CHE 211 Study Guide - Midterm Guide: Boiling Point, Filter Paper, The Technique
Organic Chemistry 2 – Chemical Purification & Separation
Centrifugation
o Centrifugation is another technique used to separate different phases
o Takes advantage of centrifugal force
o Mixture is placed inside the centrifuge and is whirled around at high speed
▪ A mixture is placed within a centrifuge tube, which is that placed in the
centrifuge --> then swirled around a circular path at high speeds
▪ This forces more dense phases towards the bottom of the tube, while the less
dense phases is on top
o Denser phase (usually a solid) is forced to the bottom
o Lighter phase will be on the top
o Commonly used to:
- Recover a solid after a crystallization
- Separate a finely divided solid from a liquid
- Separate two immiscible liquids separating two immiscible liquids after an
extraction
Recrystallization
Recrystallization is a method used to PURIFY chemicals based on their solubilities
▪ When a solid organic compound is synthesized in the lab or it is isolated from its
naturally occurring matrix... it is usually NOT PURE --> technique of
RECRYSTALLIZATION is used to purify it
Common way purify a solid organic compound is called recrystallization
Recrystallization is the slow formation of a crystalline solid
o The technique is to dissolve the solid to be crystallized and a minimum amount
of hot solvent; all insoluble impurities are filtered off and the unwanted colour
can be removed by using decolourizing charcoal, which is also removed through
filtration
Precipitation is the rapid formation of an amorphous solid
The hot solution is allowed to be cooled slowly so that the desired compound
crystallizes at a slow to moderate rate; if the hot solvent is cooled too quickly,
the compound tends to precipitate out of solution, rather than to crystallize out
of the solvent --> compounds that precipitate out will still contain impurities that
were trapped in the solid matrix
Recrystallization is a SLOW process that allows the crystals to form while the
impurities that are present in low concentrations stay in the solvent, even when
the solvent cools;
Crystallization is MAXIMIZED by placing the container in an ice water bath for a
short period of time --> after this process is complete, the crystals then dried
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Stages of Recrystallization
➢ Hot saturated solution cools
➢ Becomes supersaturated as crystal nuclei forms.
• After the hot saturated solution cools, it becomes supersaturated --> this means
that the solvent has exceeded its maximum normal solute capacity --> the crystal
nuclei starts to form and they stick to the side of the container
• --> as the crystals form, the other molecules might go towards it, in order to join
the growing crystal lattice
• --> the molecules must make their way through the solvent, through the crystal
lattice; therefore, the solution surrounding the crystal lattice becomes less
concentrated
• --> the formation of crystal formation is EXOTHERMIC: releases heat, which
increases the solubility of crystals near the surface of the solvent
➢ Nuclei stick to sides of the container
➢ Other molecules add to the crystal lattice
➢ Crystal growth is exothermic
All the processes occur by the process of diffusion and they take time
Premature chilling or movement can cause the rate of crystallization to increase
so quickly that the compound will precipitate out, rather than to form crystals;
The purest crystals formed when their recrystallization occurs slowly and are
undisturbed
Seven Steps of Recrystallization
1) Choosing a solvent
2) Dissolving the solute
3) Decolorizing the solution
4) Removing solid impurities
5) Crystallizing the solute
6) Collecting and washing the crystals
7) Drying the product
Characteristics of an Ideal Solvent:
θ Choosing the appropriate recrystallization solvent is very important
θ Ideal solvent should not react with the solute You DON'T want to have your
compound undergo a chemical reaction with the recrystallization solvent; we only want
to DISSOLVE it!
θ The solvent should boil at a temperature below the melting point of the compound
the ideal solvents boils BELOW the melting point of the compound; if it is higher than
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Document Summary
Organic chemistry 2 chemical purification & separation. Separate a finely divided solid from a liquid. Separate two immiscible liquids separating two immiscible liquids after an extraction. Common way purify a solid organic compound is called recrystallization. Precipitation is the rapid formation of an amorphous solid. Recrystallization is a slow process that allows the crystals to form while the impurities that are present in low concentrations stay in the solvent, even when the solvent cools; Crystallization is maximized by placing the container in an ice water bath for a short period of time --> after this process is complete, the crystals then dried. -> as the crystals form, the other molecules might go towards it, in order to join the growing crystal lattice. -> the molecules must make their way through the solvent, through the crystal lattice; therefore, the solution surrounding the crystal lattice becomes less concentrated.
