Recrystallization

PAGE 11

Recrystallization

A purification Technique for Solids

Objectives

To understand the concept behind recrystallization and its usefulness in organic synthesis.

Learn how to choose the correct solvent for recrystallization.

Introduction

Crystallization may be defined as the process in which a solid compound precipitates from a saturated solution in the form of crystals. Saturation is usually effected through cooling or evaporation.

- Purification by recrystallization depends on the following facts:

1. Different solids have different solubilities in a given solvent.

2. Most solids are more soluble in hot than in cold solvents.

Figure 1: a crystallized compound

When the impure solid is dissolved in a minimum volume of a suitable hot solvent and the resulting solution is gradually cooled, saturation and eventual crystallization of the pure compound occurs.

Impurities in a solid are of two kinds: soluble and insoluble and recrystallization involves the removal of both to purify a solid. Insoluble impurities are first removed by gravity filtration of the hot solution while the soluble impurities remain dissolved in the cold saturated solution (mother liquor) after precipitation of the desired compound. The pure crystals are separated from the supernatant liquid by suction filtration.

Generalized Experimental Procedure:

Recrystallization involves the following sequence of steps:

1. Selection of a suitable solvent.

2. Preparation of the solution.

3. Hot Filtration (Gravity Filtration).

4. Cooling.

5. Collecting and Drying of Crystals.

Each step will now be discussed more fully.

1. Selection of a suitable solvent

A suitable solvent for recrystallization should possess the following important properties:

1. Dissolve a large amount of the solid to be purified at high temperature.

2. Dissolve impurities readily at low temperatures (= soluble impurities) or not at all even at the boiling point (= insoluble impurities).

Experimentally: the suitable solvent is determined through solubility tests.

This is done by shaking about 0.1 g of the powdered solid with ~2 mL of the given solvent in a dry test tube. If the entire solid has nearly dissolved in the cold solvent, the solvent considered unsuitable. If not, the mixture is heated gently to the boiling point with stirring. If most of the solid did not dissolve, the solvent is also unsuitable.

If a substance is found to be too soluble in one solvent and insoluble in another, then a mixture of both solvents (solvent pair) may be used. In such cases, the solvents must be completely miscible. The compound to be recrystallized is dissolved in the solvent in which it is very soluble and then the other solvent is added gradually at the boiling point until a slight turbidity occurs. The solution is then allowed to stand at room temperature to effect slow crystallization before chilling in ice.

Table 1: Common solvents for RecrystallizationSolventb.p., CParticulars of Solvents

Water100To be used whenever suitable

Methanol65Flammable, toxic

Ethanol78Flammable

Acetone56Flammable

Chloroform61Non-flammable, vapor toxic

Cyclohexane81Flammable

Ethyl acetate78Flammable

2. Preparation of the solution

To prepare the hot solution, the finely divided solid is placed in an Erlenmeyer (conical) flask and the selected solvent is added in small portions. The mixture is stirred and heated to boiling after each addition, until the solid dissolves completely. A slight excess of the solvent is usually added to compensate for any losses (through evaporation) during hot filtration.

NOTE: Decolorizing charcoal may added at this stage if the solution is colored due to colored impurities.

The flask should be removed from the heat source before adding charcoal to it, otherwise bumping will occur.

3. Hot Filtration (Gravity Filtration)

Filtration of the hot solution is necessary to remove insoluble impurities. Upon addition to the funnel, the solution will cool rapidly and this can cause unwanted precipitation. This can be minimized, or avoided, by using a fluted filter paper and a stemless funnel placed on top of a beaker on a hotplate containing a few milliliters of the recrystallization solvent.

4. CoolingCooling the filtered solution will allow crystals to form. The rate of cooling plays a role in determining the size of the crystals that form: fast cooling will tend to generate more crystals of small size, slow cooling can allow largest crystals to form. The best compromise of speed, convenience, and crystal quality, is simply to let the solution cool to room temperature on the lab bench. To ensure maximum recovery of material, the solution should be cooled in an ice-water bath after the solution has reached room temperature.

NOTE: Scratching of the inner surface of the glass can help in crystal formation.

5. Collecting and Drying of Crystals

When the crystallization is complete, the crystals need to be collected by suction filtration using a Buchner funnel to ensure rapid and complete removal of the solvent. Transfer the crystalline material as a suspension to the filter, being careful to never fill the funnel over half full. Most of the time, it will be difficult to completely transfer all of the crystals to the funnel, quantitatively, so it will be necessary to add a small amount of ice-cold solvent to the flask to help facilitate the transfer of all of the crystals. The crystals are then washed with a little more ice-cold solvent to remove any final impurities that may remain on the surface of the crystals. This solvent should be as cold as possible to keep the crystals from redissolving.

The crystals are finally dried in an oven or allowed to air-dry, in case the melting point is low, by spreading them over a sheet of paper.

Experimental Selection of Solvent

Perform solubility tests on anthracene, salicylic acid, benzoic acid, and sodium benzoate in water, alcohol, and ligroin as follows:

With a spatula take about 0.1 g of the powdered solid and place in a dry test tube. Start by dissolving it in about 2 mL of solvent with stirring. If insoluble, heat the mixture to boiling (in water bath) and observe the solubility. The results should allow the selection of a suitable solvent for each compound. Recrystallization of an impure unknown Weigh about 1 g of the impure unknown and recrystallize it from the solvent you have selected.(By making a solubility tests on pure unknown to choose the suitable solvent).Make sure you use the minimum volume of solvent; otherwise the amount of recovered product will be small. Determine the weight and melting point of the purified compound.

Recrystallization

Report Sheet

Selection of Recrystallization Solvent

CompoundSolubilitySuitable Solvent

WaterAlcoholLigroin

Cold HotCold HotCold Hot

Sodium benzoate

Salicylic acid

Anthracene

Benzoic Acid

Recrystallization of an Impure unknown -Unknown letter:- Suitable recrystallizing solvent:

- Mass of crude sample:

- Mass of purified sample:

- % yield:

- Melting point of crude sample:

- Melting point of pure sample:-What is the effect of the presence of impurities on the m.p?

-Did you find insoluble impurities in the crude unknown? How can you remove it? Questions:1. What is the purpose of recrystallization?2. What properties should ideally be possessed by a recrystallization solvent?

3. How might the melting point of a material differ before and after recrystallization?4. Why should the crystals be washed with extra solvent when collected using suction filtration? Why cold solvent?

5. Why it is preferable to allow the hot solution to cool gradually instead of chilling immediately in an ice-bath?6. For what purpose is charcoal used in recrystallization?7. Results of solubility tests for compound A are shown in the following table (g/mL). Which solvent will you choose for recrystallization?Solvent Water EthanolDiethyl ether

Cold2035

Hot30255

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Procedure for CrystallizationThe photos below illustrate the process of crystallization.

Heat some solvent to boiling. Place the solid to be recrystallized in an Erlenmeyer flask.Pour a small amount of the hot solvent into the flask containing the solid.Swirl the flask to dissolve the solid.

Place the flask on the steam bath to keep the solution warm.If the solid is still not dissolved, add a tiny amount more solvent and swirl again.When the solid is all in solution, set it on the bench top. Do not disturb it!

After a while, crystals should appear in the flask.You can now place the flask in an ice bath to finish the crystallization process.

You are now ready to filter the solution to isolate the crystals. Please see the section on vacuum filtration. After your crystals are filtered from the solution, put them on a watch glass as shown below.

Here is the filter paper with crystals on it.Carefully scrape the crystals onto the watch glass.

Let the crystal finish drying on the watch glass.

You may Go to: http://orgchem.colorado.edu/hndbksupport/cryst/crystproc.html

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