Recrystallization
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Contents
Recrystallization
The general purpose of recrystallization is to crystallize the product in a fresh solvent after it has already been isolated from the extraction solvent, likely containing a considerable amount of impurities. In this technique, an impure solid compound is dissolved in a solvent and then allowed to slowly crystallize out as the solution cools. Often this process results in more well-formed crystals with less discoloration. The advantage of this method of purification is that the solvent choice for recrystallization may be different and more suitable than that chosen for extraction. Crystallization of a solid relies on slow, selective formation of the crystal lattice and is quite different from precipitation. In freeze precipitation, there is a rapid formation of a solid from a solution that causes impurities to be trapped within the solid's crystal framework. For this reason, extractions that rely on precipitation or evaporation to produce a solid product always include a final recrystallization step to give the pure compound.
The process of recrystallization relies on the property that for most compounds, as the temperature of a solvent increases, the solubility of the compound in that solvent also increases. For example, much more sugar can be dissolved in very hot water (just below boiling) than in water at room temperature. Inversely, if a hot saturated solution of sugar and water is allowed to cool, sugar will begin to crystallize out of solution as solubility decreases. Recrystallization will give your product a more sharply defined, uniform melting point and in the case of DMT allow for hard non-waxy crystals.
Process of Recrystallization
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1. Find a suitable solvent for the recrystallization;
2. Dissolve the impure solid in a minimum volume of hot solvent;
3. Remove any insoluble impurities by filtration;
4. Slowly cool the hot solution to crystallize the desired compound from the solution.
Finding a suitable solvent
The first consideration in purifying a solid by recrystallization is to find a suitable solvent. A good recrystallization solvent should fit the following criteria:
1. The compound should be very soluble at the boiling point of the solvent and only sparingly soluble in the solvent at room temperature.
2. The unwanted impurities should be insoluble in the hot solvent.
A good recrystallization solvent for DMT is heptane. DMT is not very soluble in it at room temperature but quite soluble as we add heat. Most common spice impurities, however, are not very soluble in it at all and can thus be separated via simple decanting.
Dissolve solid into hot solvent
Prepare a waterbath and heat the DMT and the heptane in their own beakers until the DMT begins to melt. Add heated solvent dropwise into the beaker containing the extract. The heptane will go cloudy almost immediately and take on a yellow color as the DMT goes into solution. Keep adding heated solvent until further addition or agitation causes no more DMT to dissolve.
Your beaker should now contain yellowish-tinged heptane with an orange-brown blob of oil and undissolved solids at the bottom of the vessel. Carefully decant the solution into another beaker, careful to leave the impurities behind. Repeat dropwise addition of heated solvent and decantation to ensure no DMT is left behind.
Decolorize with carbon
Now that we have dealt with insoluble impurities, our solution is see-through but tinged yellow. This discoloration is due to the presence of high-molecular-weight reaction by-products which may have been formed during the extraction process. A simple wash with activated carbon will get rid of decolorizing compounds. (Activated carbon is extremely efficient at absorbing impurities due to its large surface area.)
1. Add excess solvent and activated carbon, and boil the solution for a few minutes. The colored impurities will adsorb onto the surface of activated charcoal.
2. Remove the charcoal with absorbed impurities by filtration. Your solvent should now be almost clear. If the yellow color persists, repeat the charcoal wash carefully.
Note: Very little activated carbon is needed to remove the colored impurities from a solution. You must be careful in your use of decolorizing carbon: if too much is used, it can adsorb the desired compound from the solution as well as the colored impurities.
Crystallization
After the solution has been filtered cover the flask containing the hot filtrate and set it aside undisturbed to cool slowly to room temperature. As the solution cools, the solubility of the dissolved compound will decrease and the solid will begin to crystallize from the solution. Once the solution reaches room temperature, move it into the refridgerator, and finally into the freezer to freeze precipitate most of the DMT.
The slower your solution cools, the cleaner and larger your crystals will be.
