Chapter 12: Recrystallization Recrystallization is a technique that chemists use to purify solid compounds. It is one of the fundamental procedures each chemist must master to become proficient in the laboratory. Recrystallization is based on the principles of solubility: compounds (solutes) tend to be more soluble in hot liquids (solvents) than they are in cold liquids. If a saturated hot solution is allowed to cool, the solute is no longer soluble in the solvent and forms crystals of pure compound. Impurities are excluded from the growing crystals and the pure solid crystals can be separated from the dissolved impurities by filtration. This simplified scientific description of recrystallization does not give a realistic picture of how the process is accomplished in the laboratory. Rather, successful recrystallization relies on a blend of science and art; its success depends more on experimentation, observation, imagination, and skill than on mathematical and physical predictions. Understanding the process of recrystallization in itself will not make a student a master crystallizer; rather, this understanding must be combined with laboratory practice to gain proficiency in this technique. 12.1 How Recrystallization Works You are likely familiar with a tasty demonstration of recrystallization: the formation of “rock candy.” Rock candy is made by dissolving sugar in hot water to the point of saturation. The clear solution is allowed to cool and evaporate very, very slowly. Days later, beautiful large crystals of pure sugar have grown in the liquid. Three terms are important to know for this process: solvent, solute, and solution. The following definitions are taken directly from the CRC Handbook of Chemistry and Physics: • Solvent: “That constituent of a solution which is present in larger amount, or, the constituent which is liquid in the pure state, in the case of solutions or solids or gasses in liquids.” • Solute: “That constituent of a solution which is considered to be dissolved in the solvent. The solvent is usually present in a larger amount than the solute. A solution is saturated if it contains at a given temperature as much of a solute as it can retain in the presence of an excess of that solute.” • Solution: “The word solution is used to describe a liquid or solid phase containing more than one substance, when for convenience one of the substances, which is called the solvent and may itself be a mixture, is treated differently from the other substances, which are called solutes.” In the case of rock candy formation, the solvent would be water, the solute would be sugar, and the solution would be the sugar-water. Recrystallizations in the organic laboratory proceed in a similar manner to the formation of rock candy (Figure 12-1). Just enough hot solvent is added to a small amount of an impure, solid compound in a flask to completely dissolve it. The flask then contains a hot solution, in which solute molecules – both the desired compound and impurities – move freely among the hot solvent molecules. As the solution cools, the solvent can no longer “hold” all of the solute molecules, and they begin to leave the solution and form solid crystals. During this cooling, each solute molecule in turn approaches a growing crystal and rests on the crystal surface. If the geometry of the molecule fits that of the crystal, it will be more likely to remain on the crystal than it is to go back into the solution. Therefore, each growing crystal consists of only one type of molecule, the solute. After the solution has come to room temperature, it is carefully set in an ice bath to complete the recrystallization process. The chilled solution is then filtered to isolate the pure crystals and the crystals are rinsed with chilled solvent. 99 Chapter 12: Recrystallization Figure 12-1: The general scheme for recrystallization. Although the terms “crystallization” and “recrystallization” are sometimes used interchangeably, they technically refer to different processes. Crystallization refers to the formation of a new, insoluble product by a chemical reaction; this product will then precipitate out of the reaction solution as an amorphous solid containing many trapped impurities. Typically, crystallization is the method used to obtain the crude product. Recrystallization does not involve a chemical reaction; the crude product is simply taken into solution, and then the conditions are changed so as to allow crystals to re-form. Recrystallization produces a much purer, final product. For this reason, experimental procedures that produce a solid product by crystallization normally include a final recrystallization step to give the pure compound. Recrystallization is not generally thought of as a separation technique; rather it is a purification technique in which a small amount of an impurity is removed from a compound. However, if the solubility properties of two compounds are sufficiently different, recrystallization can be used to separate them, even if they are present in nearly equal amounts. Recrystallization is not a perfect technique. A percentage of compound is always lost because some of the compound will be soluble in cold solvent. If there is a lot of an impurity with the same solubility properties as the compound in which you are interested, the recrystallization will not yield pure compound. Sometimes a chemist simply cannot find a solvent with the proper solubility characteristics to recrystallize a particular compound or to prevent oiling out during the process. This is a common problem when working with low-melting compounds. 12.2 How to Choose a Recrystallization Solvent A successful recrystallization depends on the proper choice of solvent. The compound must be soluble in the hot solvent and insoluble in the same solvent when it is cold. For the purpose of recrystallization, we consider 3% the dividing line between soluble and insoluble: if 3 g of a compound dissolves in 100 mL of a solvent, it is considered soluble, if not, it is considered insoluble. Table 12-1 contains hypothetical solubility data. In choosing a solvent, the bigger the difference between hot solubility and cold solubility, the more product will be recoverable from a recrystallization. Study Table 12-1 and convince yourself that the following statements are true: 100 Chapter 12: Recrystallization • Compound A is soluble in boiling water but not in cold water, therefore water would be a good choice for a solvent to recrystallize the compound. • Compound A is not soluble in either hot or cold hexanes; hexanes would be a poor choice to recrystallize this compound. • Compound B could not be recrystallized from water, although ethanol would be a good choice. • Compound B is soluble in hexanes both at high and at low temperatures, therefore it could not be recrystallized from hexanes. • Compound C could be recrystallized from methanol but not from water. • If Compound C were recrystallized from ethanol, the yield would be low. Table 12-1: Hypothetical solubility data. Compound Solvent Solubility in Boiling Solvent Solubility in Solvent at 0°C A Water 4 g/100 mL 0.2 g/100 mL A Hexanes 0.1 g/100 mL 0.0002 g/100 mL B Water 0.3 g/100 mL 0.02 g/100 mL B Hexanes 10 g/100 mL 9 g/100 mL B Ethanol 3 g/100 mL 0.1 g/100 mL C Water 4 g/100 mL 3 g/100 mL C Methanol 3 g/100 mL 0.3 g/100 mL C Ethanol 4 g/100 mL 2.8 g/100 mL Determining an appropriate recrystallization solvent might be a trivial process if extensive solubility data for all known organic compounds was listed in easy-to-read and readily available tables. This is not the case. Sometimes solubility data for a particular compound can be found in a laboratory text, the scientific literature, or the Merck Index. The CRC (Table C) only lists qualitative values for the solubility of organic compounds. Therefore, determination of a solvent with the appropriate solubility parameters for a particular compound usually is made by a trial-and-error process. Where do you begin this trial-and-error process of finding a suitable recrystallization solvent? First, consider the rule of thumb: “like dissolves like” (Table 12-2). A polar compound dissolves in a polar solvent and a non-polar compound in a non-polar solvent. Try one solvent and if it does not work, try another. The process is not always trivial and it is not always predictable. In the undergraduate organic chemistry teaching laboratories at CU, you will usually be told which solvent to use to recrystallize your product compounds. These recrystallization choices were determined by a process of trial and error, as in the sample procedure given below. 101 Chapter 12: Recrystallization Table 12-2: Best choice of solvent for recrystallizing various types of compounds. Compound type Solvents in which the compound is likely to be soluble Hydrocarbon Petroleum ether, hexanes, cyclohexane, toluene Ether Diethyl ether, methylene chloride Halide Dichloromethane, chloroform Carbonyl Ethyl acetate, acetone Alcohol, acid Ethanol Amine Ethanol, water A. Procedure for Determining a Recrystallization Solvent Place about 50 mg of the sample in a test tube. Add about 0.5 mL of cold solvent; if the sample dissolves completely, the solubility in the cold solvent is too high to be a good recrystallization solvent. If the sample does not dissolve in the cold solvent, heat the test tube until the solvent just boils. If the sample has not completely dissolved at this point, add more boiling solvent dropwise until all the solid dissolves. If it takes more than 3 mL to dissolve the sample in the hot solvent, the solubility in this solvent is probably too low to make it a good recrystallization solvent. If your first choice of solvent is not a good recrystallization solvent, keep trying others.