What causes oiling-out during recrystallization, and how can i fix it?

[FrankGM]

I’m trying to recrystallize my final product from a reaction, but it keeps coming out as an oily mess instead of nice crystals. I’ve tried varying the cooling rate and using a seed crystal, but it just won’t solidify properly. I’m wondering if the polarity of my solvent system is off or if there’s too much impurity acting as an oiling-out agent.

[Ella_G]

That oily mess can be the classic oiling out of a solubility window. If impurities are present or the solvent pair is too polar or too nonpolar for the product, it can stay liquid instead of crystallizing. Seeds help sometimes, but only if the solvent system actually favors solid formation at lower temp.

[Brandon.B]

I ran into this too. I kept changing the solvent and still saw oiling until I tried a different polarity mix. The crude showed up on TLC and I suspect impurities were dragging the product along. Cleaning up the crude helped a bit, but the crystals still weren't clean.

[Kenneth39]

Could the real issue be that the crude contains an oiling-out impurity that dominates the liquid phase? It sometimes looks like the product but behaves like oil. Hard to tell without a quick purity check.

[Nathan.W]

Another angle is that the product might form solvates or remain liquid in that solvent because of its own properties. A solvent swap to something of different polarity or a two solvent system can shift the balance, but I haven't nailed it down yet.