Question

1) Pretend you have just synthesized a novel, relatively non-polar compound that is solid at room temperature. The last reaction you performed on route to this compound also formed small quantities of more polar impurities. You are going to attempt to purify your novel compound by recrystallization. Would you rather try using ethanol or cyclohexane? Explain why.

2) When is it necessary to use a solvent-pair recrystallization?

Answer 1

To remove the polar impurity from a non polar novel compound, we'll have to use a solvent who can dissolve the impurity in it.

in this case for removing the slightly polar impurity we'll use the slightly polar solvent ethanol.

When the compound gets dissolve in ethanol, all the impurities which are polar dissolve in it and non polar compound remains unaffected. However after crystallization,w can get the non polar pure compound.

2.

The important thing is that the solvent is not miscible with water.. Solvents which are more
dense (e.g, dichloromethane) or less dense (e.g., ether) than water can be used; thus, the aqueous phase
may turn out to be the upper or the lower layer, depending on the circumstances. Since you will usually
want one of the layers, because it contains your product, and will want to discard the other, since it
contains impurities, it is critical that you know which layer is which. Now, if you know the densities of the
solvents you are using (something to look up for your lab notebook), you will be able to predict which layer
is on top, and which is on the bottom.