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A curriculum resource for post-16 chemistry courses Published and distributed by The Royal Society of Chemistry The preparation of aspirin Nearly everyone has used aspirin at sometime in our lives, but few know that a relatedcompound derived from willow bark hasbeen used to relieve pain and treat fevers forover 2,000 years.
This book contains eight activities for bothacademic and vocational courses. It has beenadapted by The Royal Society of Chemistryfrom a unit written by David Paterson, andoriginally produced by the New ZealandAssociation of Science Educators inconjunction with the Royal Society of NewZealand. The activities can be used singly or as acoherent package in a variety of learning andteaching situations. There are two main sections: • a series of student activities covering both written and practical tasks including preparative/analytical work and problem solving exercises; and • a guide for teachers and technicians giving equipment lists and answers.
How to use this book .
1. The aspirin story . 1 Experimental and investigation section
2. The preparation of 2-hydroxybenzoic acid . 4 3. The preparation of aspirin . 6 4. Purifying by recrystallisation . 8 5. The melting point of aspirin . 11 6. Using thin-layer chromatography to investigate aspirin . 13 7. The solubility of aspirin . 15 8. Thin-layer chromatography of pain killers . 16 Teacher's and technician's guide to the activities
1. The aspirin story . 18 2. The preparation of 2-hydroxybenzoic acid . 21 3. The preparation of aspirin . 22 4. Purifying by recrystallisation . 24 5. The melting point of aspirin . 26 6. Using thin-layer chromatography to investigate aspirin . 28 7. The solubility of aspirin . 30 8. Thin-layer chromatography of pain killers . 32 How to use this book
This book consists of eight free-standing activities that can be used singly or as acoherent package in a wide range of teaching and learning situations in bothacademic and vocational courses.
The book is organised in two main sections – a series of student activities and a guide for teachers and technicians with equipment lists and answers. Within thesection for students there are both written and practical tasks. The practical tasks areboth preparative/analytical and problem solving.
For ease of use some traditional names are retained for chemicals throughout thisbooklet. Below are listed the traditional names against the systematic names that arecommonly used in post-16 chemistry courses in the UK.
Traditional name/trade name
Methyl salicylate Methyl 2-hydroxybenzoate 2-Hydroxybenzoic acid Sodium salicylate Teachers and students should do a risk assessment before commencing any practicalactivity. Safety glasses and other appropriate protective equipment should be wornfor all the experiments in this booklet.
Dichloromethane is harmful by inhalation. Avoid breathing vapour and avoid
contact with skin and eyes.
Ethanoic anhydride is flammable and causes burns.
Ethanol is flammable.
Ethyl ethanoate is highly flammable and the vapour may irritate the eyes and
respiratory system. Avoid breathing the vapour and avoid contact with the eyes.
Hydrochloric acid can cause burns. It gives off an irritating vapour that can damage
the eyes and lungs.
2-Hydroxybenzoic acid (salicylic acid) is harmful by ingestion and is irritating to the
skin and eyes.
Phenol is toxic by ingestion and skin absorption. It can cause severe burns. Take care
when removing hydroxybenzene from the bottle because the solid crystals can be
hard to break up. Wear rubber gloves and a face mask.
Phosphoric acid is irritating to the eyes and causes burns.
Sodium hydroxide can cause severe burns to the skin and is dangerous to the eyes.
6. Using thin-layer chromatography
to investigate aspirin
You have probably used a simple chromatography experiment as part of your earlierstudies to separate the dyes in a coloured ink. The same technique can be used toseparate substances which are not dyes but in such experiments the chromatogrammust be developed to show up the various different substances that have beenseparated.
Chromatography techniques are used a great deal in industry because they can be controlled very precisely and use very small amounts of substance. In this activityyou investigate the purity and identity of your laboratory prepared aspirin samplesusing thin-layer chromatography (tlc). In this activity all the substances are white orcolourless so you will need to develop the plate before you can see what hashappened.
Thin-layer chromatography is a powerful tool for determining if two compounds are identical. A spot of the compound being investigated is placed on achromatography plate, and a spot of a pure manufactured sample of the samesubstance is placed next to it. The plate is then allowed to stand in a suitable solvent,which travels up the plate. If the compound to be identified leaves exactly the samepattern on the chromatography plate as the known pure compound it is reasonable toconclude that they are the same. However, if extra spots are observed as well as thecharacteristic pattern of the known compound, then impurities are likely to bepresent in the sample.
In this experiment both crude and recrystallised samples of aspirin are compared with a known sample of aspirin.
Make sure that you do not touch the surface of the tlc plate with your fingersduring this activity. Handle the plate only by the edges and use tweezers ifpossible.
Take a tlc plate and using a pencil (not a biro or felt tip pen) lightly draw a lineacross the plate about 1 cm from the bottom. Mark three equally spaced pointson this line.
Place small amounts (about 1/3 of a spatula measure) of your crude aspirin,your recrystallised aspirin and the commercial sample of aspirin in threeseparate test-tubes. Label the test-tubes so that you know which is which.
Make up 5 cm3 of solvent by mixing equal volumes of ethanol anddichloromethane in a test-tube. Add 1 cm3 of the solvent to each of thetest-tubes to dissolve the samples. If possible do this in a fume cupboard.
Use capillary tubes to spot each of your three samples onto the tlc plate. Allowthe spots to dry and then repeat three more times. The spots should be about1–2 mm in diameter.
After all the spots are dry, place the tlc plate in the developing tank makingsure that the original pencil line is above the level of the developing solvent –ethyl ethanoate. Put a lid on the tank and allow to stand in a fume cupboarduntil the solvent front has risen to within a few millimetres of the top of theplate.
Remove the plate from the tank and quickly mark the position of the solventfront. Allow the plate to dry.
Observe the plate under a short wavelength UV lamp and lightly mark with apencil any spots observed.
Place the plate in a jar or beaker containing a few iodine crystals. Put a coveron the jar and warm gently on a steam bath until spots begin to appear. Dothis in a fume cupboard if possible.
Draw a diagram to show which spots appeared under UV light and which appearwith iodine.
Write a short paragraph explaining why some substances move further up thetlc plate than others and how the results are made visible.
What conclusions can you draw about the nature of the three samples tested? 6. Using thin-layer chromotography
to investigate aspirin
Revision of chromatography ideas introduced pre-16 could be useful – eg separationof coloured inks using filter paper. Emphasise the ability of chromatograms to identifysubstances from very small samples.
Thin-layer chromatography is a powerful tool for determining if two compounds are identical. If the compound to be identified leaves exactly the same pattern on achromatography plate as a known compound it is reasonable to conclude that theyare the same. However, if extra spots are observed as well as the characteristicpattern of the known compound, then impurities are likely to be present in thesample.
In this experiment both crude and recrystallised samples of aspirin are compared with a known sample of aspirin.
Thin-layer chromatography plate and a pencil (not a biro or felt tip pen) Four test-tubes in a stand: method of labelling the test-tubes Three capillary tubes for use as micropipettes Chromatography chamber. Either a screw top jar tall enough to take the tlcplate, a small beaker with a petri dish for a lid, or a commercial tank Access to a fume cupboard and short wavelength UV lamp.
Ethanol and dichloromethane Access to a few iodine crystals (three or four per experiment is enough) Samples for testing Ethyl ethanoate as chromatography solvent.
A good separation is obtained which can be seen by using UV light to observe theplate. Impurities should be clearly visible in the crude sample.
Separation of substances is based on the many equilibrations the solutesexperience between the moving (ethyl ethanoate) and stationary (silica) phase.
Less polar substances move more quickly than more polar ones. In general thestationary phase is strongly polar and strongly binds polar substances. Themoving liquid phase is usually less polar than the adsorbent and most easilydissolves substances that are less, or even non-polar. The results are madevisible by UV absorption or by chemical reaction with iodine.
The recrystallised aspirin and the commercial sample should only show onespot with the same Rf value. Other spots should be seen in the crude sample.
try using different solvents to see whether better separation of impurities in thecrude product is possible; and find out how iodine development involving reactions with double bonds canbe used to find the degree of unsaturation in fatty acids such as margarine andbutter.

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