RECRYSTALLIZATION

 

Most of the organic substances recovered from organic reactions or collected from natural sources are impure and require purification to obtain the desired pure product.  One technique commonly used to purify solid organic substances is called crystallization.  The general technique involves dissolving the material in a minimal amount of a suitable hot solvent.  This creates a saturated solution, which upon slowly cooling to room temperature the solid will come out of solution.   The dissolved material has a decreased solubility at the lower temperatures and will crystallize out of solution as it is cooled.  If this process is allowed to cool slowly the pure crystals of the compound will form, it is called crystallization.  However, if the solution is cooled too quickly nonselective crystals will form, it is called precipitation.  This crystallization process helps in the purification because as the crystals form slowly, they select out the correct molecules, which fit into the crystal lattice and the undesired molecules would not fit the same way. 

 

Required Reading: Mayo 90-93, 110-115..

 

Purpose of the experiment: 

 

This experiment consists of two parts.  The first part of the experiment is to introduce you to a technique called recrystallization.  This technique can be used to purify solids.  You will perform two recrystallizations, which simulate two common situations.  Crude benzoic acid contains a colored impurity, which is more soluble in the chosen solvent than pure benzoic acid.  The other situation is fluorene, which has a higher solubility in the hot chosen solvent than the impurity.  The second part is a problem set of conformational analysis of alkanes and cycloalkanes.

 

Procedures:

 

Benzoic acid from water

 

Heat a small amount of water in a 50 mL Erlenmeyer flask.  Weigh out approximately 200 mg of crude benzoic acid, and place this material and a boiling stick in a 25 mL or 50 mL Erlenmeyer flask.  Add approximately 2-3 mL of hot water.  Heat the mixture on a hot plate until the water boils or until the solid has gone into solution.  Use a Pasteur pipet to add additional hot water if necessary until all of the benzoic acid dissolves.  Record the exact amount of solvent  used for this recrystallization.

 

After all the benzoic acid dissolves, take the mixture off the hot plate and let it cool slowly to RT and then place the flask in an ice bath to maximize the recovery of the solid.  Filter the crystal using the Hirsch funnel (Fig 5.23) fitted with a filtered paper and apply a vacuum (A vacuum can be generated by connecting the thick walled rubber hose to the side arm of the test tube and the aspirator).  Rinse the beaker 2-3 times with cold water and pour the solution over the crystals on the Hirsch funnel to wash them.  Continue washing the crystal with cold water until the solid becomes white.  Dry the crystals on the funnel by applying a vacuum for at least 5 minutes.  Weigh your purified sample, calculate the % recovery and determine the melting point of crude and purified benzoic acid.

 

Fluorene from 95% ethanol

 

Place approximately 100 mg of crude fluorene in a 25 mL Erlenmeyer flask and added 1-2 mL of 95% ethanol.  Heat the mixture until boils.  Add additional hot ethanol if necessary.  In another 25 mL Erlenmeyer flask, set up a cotton stuffed broken-tipped pipet[1] with a piece of glass wool inside and place the flask on the hot plate.  Rinse the broken-tipped pipet with 1 mL of hot ethanol.  Filter the mixture through the pipet to remove any insoluble impurities from the solution.  Bring the solution to boil and then remove it from the hot plate and let it slowly cool down to RT.  If crystals do not form, induce crystal formation by scratching the bottom of the flask very lightly with a glass rod or a spatula while chilling the solution in an ice bath.  Filter the crystal and wash them with cold ethanol.  Dry them on the funnel by applying a vacuum.  Collect the crystals, weight your sample and calculate the % recovery.  Determine the melting point of the crude fluorene and your pure fluorene. 

 

Questions:

 

1)      Why is it important to use the minimal amount of solvent?  What would happen if you use lots of solvent to dissolve your solid?

 

 

2)      Make a prediction on which of the following three solvents would be the best solvent to crystallize naphthalene: water, diethyl ether, or toluene.  Give your reasoning.  Draw out all three of the solvent structures.

 

    Naphthalene

 

3)      Could the following solvent pairs be used in a recrystallization?  Why or why not?

(a)    Acetone and ethanol

(b)   Hexane and water

(c)    Hexane and diethyl ether

 

4)      A student was asked to crystallize fluorenol (mp 153-154ēC), and he/she selected benzyl alcohol (b.p. 205°C) because of its solubility characteristic.  Do you think this solvent is a good choice?  Why or why not?

 

References: 

 

1)      Mayo, D. W.; Pike, R. M.; Trumper. P. K. Microscale Techniques for the Organic Laboratory; Wiley & Sons: New York 2001.

 



[1]  Insert a very small pellet of cotton or glasswool through the top and wedged gently in the constriction of a pipet.  Break off the tip of the pipet to a length of about 2 cm.  Use a paper towel to cover the narrow part of the pipet tip when breaking to avoid cutting yourself.  Transfer the solution to be filtered through the top with another pipet.  If necessary, apply pressure using a pipet bulb so the solution in the filtered pipet can drain faster.