Recrystallization: Solvent and Solubility Essay

ABSTRACT

Recrystallization is the primary method for purifying solid organic compounds through the differences in solubility at different temperatures. In this experiment, a suitable solvent was first determined. Acetanilide was produced by acetylation of aniline with acetic anhydride. The crude acetanilide was dissolved in a solvent in a heating water bath. The hot solution was filtered and the filtrate, cooled slowly in an ice bath as crystals started forming out. As the compound crystallizes from the solution, molecules of other compounds were excluded from the crystals to obtain pure acetanilide.

INTRODUCTION

Recrystallization, referred to as the second crystallization, is a method in which two crystallization processes are performed. It is a process of dissolving the solid to be crystallized in a hot solvent or solvent mixture and then cooling the solution slowly. This technique can be used for large or small quantities of materials, and is usually very effective and efficient. The most important aspect of the recrystallization technique is the selection of the solvent. A large number of solvents are commonly used for recrystallization of organic compounds. Among these are water, ethanol, ethyl ether, methanol, and hexane to name just a few. A solvent, which works beautifully for one recrystallization, may be totally unsuitable for purification of a different compound – the choice for recrystallization solvent is made on a case-by-case basis.

This is because of the variation of solubility of different organic compounds in these solvents. A desirable recrystallizing solvent would increase the solubility of the solute as the temperature rises. Aniline is a clear to slightly yellow liquid with a characteristic odor. It is slightly soluble in water and mixes readily with most organic solvents. When exposed to light and air, it slowly changes color. It darkens because of atmospheric oxidation of an impurity usually present in the benzene from which the aniline was prepared. It is acetylated by warming a mixture of aniline and acetic anhydride, forming N-phenylacetamide, which is more commonly known as acetanilide.

When a solid material is placed in a solvent in which it has a low solubility, not much happens. A small fraction of the solid will dissolve, but the rest will not. They will not just sit there, though, since equilibrium is in effect, with solid molecules going into solution, and an equivalent number of dissolved molecules reforming solid, but the same amount of undissolved material is seen. One way to get the entire solid to dissolve is to heat the solution — most materials are more soluble in hot solvent than in cold. Suppose it is subjected to heat at the boiling temperature of the solvent (the highest temperature which is practical), and the entire solid goes into solution.

At some temperature, the concentration of solute will exceed its solubility, and the solid will start to come back out of solution – it will crystallize. By the time it returns to the initial temperature, nearly all of the original material has formed crystals, which can be removed from the solvent by filtration. If the original solid contained small amounts of impurities, the impurities would go into solution as well, but since the impurities are present only in small amounts, they remain soluble when the temperature is lowered again. Thus, the crystals, which come out of solution, are more pure than the original sample. This is the essence of purification by recrystallization (Fig. 1).

Figure 1: An impure solid in: (A) cold solvent in which it has low solubility, (B) solvent that has been warmed up to dissolve all of the solid, and (C) solvent that has cooled down to the original temperature. The crystallized solid is now pure, while the impurities remain in solution. The objectives of the experiment were, (a) to synthesize acetanilide by the acetylation of aniline, (b) to purify crude acetanilide product by recrystallization, and (c) to calculate the percentage yield of pure acetanilide.

EXPERIMENTAL

A. Compounds tested (or Samples Used)
For the determination of recrystallizing solvent, water, methanol, and hexane were tested. Aniline was allowed to react with acetic anhydride to produce the acetanilide. Activated charcoal was added for discoloration.

B. Procedure
In choosing the recrystallizing solvent, a corn-grain amount of pure acetanilide was placed into each of three test tubes. 1 mL of water, methanol, and hexane were added to each test tube. They were shaken, placed in a warm water bath (37 – 40o C) for 1 – 5 minutes, and allowed to cool. Observations were recorded. For the crude acetanilide, 2mL of aniline and 20mL of distilled water were mixed in an Erlenmeyer flask. 3mL of acetic anhydride was then slowly added. Noting the changes, the flask was cooled in an ice bath to hasten the crystallization. After cooling, the mixture was filtered, dried, and the crude acetanilide, weighed. In a beaker, the crude acetanilide was dissolved with 10mL of the recrystallizing solvent (water) for every gram of the crude acetanilide and then subjected to heat in a water bath until all the solid particles dissolved.

Figure 2: Fluted Filter Paper Folding

Removing the solution from the water bath, activated charcoal was then added to remove the colored impurities. After this, the solution was quickly filtered while it was still hot using a fluted filter paper. The residue was discarded and the filtrate was subjected to an ice bath for 5 – 10 minutes. It was then filtered to separate the crystals. When dried, the pure acetanilide was weighed.

The following set – ups were used:

Distilled Ice
Acetanilide
Erlenmeyer flask

Figure 3: Ice Bath

Figure 4: Filtration of Hot Solution Using Fluted Fillter Paper

RESULTS AND DISCUSSION

In selecting a suitable solvent, there is a set of characteristics to consider. First, the substance to be purified must have a high solubility in the solvent at its boiling point, and a significantly diminished solubility at lower temperatures (limited only by the freezing temperature of the solvent). Second, the solvent should dissolve the impurities at all temperatures. Third, the solvent must not react with the substance to be purified. Fourth, a reasonably volatile solvent is preferred.

This consideration is dictated by the need to ultimately remove traces of solvent from the crystals, and this is most easily accomplished by evaporation, either at room temperature or in an oven. Lastly, the solvent should be inexpensive, non-toxic, and non-flammable. These are not absolutely required, but desirable characteristics, which can help to determine the choice of a solvent. Water is an ideal choice based on these criteria, and is the preferred solvent if it satisfies conditions 1-4.

REFFERENCES

From books:
Bayquen, A. et al. (2009). Laboratory Manual in Organic Chemistry. Quezon City : C&E Publishing INC.

From the Internet:
ACETANILIDE (N-PHENYLACETAMIDE)
chemicalland21.com/lifescience/phar/ACETANILIDE.htm 08/04/12
Bunnelle, W. et. al. Recrystallization and Melting Point Determination Version 3. www.chem.missouri.edu/chem2050/Recrystallization%20notes.doc 08/04/12