You are given a unknown white solid to recrystallize in lab. The white solid cou

Question

You are given a unknown white solid to recrystallize in lab. The white solid could be Compound A (melting point 110-112°C), Compound B (melting point 66-68°C) or Compound C (melting point 92-93°C). Compound A, B and C are significantly soluble in ethyl acetate (bp: 78°C), even at room temperature and not very soluble in water (bp: 100°C) or toluene (bp: 111°C) (difference in solubility of solid in water and toluene is not significant) . Ethyl acetate is miscible with toluene but not with water.

a) Why would one choose toluene over water to pair with ethyl acetate to use as a solvent pair?

b) If you decide to use toluene and ethyl acetate 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.

c) If you could use ethyl acetate as the single recrystallization solvent for the solid, which compound might have a problem of oiling out and why?

Explanation / Answer

a)

One would choose Toluene over water to pair with ethyl acetate because toluene is miscible with ethyl acetate and therefore those 2 solvents would completely mix with each other and we would not have any problems with aq. and organic layers. Also this would result in higher % recovery of the solid

If water is used along with ethyl acetate, then we will see 2 separate layers , aq. layer for water and organic layer for ethyl acetate and then we will have to recover solid from each layer which would result in lower % yield.

Therefore Toluene is preferred over water.

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b) For a toluene-ethyl acetate system, we would first dissolve the solid compound in ethyl acetate because the compound has considerably high solubility in ethyl acetate even at room temperature.

We would slightly warm up the solution in order to completely dissolve the whole solid.

After the solid is completely dissolved, we will start adding toluene drop wise. Because the compound is insoluble in toluene, as we mix the solvents, compound would start to precipitate out and this will be seen in the form of cloudiness being developed in the solution. We will continue to add toluene until no more cloudiness develops in the mixture

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c)

Boiling point of ethyl acetate is 78 C .

If we decide to use ethyl acetate as the only solvent, then we would have to evaporate the solvent by boiling it off because all three compounds are soluble in ethyl acetate even at room temperature.

This would create problem for compound B which has a melting point of 66-68 C .Because ethyl acetate would start boiling at 78 C and by that time melting point of compound B is exceeded, so it would melt before ethyl acetate start boiling and the whole mixture will show oil like drops of melted compound B in ethyl acetate which is known as oiling out

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