A Mixed-Solvent System—The Bad Part

Every silver lining has a cloud. More often than not, compounds recrystallized from a mixed-solvent system don t form crystals. Your compound may form an oil instead. 

Oiling out is what it s called more work is what it means. Compounds usually oil out if the boiling point of the recrystallization solvent is higher than the melting point of the compound, though that s not the only time. In any case, if the oil solidifies, the impurities are trapped in the now solid oil, and you ll have to purify the solid again. 

Don t think you won t ever get oiling out if you stick to single, unmixed solvents. It s just that with two solvents, there s a greater chance you ll hit upon a composition that will cause this. 

Add more solvent. If it s a mixed-solvent system, try adding more of the solvent the solid is NOT soluble in. Or add more of the OTHER solvent. No contradiction. The point is to change the composition. Single solvent or mixed solvent, changing the composition is one way out of this mess. 

Redissolve the oil by heating, then shake up the solution as it cools and begins to oil out. When these smaller droplets finally freeze out, they may form crystals that are relatively pure. They may not. You ll probably have to clean them up again. Just don t use the same recrystallization solvent. 

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