Question

A student was attempting a recrystallization in an ethanol-water mixture (about 1:4) of an impure compound (quite soluble in ethanol) with a melting point when pure of 48 Degrees Celsius, but the substance kept oiling out when the temperature was lowered despite scratching, putting in seed crystals, and ardent prayer. Can you suggest an adjustment of the solvent composition that might improve the chances of getting crystals rather than oil to form when saturation is reached by cooling?

Answer 1

If the solvent is not specified in that case we need to check a variety of solvents to make sure that it is good for recrystallization of given compound. As we now that water has a dielectric constant of 78.4 whereas 24.5 for ethanol so generally the ionic compounds are more soluble in the compound with more dielectric constant but it's given that given compound is quite soluble in ethanol. So you should try with only ethanol as a solvent rather than the mixture of solvent.

So do the recrystallization of given compound in ethanol. And remember we should take such solvent which doesn't dissolve the compound when it's cold but dissolve when it is warm or hot.