Microscale Recrystallization

Microscale recrystallizations take place in the same way the big ones do. Youjust use teeny-tiny glassware. Read the general section on crystallization first. Then come back here and ... 

List the steps in the systematic procedure for microscale recrystallization, briefly explaining the purpose of each step. 

Trial and error is usually required in selecting a second solvent for a mixture. There are some generally successful mixtures, such as diethyl ether/methanol (or ethanol) for highly associated solids (especially amides and alcohols) and many natural products, and diethyl ether/petroleum ether (or benzene) for dipolar compounds (especially esters and alcohols) and hydrocarbons . Cf. also J. B. Baumann Solvent Selection for Recrystallization. J. Chem. Educ. 56, 64 (1979) R. E. R. Craig Rapid, Efficient Determination of Recrystallization Solvents at the Microscale Level, J. Chem. Educ. 66, 88 (1989). 

I ve put drawings of microscale equipment I ve had occasion to use in this section, along with some discussion ofthe 0-ring seals, conical vials, drying tubes, and so on. I ve put full descriptions of certain microscale apparatus with the operations they re used in. So Craig tubes show up with recrystallization the Hickman still is with distillation. 

After a recrystallization, you usually collect the new crystals by suction on a Buchner funnel (see Chapter 13, Recrystallization ). For microscale quantities you may have to use a Hirsch funnel—a tiny Buchner funnel with sloping sides and a flat porous plate (see Chapter 5, Other Interesting Equipment ). Or you might need the high-tech power of Craig tubes (see Chapter 14, RecrystalUzation Microscale ). Or you might be able to get away with a Pasteur pipet (Fig. 42). 

The procedures for performing dissolutions in single solvents at the mini- and microscales differ slightly, as described below. Discussion of the strategies for using mixed solvents for recrystallizations is presented as well. 

Analysis and Recrystallization Determine the weight and melting point of each crude product. If you know the relative amounts of benzoic acid (5) and naphthalene (7) in the original mixture, calculate the percent recovery of each compound. Recrystallize them according to the general microscale procedures provided in Section 3.2 and determine the melting points of the purified materials. 

Consult with your instructor before performing this experiment, in which you will determine the stereoselectivity of the reduction of benzoin with sodium borohydride. Follow either the Miniscale or Microscale Procedure described for reducing 9-fluorenone to reduce benzoin with sodium borohydride. but use ethanol rather than methanol as the solvent. After slowly adding 3 M HCI to decompose the excess borohydride. add enough water to adjust the solvent composition to 50% v v) ethanol and water. You may recrystallize the crude product from 50% (v v) ethanol and water. Obtain the melting point and the IR. H. and NMR spectra of the purified product for characterization. Compare these data with those for racemic and meso-hydrobenzoin to determine the identity of the product and the stereochemistry of the reduction. If authentic samples of racemic and meso-hydrobenzoin are available, determine mixed melting points to support your assignment. 

Design and implement an experimental protocol for performing the reaction of Equation 18.8. Because 9-anthraldehyde is relatively expensive, you should plan to carry out the reaction at the microscale level unless instructed to do otherwise. You should be sure that your proposal includes a means to analyze for the possible presence of both isomeric products in the reaction mixture. Appropriate quantities of reagents are 0.97 g of benzyltriphenylphosphonium chloride (4), 0.57 g of 9-anthraldehyde, and 1.5 ml of 50% (by mass) of aqueous sodium hydroxide solution. Use 2-propanol as the solvent for recrystallizing the crude product. 

Use of the Hirsch Funnel. The standard filtration system for collecting products purified by recrystallization in the microscale laboratory is vacuum (suction) ffltration with an 11-mm Hirsch funnel. Many reaction products that do not require recrystallization can also be collected directly by vacuum filtration. The Hirsch funnel (Fig.5.23a) is composed of a ceramic cone with a circular flat bed perforated with small holes. The diameter of the bed is covered by a flat piece of filter paper of the same diameter. The funnel is sealed into a filter flask with a Neoprene adapter (Fig. 5.23b). Plastic and glass varieties of this funnel that have a polyethylene or glass frit are now available. It is stfll advisable to use the filter paper disk with these funnels to prevent the frit from clogging or becoming discolored. Regardless of the type of fflter used, always wet the filter paper disk with the solvent being used in the crystallization and then apply the vacuum. This ensures that the filter paper disk is firmly seated on the bed of the filter. 

Another common purification method is recrystallization. It is operationally simple and can be done on quantities down to the milligram scale using conventional microscale techniques and apparatus. It is relatively easy to conduct the required manipulations so as to avoid the inadvertent dispersal of particulates. Compounds sensitive to radiationgenerated oxygen radicals in solution can be protected by working under an inert gas atmosphere. ... 

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