic anhydride in excess, hence there is no salicylic acid left in the product solution.

While acetic acid is not used due to the formation of water as a by-product. Water causes hydrolysis of the product during the isolation steps. The hydrolysis reactions of acetylsalicylic acid produce salicylic acid and acetic acid, hence causing impurities in acetylsalicylic acid and lessen the yield of acetylsalicylic acid.

C6H4(OH)COOH + CH3COOH C9H8O4 + H2OC9H8O4 + H2O C6H4(OH)COOH + CH3COOHTherefore acetic anhydride is more recommended to be use as a reactant.

4.What is percentage yield?Percentage yield is the ratio, in percent terms, of the actual yield to the theoretical yield. It serves to measure the effectiveness of a synthetic procedure. In this experiment we get a 46.6% yield. This means that we obtain fair result. As according to Vogel’s Textbook of Practical Organic Chemistry, yields around 100% are called quantitative, yields above about 90% are called excellent, yields above about 80% very good, yields above about 70% are called good, yields below about 50% are called fair, yields below about 40% are called poor. Purification steps always lower the yield, hence the yield we achieve are acceptable.

5.Why find the number of mole of salicylic acid and not the number of mole of acetic anhydride?Number of mole of salicylic acid is used for calculation of percentage yield as it is the limiting reactant while acetic anhydride is in excess. Taking into account the stoichiometry of the reaction, the theoretical yield is therefore calculated based on the molar amount of the limiting reactant.

6.What is recrystallisation?Recrystallisation is a procedure for purifying compounds where the substance is dissolved in solvent and precipitated out in crystalline form. The desired substance crystallises, while impurities stay in solution as the solvent can dissolve the impurity. The purity is much higher afterwards, but the yield will have fallen as some of the desired substance may remains in solution. Recrystallisation only applies when all unwanted materials are much more soluble than the required compound in certain solvent and depends mainly on the differential solubility of a substance in a hot and a cold solvent.

7. What are the possible impurities that may be present with aspirin?The most likely impurity in the product after purification is salicylic acid, which arise from incomplete reaction of the reactant and from hydrolysis (reaction with water) of the product, acetylsalicylic acid, during isolation steps. Another impurity might be acetic anhydride which was added in excess in the reaction and traces of acetic acid.

8.Why did we use ethanol and water during recrystallisation?Ethanol and water combinations are used because ethanol has good dissolving ability for many organics, but is also considerably co-soluble with water. Addition of water can rapidly and dramatically reduce the solubility of many organics and thus induce crystallization. While organic mixtures are also frequently useful, the difference in character between two organics is rarely as dramatic as the difference between water and an organic solvent.

4.Procedure1.An estimated of 2.4g of salicylic acid was weighed and recorded in a dry 100ml conical flask.

2.In the fumehood, 6mL of acetic anhydride was added to the salicylic acid in the flask.

3.3-4 drops of concentrated sulphuric acid was added to the mixture and swirled.

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

5.The flask was removed from the water bath and 1mL of distilled water was
added from a dropper to decompose the excess acetic anhydride.

6.Additional 40mL of cold water was added. The mixture was stirred and rubbed with stirring rod to induce crystallisation.

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

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

9.Approximately 30mL of hot distilled water was added to the solution.

10.The solution is warmed until all the solid dissolves.

11.The solution was allowed to cool12.A clean dry watch glass together with a filter paper was weighed and recorded.

13.The recrystallised product was obtained by suction filtration using weighed filter paper.

14.The crystal and filter paper were transferred onto the weighted watch glass and dried in the oven for approximately 20 minutes.

15.The crystals, filtered paper and watch glass were placed in a desiccator for 5-10 minutes.

16.The dried crystals together with the filter paper and watch glass is weighed and recorded. Weight of dried, recrystallised aspirin was then calculated.

17.Percentage yield of dried, recrystallised aspirin was obtained by series of calculation.

18.Melting point of aspirin is determined by using OptiMelt.

19.Aspirin was discarded in the ‘Aspirin Waste’ container.

5.Results and CalculationsMassMass of salicylic acid = 2.50 gMass of filtered paper & watch glass = 41.82 gMass of dried, recrystallised aspirin, filter paper & watch glass = 43.34 gMass of dried, crystallised aspirin = 1.52 gPercent yieldNumber of moles of salicylic acid used = 0.01812 molExpected number of moles of aspirin= 0.01812 molExpected mass of aspirin = 3.262 gPercent yield = 46.6 % yieldMelting pointTemperature range 135.8 – 136.7 °CAppearanceThe crystals are white and needle-like.

6.DiscussionFrom this experiment, we get a rather fair yield. The followings might be the reasons that we did not get a high yield.

1.Air pocket appearing during vacuum filtration leading to crystal seeping through the air gap, hence lowered the yield.

2.Not all aspirin has crystallised during the cooling period.

3.Some crystals stick on the side of the Buchner filter funnel or on the conical flask, leading to lesser yield.

4.Too much water might be added causing aspirin to recrystallise at a longer period.

5.Some of the aspirin might be hydrolyzed to acetic acid and salicylic acid after water added and is left for isolation. Hence lessen the yield.

Figure 1: Hydrolysis of acetylsalicylic acid to acetic acid and salicylic acid (ChemistryExplained, 2009)The following might be the ways to improve the yield of aspirin.

1. Air pocket must be checked before starting suction filtration.

2.Do not leave aspirin with water for long isolation.

3.Use ethyl acetate instead of ethanol and water for recrystallisation so as to prevent aspirin from decompose.

4.Lessen the water added to the aspirin solution help to induce crystal at shorter period.

5.Longer cooling period or place the flask in an ice bath might help to induce crystal at faster rate.

In this experiment, we get a rather pure aspirin. As the melting point obtained (135.8-136.7°C) was close to the theoretical melting point, 140°C. In addition, the melting point range was quite narrow. As the more impurity are present, the lower the melting point and the broader the range. Hence indicate the high purity of the synthesized asprin.

In this experiment, the safety precautions to take note are to wear safety goggles and disposable plastic gloves so as to prevent strong irritant and corrosive chemical from reaching our eyes or skin. Cotton gloves were used when handling glassware.

7.ConclusionWith reference to Introduction to Organic Laboratory Techniques A Small Scale Approach, experiment 7, pp. 60, pure aspirin has a melting point of 135-136°C. We can conclude that recrystallisaion was indeed an efficient way to increase purity as we get a melting point of 135.8-136.7°C.

To ensure better accuracy on the purity, we can test the purity by doing Ferric Chloride test, so as to determine if there is any salicylic acid remaining in our product. As salicylic acid is a phenol (compound that contain -OH on benzene ring). Phenols form a highly coloured complex with ferric chloride (Fe3+ ion). The result will appear purple if salicylic acid is present. A sample of pure aspirin should not exhibit any colour change. Infra-red spectrometry could also be used to check the purity of the compound for any presence of the spectrum of the -OH group of salicylic acid.

To have better purity, we could use hot ethyl acetate instead of ethanol and hot distilled water during recrystallisation. So as to prevent aspirin from partially decompose. In addition, we can substitute cold petroleum ether with water for rising remaining material from the flask. In conclusion, water is not recommended to be use with aspirin as it hydrolyzed it.

Reference

1.’aspirin’ 2005, in Collins Dictionary of Medicine, Collins, London, United Kingdom, viewed 20 November 2009, http://www.credoreference.com.ezp2.lib.sp.edu.sg/entry/collinsmed/aspirin2.’Aspirin’ 2004, in McGraw-Hill Concise Encyclopedia of Science and Technology, McGraw-Hill, New York, NY, USA, viewed 21 November 2009, http://www.credoreference.com.ezp2.lib.sp.edu.sg/entry/conscitech/aspirin3.Mary Bellis, History of Aspirin, About.com, a part of The New York Times Company, viewed 20 November 2009, http://inventors.about.com/library/inventors/blaspirin.htm4.’recrystallization’ 2004, in The New Penguin Dictionary of Science, Penguin, London, United Kingdom, viewed 21 November 2009, http://www.credoreference.com.ezp2.lib.sp.edu.sg/entry/penguinscience/recrystallization5.Donald L. Pavia, Gary M. Lampman, George S. Kriz, Randall G Engel, 2005, Introduction to organic laboratory techniques a small scale approach, 2nd ed, USA.

6.Vogel, Arthur Israel , 1989, Vogel’s Textbook of practical organic chemistry, 5th ed, London : Longman Scientific & Technical ; New York : Wiley7.Minnesota State University Moorhead , Recrystallization II, pp.23-27, Moorhead ,Clay County, Minnesota, United States, viewed 22 November 2009, http://www.mnstate.edu/jasperse/Chem355/Recrystallization-2.doc.pdf