Activities - Benjamin-Mills

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WM2Extraction of salicylicacidIn this activity you can extract some salicylic acid, apharmacologically active compound from a naturalsource, oil of wintergreen, which is obtained in turnfrom the shrub Gaultheriae procumbus. You then learnthe technique of thin-layer chromatography and use itto compare the extract with a pure sample of salicylicacid.Requirements● oil of wintergreen (2 cm 3 )● apparatus for heating under reflux (see Figure 1)● sodium hydroxide solution, 2 mol dm –3 (25 cm 3 )● measuring cylinder (25 cm 3 )●●anti-bumping granulessmall Bunsen burner or electric heating mantle● 100 cm 3 beakers (2)●ice (if available)● concentrated hydrochloric acid (a few cm 3 )●●●●Universal Indicator paperglass rodBuchner funnel and apparatus for vacuum filtrationwatch glass● ethanol (a few cm 3 )●●●t.l.c. plates (silica-coated)small beaker to hold t.l.c. platecover for the beaker● solution of salicylic acid in ethanol (1 cm 3 )●solvent for chromatography – cyclohexane, ethylethanoate, ethanoic acid (200:100:1)● dropping tubes or melting-point tubes (2)●●●u.v. light sourceiodine crystalsaluminium foil or clingfilmcyclohexaneglacial ethanoic acidethanolethyl ethanoateconcentrated hydrochloric acid2-hydroxybenzoic acid(salicylic acid)iodineHIGHLY FLAMMABLECORROSIVEHIGHLY FLAMMABLEHIGHLY FLAMMABLECORROSIVEIRRITANTHARMFULFLAMMABLECARE Ultraviolet radiation is harmfulto the eyes. Do not look directly atthe lamp. Follow the recommendedprecautions concerning eye protection.ULTRAVIOLET RADIATIONmethyl 2-hydroxybenzoate,methyl salicylate(oil of wintergreen)HARMFULCARE Eye protection must be worn.WEAR EYE PROTECTIONsodium hydroxide solutionCORROSIVEIntroductionThe ester methyl salicylate found in nature can be convertedinto salicylic acid. It is obtained as oil of wintergreen whichused to be made by dry distillation of the leaves of theevergreen shrub Gaultheriae procumbus prevalent in the US.You may be familiar with the smell of the ester for it is presentin many medicinal preparations (for example ‘Deep Heat’ andother similar ointments used to relieve muscle pain).In this experiment you will obtain salicylic acid from themethyl ester by hydrolysis, using dilute sodium hydroxidesolution.AS LEVELWhat you do„ Salters Advanced Chemistry 2000 – see Copyright restrictions1 Put about 2 cm 3 of oil of wintergreen (CARE Harmful) intoa 50 cm 3 pear-shaped flask and add 25 cm 3 of 2 mol dm –3sodium hydroxide solution (CARE Corrosive) togetherwith a few anti-bumping granules.2 Attach a water condenser to the flask in a vertical position(see Figure 1) and gently heat the mixture under reflux for 30minutes. Make sure the condenser water supply is on. Youcan heat the flask with a small Bunsen flame below a wiregauze or use an electric heating mantle. When the mixture isboiling, condensed droplets should be falling back into theflask at a rate of about 1 drop per second.139
WM2EXTRACTION OF SALICYLIC ACID3 Allow the mixture to cool and pour it into a 100 cm 3 beaker surrounded bycold water (and ice, if available). Using a dropper, add concentratedhydrochloric acid drop by drop (CARE Corrosive) until the mixture is acidic.Test the mixture, as you add the acid, with Universal Indicator paper, using aglass rod and small drops of the mixture.4 Using a Buchner funnel, filter the product using vacuum filtration. Wash thesolid with a little cold water and transfer it to a watch glass.5 Take a few crystals of the product and dissolve them in a minimum of ethanol.6 Take a pre-dried thin-layer chromatography plate which will fit into a smallbeaker (see Figure 2). About 1 cm from the bottom of the plate draw a finepencil baseline. On the line place a small spot of the extract from stage 5. Onthe baseline also put a small spot of a solution of salicylic acid in ethanol.The spots are best made using a very fine dropping pipette or a drawn-outmelting-point tube. Apply a small quantity of the solution at a time; let it dry,and then add more. Try not to let the diameter of the drops exceed 5 mm.7 Place some solvent for the chromatograpy in the beaker to a depth of about5 mm. (CARE Highly flammable. Avoid breathing the vapour.)8 Place the chromatography plate in the beaker, making sure the solvent isbelow the pencil line on the plate.9 Cover the beaker, for example with aluminium foil or a watch glass. Leavethe solvent to rise up. This will take about 15–25 minutes.10 When the solvent has nearly reached the top of the plate, take thechromatogram out of the beaker. (CARE Avoid breathing the solvent vapour.)Place the plate in a fume cupboard and allow the solvent to evaporate.11 You can locate the positions of the substances on the plate by examining itunder u.v. light. (CARE Do not look directly at the light source.) View theplate by reflected light.12 Alternatively, place the sheet in another beaker with a few crystals of iodine(CARE Harmful. Avoid skin contact. Use in a fume cupboard.) Cover thebeaker with aluminium foil or clingfilm. The spots which were probably justvisible before should show up more clearly now. After the experiment,dispose of the solvent as directed by your teacher.water inantibumpinggranulesHEATwater outoilwintergreenofsodium hydroxide+solutionFigure 1 Apparatus for heating under refluxthin-layer platesalicylicsolutionacidextractfromwintergreenoil ofsolventlidFigure 2 Apparatus for thin-layer chromatographyQUESTIONSa Explain what is meant by heating under reflux.Why is this often necessary when heatingorganic liquids?b Why is it necessary to heat the flask for sucha long time?c What do the results from the thin-layerchromatography separation tell you about thecomposition of the extract from oil of wintergreen?d The equations for the reactions occurring in theexperiment are shown on the right. How couldyou have obtained the methanol formed duringthe experiment if you had wished to use it?e Suggest why the mixture formed by the hydrolysisof the ester was kept as cool as possible.OCOHoil of wintergreenCOHCOO++ CH 3 OHCH 3O+H +OHOHCOHsalicylic acidO140AS LEVEL„ Salters Advanced Chemistry 2000 – see Copyright restrictions
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