starting material of benzion = 1.027 and when recrystallized .208. A. The goal o
ID: 1006598 • Letter: S
Question
starting material of benzion = 1.027 and when recrystallized .208. A. The goal of any separation or purification step is to completely remove unwanted material (maximize purity) and recover all of the desired material (maximize yield or recovery). Unfortunately, you often sacrifice one to maximize the other, so it ’s always a balance between the two. a) Is there any point during the extraction or recrystallization that you could visually tell that you were losing benzoin? b) Is there any information on the TLC plates that indicated that benzoin was lost? c) What changes could you make to minimize the loss of benzoin? B. After running a reaction you are asked to extract the product into diethyl ether (3 x 5 mL). a) How do you do this? (give a detailed, step - wise procedure) Keller/Stickley/Vaughn Chem 2255 b) Why should you then wash your extract with water? c) If you can't see any water droplets, do you still have to dry the solution with sodium sulfate? Why or why not? d) You want to evaporate the solvent by leaving it in the back of the hood until the next lab period. Should you stopper the flask? Why or why not? e) Upon returning the next lab, you weigh your crude sample (1.538 g) and recrystallize with hot isopropa nol. How do you know how much isopropanol to add?
Explanation / Answer
B) a) The extraction of reaction mixture to get benzoin can be done in following steps.
i) Transfrer te reactino mixtue to a separating funnel.
II) Add suitable amount of Diethyl ether to the flask and tranfer all the content to the separating funnel.
iii) Stopper the flask and shake the flask vigourously and carefully.
iV) Allow the layers to separate and collect the Diethyl ether layer containg the Benzoin.
b) The ether layer should be washed in water to remove any remaining watr soluble (polar ) impurities like salts.
c) Yes you still have to be dried with sodium sulfate becaus water may not be visible but some little amount of water may get miscible with ether to removethat sodium sulfate treament is nessasary.
d) Eventhough you want to evapourate the solvent opening it in the air is not a good idea becaus dust or other particle may get added and make the material impure. So its better close the flask or beaker gently so that there should be enough space for solvent vapours to get out as well as no spece to dust and other impurities to enter.
e) Always during recrystalisation you have to add minimal amount of slovent just to dissolve so add the solvent dropwise and check solubility once its soluble keep it for recrystalisationwith out further addition of solvent.
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