3What i the advantage and disadvantage of the low boiling point of acctone when
ID: 576276 • Letter: 3
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3What i the advantage and disadvantage of the low boiling point of acctone when it is used as a recrystallization solvent? 4) What can one What is the drawback of this method? do to obtain a second crop of crystals after initial crystals have been collected? 5) You are given a unknown white solid to recrystallize in lab. The white solid could be Compound A (melting point 74-76°C), Compound B (melting point 91-93°C) or Compound C (melting point 85-86°C). Compound A, B and C are significantly soluble in ethanol (bp: 78°C), even at room temperature and not very soluble in 2-butanone (bp: 80°C). Ethanol and 2-butanone are miscible. as the solvent pair for the unknown white solid, a) If you decide to use ethanol and 2-butanone describe which solvent you would heat up and dissolve the solid in initially and which solvent you would add second and at what point would you stop adding the second solvent. b) If you could use ethanol as the single recrystallization solvent for the solid, which compound might have a problem of oiling out and why?Explanation / Answer
3) the low boiling point of acetone means you cannot heat the solution which is ideally dont in recrystallization method. It will dissolve readily at RT which is good. But since acetone is a very good solvent at all temperatures there will be nothing like "cooling down" the solution for crystals to form. Most of the compound will stay in the solution and the yield will be very less.
4) to recollect you can either reheat the solution and repeat the process again or you can do seeding. Seeding means you van add an already formed crystal from your previous batch which will induce crystallization again. But the recovery will not be much. Another drawback is that you might get impurities then.
5a) I would add ethanol first since all the compounds are readily soluble in ethanol. In recrystallization process the key is to dissolve your compound perfectly in the solvent. And while cooling the solubility should decrease so that maximum crystallization can happen resulting in maximum yield. So i would add 2 butanone later on. Once the crystallizatio would start i would stop adding 2 butanone because if it happens too fast we will get impurities.
B)compound A would oil out since it has the lowest boiling point. Its m.p. is lower than the b.p of both of the solvents so while cooling down when the solution reaches saturation the compound A will still be in liquid phase and thus seperate in that form only along with impurities which can make the solution gooey or cloudy.
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