1.) Pick the solvent.In the introductory organic lab course,the solvent for recrystallization is usually determined for you.The criteria used to choose an appropriate recrystallization solvent includes:
a.) finding a solvent with a high temperaturecoefficient. The solvent must notdissolve the compound at low temperatures (that includes room temperature),but must dissolve the compound at high temperatures. The solute mustdissolve in order to rid its lattice of impurities, but must not remaindissolved at room temperature (after all, recovery of the solid is essential!).
b.) using a solvent that dissolves impuritiesreadily or not at all. If the solventdissolves the impurities readily (even at room temperature) then the impuritieswill not become trapped in the developing crystal lattice, but will remaindissolved in the solvent. If the impurities do not dissolve (evenat elevated temperatures) then they can be easily removed by gravityfiltration. (seethe section on gravity filtration)
c.) insuring the solvent willnot react with the solute. As mentionedearlier, recrystallization does not chemically alter a molecule.No chemical bonds must be broken in the solute molecule. The crystallattice is dissolved at elevated temperatures, but this only involves overcomingthe intermolecular attractive forces.
d.) using a solvent that is nonflammable,inexpensive and volatile. Solventswith low boiling points (i.e., volatile) can be easily removed from theresultant crystals by simply allowing the solvent to evaporate.
2.)Dissolve the solute. Remember thatthe solute should dissolve only when the solvent is heated. Therefore,the solvent is heated to its boiling point (rememberto use boiling stones!) and then slowly addedto completely dissolve the solute. If too much solvent is added,the solution will not be saturated upon cooling and no crystals will form.
Dissolving the solutegenerally involves adding a small volume of hot solvent, swirling the flask(or stirring the solution), and watching to see if the solute dissolves.
3.)Decolorize the solution. If thesolute is supposed to be white in its pure solid state (most organic solidsare) and the solution is colored after dissolving all the solute, it willbe necessary to add decolorizing carbon to the solution. This willcause the colored molecules to adsorb onto the surface of the decolorizingcarbon, thereby ridding the solution of these impurities. Shouldthese impurities remain in solution, they may become trapped in the developingcrystal during cooling. Review the materialabout decolorizing carbon.
4.)Filter any solids from the hot solution. Ifdecolorizing carbon was used (as in step 3) or undissolved impurities remainin the hot solution, it is necessary to gravity filter the solution whileit is still hot. Review the information about hot gravityfiltration and decolorizing carbon.Under no circ*mstances should the hot solution be vacuum filtered witha Buchner funnel. This leads to premature crystal development asthe solution passes through the vacuum filter. (The vacuum reducesthe pressure, but also the temperature.) Impurities will be trappedin the crystal lattice and steps 1 through 3 will need repeated!
5.)Crystallize the solute. This involvesallowing the hot solution with the solute dissolved to return to room temperatureslowly. The slower the cooling process, the less chance of trappingimpurities in the developing crystal lattice. Allow the solution to reachroom temperature. If crystals have not formed by the time the solutionreaches room temperature, further steps may be necessary to inducenucleation. Try the following
a.) Take a clean, glass rod and scratch the inside surface of the Erlenmeyerflask. This provides a small speck of glass upon which nucleationmay occur.
b.) Ask a classmate who has recovered the pure solute to 'donate' a smallamount of the solute. Add a small sample of the pure solute to theflask. Again, this is thought to provide a site for nucleation.
c.) Place the Erlenmeyer flask in an ice water bath. This will dramaticallyreduce the temperature of the solution. Saturation conditions maybe met at this lower temperature, allowing the formation of crystals.
6.)Collect and wash the crystals. Theresultant crystals formed via this process can be collected by vacuum filtration,provided the solution is at room temperature and no further crystal growthis evident. To transfer all of the crystals to the Buchner funnel,add a small amount of the cold recrystallization solvent.Remember, the solute is not soluble in the cold solvent, so it safeto use this solvent to transfer the crystals. Wash the crystalswith a small quantity of the cold solvent to rinse any impurities off thecrystal surface.
7.)Dry the crystals. Usually a meltingpoint is taken directly following thepurification process, so it is necessary to quickly dry the crystals.This is accomplished by allowing the crystals to remain in the Buchnerfunnel and keeping the vacuum on for a few minutes. Alternatively,if more time is allotted, the crystals can be stored safely and allowedto sit for a few days, allowing the solvent to evaporate over time.
