Recrystallization: Solvent and Solubility

Topics: Solvent, Solubility, Temperature Pages: 6 (1376 words) Published: February 1, 2013

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.

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...
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