Recrystallization

Recrystallization Manrique, Gilbert Patrick L., Manuel, Roselle M., Munsayac, Rose Anne M., *Ngohayon, Hannako Faith T. , Ollica, Emmanuel John C.

Abstract Recrystallization is a common method of purifying organic compounds through their differences in solubility at different temperatures. The suitable solvent for crystallization carries the characteristic of changing its solubility in different conditions in temperature. In this experiment, acetylation of aniline with acetic anhydride was used to synthesize crude acetanilide. The crude acetanilide was purified by dissolving it in a hot water and then cooled slowly by placing in an ice bath. The pure acetanilide is then gathered as crystals since molecules of other compounds present in the crude acetanilide are excluded from growing crystal lattice.

Introduction (1)

 Natural organic compounds, whether solid or liquid, contain impurities, thus requiring different and various methods of purification. Purification methods to be applied are determined by the specific chemical and physical characteristics of certain compounds. For liquids, distillation is most commonly used and practiced on the  basis of the boiling points p oints of the compounds involved. For solids, purification can be achieved either by the process of sublimation or recrystallization. Organic compounds that undergo sublimation pass directly into the vapor phase without passing through the liquid phase. Organic compounds that lack this property and are solid at room temperature are purified by recrystallization which is the simplest and most widely used  purification method. (2) Recrystallization is a method in which two crystallization processes was  (3)  performed. Crystallization is the deposition of crystals from a solution. During the  process of crystal formation, a molecule have the tendency to be attached to a growing crystal composed with the same type of molecules because of a better fit in a crystal lattice of a molecule of the same structure (4) than for other molecule. Lattice is a fixed and rigid arrangement of atoms, molecules or ions.

The main factor that affects crystallization is solubility, which is the degree of substance that the solute is dissolved in a solvent. Before performing the recrystallization process, it is important to identify first a suitable recrystallizing solvent. The substance to be crystallized should show the ideal solubility behavior in the solvent. Thus, a substance can be purified when both the desired substance and the impurity have similar solubility at its boiling  point temperature, and when the impurity represents only a small fraction of the total solid. The desired substance will crystallize upon cooling, but the impurities will not. Methodology

In this experiment, the recrystallizing solvent was first determined. A corn-grain amount of pure acetanilide was placed into three test tubes. To the first test tube, distilled water was added. Methanol was added to the second and hexane was then added to the third test tube. After adding different solvents, the three test tubes were  placed in a water bath with a temperature of 37°C-40°C for 5 minutes. After heating, the test tubes were cooled by placing them in an ice bath. The solubility of the pure acetanilide in the different solvents at varying temperature was noted. This process

will determine the appropriate and the best solvent that will yield pure Acetanilide through the acetylation of Aniline and Acetic Anhydride.

fluted filter paper while it’s hot. The filtrate was allowed to cool by placing the receiver in a beaker containing tap water. The crystals were collected, washed with distilled water, and dried.

Figure 1. Determination of suitable recrystallizing solvent

In the preparation of Acetanilide, two milliliters of aniline and twenty milliliters of distilled water were mixed in a beaker. Then, three milliliters of acetanilide was slowly added to the mixture. The mixture was cooled with the use of ice bath to hasten the crystallization. When the crystals are fully formed, the solution was filtered which will  be the crude acetanilide was collected. After filtering, the crystals were dried and weighed. Twenty milliliters of the recrystallizing solvent was poured into the crude acetanilide. It was heated in a water  bath until the solid was dissolved. After that, the solution was filtered with the use of

Figure 2. Dried crude acetanilide from the first crustalllization process

Figure 3. Solution of the recrystallization  process is filtered using a fluted filter paper.

Results and Discussion

In picking the suitable recrystallizing solvent, some things should be considered. The compound should be very soluble at the  boiling point of the solvent and only sparingly soluble in the solvent at room temperature. This difference in solubility at hot versus cold temperatures is essential for the recrystallization process. If the compound is insoluble in the chosen solvent at high temperatures, then it will not dissolve while if the compound is very soluble in the solvent at room temperature, then getting the compound to crystallize in pure form from solution is difficult. Table 1 shows the solubility of pure acetanilide in various solvents. And based on the given properties, water is the suitable solvent for recrystallization because it changes the solubility of the pure acetanilide at various temperatures. If Methanol is used instead of water, pure acetanilide cannot regenerate to its crystal form. On the other hand, if hexane is used, pure acetanilide cannot be purified. In dissolving the solute, add just enough solvent to dissolve the solute at its  boiling point. If there is too much solvent used to dissolve the solute the tendency would be, when cooling the hot solution little or no solute will crystallize. The speed at which you allow a saturated solution to cool affects the size of the crystals that forms. If you take a hot solution and slam it into an ice  bath, you’ll get smaller crystals. If you allow a hot solution to cool without any added heat for 10-20 minutes, and then place it in an ice

Aniline

Table 1. Solubility of Acetanilide in various solvents

Water

Room Temp  Not Soluble

60°C Water Bath Soluble

Ice Bath  Not Soluble

Hexane

 Not Soluble

 Not Soluble

 Not Soluble

Met OH

Soluble

Soluble

Soluble

 bath to get larger crystals. This makes an important difference because if you have smaller crystals, they are more likely to clog a filter. Now,  in order to remove colored impurities when conducting crystallization, activated charcoal can be used to decolorize a solution as seen in Figure 3. Sometimes the sample collected is contaminated with a small quantity of insoluble impurity. To remove these impurities you can dilute the solution with excess solvent, use a pasteur pipette to vacuum the impurities out, centrifuge, evaporate excess solvent or perform filtration. Since the impurities in the solution are big, filtration would be enough to remove them. In filtering, consider the fastest way to let the solute pass through the filter to avoid the regeneration of the crystals upon filtering.

Acetic Anhydride

Figure 4. Reaction in the Synthesis of Acetanilide

Acetanilide

Acetic Acid

To retain the solubility of the solute make sure to filter it while its hot. And technique to avoid the unnecessary recrystallization is to use a fluted filter paper as shown in Figure 3. A fluted filter paper is a faster way to filter the crystals because it covers a greater surface area created by the grooves in the  paper. If the two did techniques did not work and crystals begin to form in the filter, just add excess solvent to the solute. Allow the solution to cool slowly in room temperature. This will result in a purer and bigger. Once recrystallization starts do not disturb until recrystallization is complete to achieve bigger crystals and better  purification. Once the solution has cooled to room temperature, cool it in an ice bath to maximize the amount of product that crystallizes out of the solution. Collect now the crystals through filtration let it dry and then weigh. Table 2. Weight of crystals collected Weight of crystals Crude acetanilide 0.8g Pure acetanilide 0.3g

To calculate for the percentage yield, the actual yield is divided to the theoretical yield. Theoretical yield is the maximum yield of pure crystals that could be obtained  by cooling or evaporating a given solution. This refers to the quantity of pure crystals deposited from the solution.

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      

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= 13.39%

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The percentage yield calculated is 13.39%. This is the pure acetanilide obtained

from the 2ml aniline and 3ml acetic anhydride. This represents how efficient is the synthesis of purification as well as measure the purification itself.

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     

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= 37.5% The percentage recovery calculated is 37.5%. This measures only the purification of the crystals.

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References

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