Problems

Recrystallisation is a very useful technique, but does not always work as planned. There are several reasons why crystals may not appear in the flask, with different approaches that may help.

1. Too much solvent. This is the most common cause of crystals failing to appear, or of only a small amount of crystals appearing. Since it can be hard to judge the correct amount of hot solvent for dissolution, an excess of solvent can cause this problem during crystallisation. This can be remedied by reducing the volume of solvent (using a rotary evaporator), then attempting the crystallisation again from a smaller volume.

2. Super-saturation. The cooled solution contains more compound than it should be able to dissolve (it is “super-saturated”). Crystals require a nucleation centre to begin growing. Addition of a “seed crystal” of the pure compound may help, if one is available. Gently scratching the inside of the conical flask with a glass rod may help. Alternatively, cooling of the flask (e.g. in a salt/ice bath) , combined with occasional scratching, may help.

3. Oiling out. Sometimes, the solid material may separate from the solution, forming an oil instead of crystals. This most frequently occurs when the melting point of the compound is low relative to the solvent boiling point. It can also happen if the compound is significantly impure, and is more likely to occur when mixed solvents are used. This may be helped by warming the flask to redissolve the material, adding a small amount more solvent, and re-attempting crystallisation. Very slow cooling may favour the formation of crystals instead of an oil; this can be achieved by leaving the hot solution on a cooling hot plate, instead of placing it on a cold surface. If the compound does not crystallise, it may be necessary to purify it using a different technique e.g. chromatography.

Recovering compound

Recrystallisations rarely result in loss of compound if things go wrong as the material remains in the flask with solvent. If crystallisation is proving impossible, it may be necessary to recover the crude compound and restart purification. Depending on the recrystallisation this may be achieved by:

1. Rotary evaporation of solvents. If the compound has only had volatile solvents added, the solution may be concentrated on a rotary evaporator to recover the compound.

2. Aqueous extraction, drying and concentrating. If the compound has been recrystallised using water as one of the solvents, then rotary evaporation is a poor choice as water is difficult to remove this way. It is easier to re-extract the compound using an aqueous workup. The specifics of this will depend on the nature of the compound, but typically the compound is taken into an organic solvent and separated from the aqeuous layer (often brine is added to aid the partition) and the organic layer dried, filtered and concentrated on a rotary evaporator.