1. What can one do to obtain a second crop of crystals after initial crystals ha

Question

1. What can one do to obtain a second crop of crystals after initial crystals have been collected? What is the drawback of this method? 2) 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. a) If you decide to use ethanol and 2-butanone as the solvent pair for the unknown white solid, 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? 3)  What is the advantage and disadvantage of the low boiling point of acetone when it is used as a recrystallization solvent?

Explanation / Answer

1. In case of the first crop, the mother liquor (the liquid remaining after a compound has crystallized out) has to be filtered (without disturbing the crystals obtained) to another flask (flask-2). The solution (filtrate) in flask-2 should be continued (without disturbing) for crystallization to obtain the second crop of crystals.

The disadvantage of this method is that the purity of crystals decreases in the second crop because the viscous (sticky) impurity in the solution (filtrate) partially sticks to crystals of the second crop.

2. a) If you decided to use ethanol and 2-butanone as the solvent pair for the unknown white solid, initially you would heat up and dissolve the solid in ethanol until the solution becomes saturated, followed by drop-by-drop addition of 2-butanone from the walls of the flask (without much disturbing the solution), and you would stop adding 2-butanone at the point, where the precipitate appears to form.

2. b) If you could use ethanol as the single recrystallization solvent for the solid, compound A might have a problem of oiling out because it has the melting point almost same as the boiling point of ethanol.

3. An advantage of the low boiling point of acetone when it is used as a recrystallization solvent is that the crystals grow quickly from the solution and the disadvantage is that the solvent (acetone) evaporates quickly such that the impurity in the solution can stick to the crystals.

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