Actual ExpCriMCNt: HAND OUT SHEET

Transcription

1 ORGANIC CH E MISTRY LABORATORY - Actual ExpCriMCNt: HAND OUT SHEET,D a J Cncepts Include: Reactin mechanism fr preparing aspirin, calculatin f excess and limiting reagent, theretical yield, actual yield, percent yield f aspirin, filtratin, recrystallizatin, and melting pint. Data: Calculatin f reagent in excess and limiting reagent _ Theretical yield aspirin Actual yield aspirin Percent yield f aspirin (wet cake) Percent yield f aspirin (recrystallized) Melting pint recrystallized aspirin uestins Answer questins 1-5.

2 Aspirin (acetylsalicylic acid) can be acid and acetic anhydride: Salicylic acid +ll H AA Acetylsalicylic Acid ll acetic asrhydride prepared by the reactin between salicylic Acetylsalicylic Yacid H- ----'-----> OH tl + A" acetic acid In this reactin, the hydrxyl grup (-OH) n the benzene ring in salicylic acid reacts with acetic anhydride t frm an ester functinal grup. Thus, the frmatin f acetylsalicylic acid is referred t as an esterificatin reactin. This reactin requires the presence f an acid catalyst, indicated by the H* abve the equilibrium arrws. Prcedure Preparatin f Acetylsalicylic Acid (Aspirin), Weigh f salicyclic acid (MW ) and place this in a 125-mL Erlenmeyerflask. Add 5.0 ml f acetic anhydride (MW= 102.1, d /mL), fllwed by 5 drps f cncentrated sulfuric acid, and swirl the flask gently until the salicylic acid disslves. Heat the flask gently n the steam bath r in a ht water bath at abut 50C fr at least 10 minutes. Allw the flask t cl t rm temperature, during which time the acetylsalicylic acid shuld begin t crystallize frm the reactin mixture. If it des nt, scratch the walls f the flask with a glass rd and cl the mixture slightly in an ice bath until crystallizatin has ccurred, After crystal frmatin is cmplete (usually when the prduct appears as a slid mass), add 50 ml f water and cl the mixture in an ice bath. Vacuum Filtratin. Cllect the prduct n a BLichner funnel The filtrate can be used t rinse the Erlenmeyer flask repeatedly until all crystals have been cllected. Rinse the crystals several times with small prtins f cld water. Cntinue drawing air thrugh the crystals n the Brichner funnel by suctin until the crystals are free f slvent (5-10 minutes). Remve the crystals fr air drying.. Weigh the crude prduct, which may cntain sme unreacted salicylic acid, and calculate the percentage yield f crude acetylsalicylic acid(mw 180.2) assuming that the wet cake is abut 50/ slids.

3 Acetylsalicylic Acid Recrystallizatin. Water is nt a suitable slvent fr crystallizatin because aspirin will partially decmpse when heated in water. Disslve the prduct in a minimum f ht ethyl acetate (n mre than 2-3 ml) in a 25-mL Erlenmeyer flask, while gently and cntinuusly heating the mixture n a steam bath r a ht-plate. When the mixture cls t rm temperature, the aspirin shuld crystallize. If it des nt, add petrleum ether t the slutin and cl the slutin in ice water while scratching the inside f the flask with a glass rd until the prduct crystallizes. cllect the prduct, using a Buchner funnel. Any remaining material can be rinsed ut f the flask with a few milliliters f cld petrleum ether, Dispse f the residual slvents in the waste cntainer fr nnhalgenated rganic waste. Determine the melting pint f yur prduct. The melting pint must be btained with a cmpletely dried sample. Pure aspirin has a melting pint F C. Questins. 1. What is the purpse f the cncentrated sulfuric acid used in the first step? 2. What wuld happen if the sulfuric acid was left ut? 3. If yu used g f salicylic acid and excess acetic anhydride in the preceding synthesis f aspirin, what wuld be the theretical yield f acetylsalicylic acid in mles? In grams? 4. Mst aspirin tablets cntain five grains f acetylsalicylic acid. Hw many milligrams is this? 1 grain = g 5. If the aspirin crystals were nt cmpletely dried befre the melting pint was determined, what effect wuld this have n the bserved melting plnt? 3

4 Charge 1st 2.0 g Salicylic acid 2nd 5 mlacetic anhydride 3rd 5 drps cnc. H2SO4 Cautin: Cnc. H2SO4 is crrsive Acetic anhydride is a lachrymatr!!! lf n crystals frm, scratch with glass rd --* _.-->._> 125 ml Erlenmeyer Flask Swirl mixture until disslved Heat gently with water bath Keep temp - 5OC fr 1O min Cpl t rm temperature -. Crystallizatin ccurs Further cl crystallized mixture with ice bath Add 50 ml H2O Cllect crude crystals Filtrate Waste -> m acidic sln. Rinse crystals with small amunt f cld water Vacuum dry crude crystals 1. Obtain crude mass (wet cake) 2. Calc. / yield 3. Run test fr Phenl 4

5 Preparatin f Acetyl Salicylic Acid Flw Diagram: Recystrallizatin f Crude prduct Chare l st 2-3 ml ethyl acetate 2nd crude prduct Cautin: Ethyl acetate is flammable z5 ml Erlenmeyer flask Heat gently with ht plate Prduct shuld disslve Plymer side prduct is insluble in ethyl acetate lf n crystals frm add petrc,leum eth until crvsltals frm r Cl t rm temperature and wait until crystals frr Cl slutin with ice bath until cystallizatin is cmplete Cllect crude crystals Press ut extra slvent Vacuum dry recystallized slid lfa lt f slid remains, CAA ftnte 1 pg 51 Add 5 ml mre f ethyl acetate Gently heat t biling with ht plate Gravity filter thrugh fluted paper Add petrleum ether drpwise t filtrate t tntil cnrctelc fnrrn Futher cl mixture with an ice bath 1. Obtain recrystallized mass 2. Determine melting pint 3. Run test fr Phenl 4. Submit f samle in vial Cllect crude crystals Press ut extra slvent 5