Question

What effects crystallization? What do you have to consider (solvent, temperature, solubility, etc.)? Which methods yield a purer product, extraction or crystallization? How is crystallization different than extraction?

Answer 1

Seven Steps of Crystallization

• Solvent selection.
• Dissolution.
• Decolorization.
• Filtration (Impurities)
• Crystallization.(cooling)
• Filtration (Compound)
• Drying.
• There are seven steps in crystallisation, and all above effects the crystalization. We have to consider all the seven factors stated above.
• Example:
• Starting from the top solvent selection is vital,
• An ideal solvent
• Should not associate or dissociate the compound. Means should not react.
• The tempratutem at which compound gets dissolve and at cooling it reprecipitates (recrystalize) should be not with much difference. Example- (Hypothetically - if acetanilide gets dissolved in hot water at 70°C then it should precipitate out at 40°C and not at 10°C. If we use that kind of solvent then crystals wontw grow in large.
• Solvent should be commercially available, cheap,enciromental nonhazardous.
• Solvent should have low melting point so it can be dried off.
• Dissolution
• As stated above dissolution should not result in dissociation or association.
• Decolourization:
• If solution is coloured then using decolourizer like animal charcoal is necessary for purity. But it eventually leads to loss of product. Which we don't want.
• Filtration
• While filtering hot things like loss of product in filtering medium can occur. When hot solution is poured on funnel or filter paper which is at room temperature then crystaliscrysta can occur. Almost immediately.nd impurities can't be filtered.
• Recrystalization: cooling is the most important thing, it will determine whether crystals of compound will be large shiny or not. If we let the filtration cool for long time without any disturbance then it will grow large crystals and needle shape or whatsoever. Here solvent quantity is also important.(Solvent quantity should be minimum amount or else the compound will stay soluble even at the ice temprature.)
• Drying
• Solvent used for solubilizing should have lower MP. Or for drying, excess heat should not be given to compound or it will get decompsed, if compound has lower Melting point.
• Pure product depends upon the compound and technique used for it.
• Of course extraction!.
• Recrystalization is inefficient in many ways as compared to extraction. Extraction is more efficient in a way that it can extract even two enantiomers which are optically active.
• While, with recrystalization even two diastereomer can't be separated which have different melting point.
• Crystalization is different from extraction in a way that...
• With extraction one can separate all the possible isomers . And recrystalize is the process we generally use before using extraction. We use extraction generally after recrystalization.
• Recrystalization involves heating as an inevitable step. While extraction, it's not necessary.