[Tootella]

I understand what "oiling out" is but I don't understand how to fix this problem when doing re-crystallization.  I'm working on my lab report for the re-crystallization of acetanilide.  Does anyone know how to fix this problem?  Thanks.

[DaveD]

There are a number of reasons why oiling out occurs when attempting a recrystallization.  If you are using an antisolvent it could be that you added too much of this too quickly, forming a solution that was too supersaturated to crystallize.  Sometimes the presence of residual solvents can result in oiling out.  I would recommend assaying your acetanilide to determine the level of residual solvent (including water), and then screening different solvent systems than to one which resulted in oiling out.  A general rule in crystallizations is to generate supersaturation slowly for best results; this can be accomplished by slow addition of antisolvent, slow cooling, or slow distillation/evaporation.