J a c o b B. Baumann Schuylkill Campus Schuylkill Haven. Pennsylvania 17972
Solvent Selection for Recrystallization An undergraduate organic experiment
Many introductory organic chemistry laboratory manuals provide the be~inningstudent with a background to the selection of a recrystallization solvent. Such a discussion usually relates the polarity of the solvent to the polarity of the molecule of the compound to be recrystallized. This leads to the generalization that polar solvents work well for the purificas n o n - ~ o l a solvents r are best for tion of polar c o m ~ o u n d and non-polar vompohds. In practice:the initial choice of a solvent for a recrystallization is often hased uDon the chemist's comparison of the polarity of each solventto his estimate of the polarity of the compound to be purified. However. in anv recr&alliiation finding solvent to dissolve thk compound is only part of the problem, since the substance must then be sufficiently insoluble to crystallize from solution upon cooling. Final selection must be based upon a determination in the laboratory of the solvent which actually works best. We have devised an experimental procedure which develops the student's ability to carry out a simple recrystallization effectivelv. and which demonstates how a solvent mav be selected as "sable or rejected as unusable for the recrystallization of a specific oraanic c o m ~ o u n dSolvents . are reiected if the comp"und diss(~~lvea easil;. at room temperature*(part a ) or is insuluhle in a volume of boi!ine solvent which is laree relative to the weight of the sample (part c). In addition, t i e empirical nature of the choice of a solvent and the determination of the volume of solvent to he used are emphasized, especially since the identity of the solid is unknown. T h e experiment also provides answers to such recurring questions as "What does i t mean 'recrystallize from this solvent'?" and "How much solvent should I use?"
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Experlmental Obtain a solid recrystallization unknown. Place a small amount (about enough to fill the curved bottom of a 10 X 15-mm test tube) in each of several graduated 10-ml Erlenmeyer flasks (one for each solvent to be tested). a) Add 2 ml of a differentsolvent to each flask and tap or swirl the flask for a minute or two to attempt to dissolve the solid. The contents of thnse flasks in which the solid has dissolved may be discarded. b) Warm each flask containing undissolved unknown gently and see if the solid dissolves. The flasks in which the unknown dissolves completely should he corked and allowed to stand at room temperature for 15'min. c) Treat each of the flasks whieh still contain undissolved solid as follows. Heat the mixture in each flask to boiling. Slowly increase the volume of the bailing mixture by gradual addition of solvent,keeping the mixture at or near the boiling point. Continue until the solid dissolves or until the total volume is 5 ml. Each solution which resulh should be corked and let stand at room temperature for 15 min. Thwe
84 / Journal of Chemical Education
flasks in which the solid is not completely dissolved after heating may be set aside. After the solutions from parts (b) and (e) have been standing for 15 min, check each flask to see if crystals have formed. Failure of crystallizationto take place may be the result of supersaturation: some of the compound may become insoluble as the solution cools to room temperature, but may not crystallize because equilibrium between solution and solute has not been reached. Crystal formation can sometimes be initiated by scratching the side of the flask with a glass rod which has not been fire-oolished.Place each flask in s~. Dan of ice and water slush tor 10 min. Check again 18, see ~t'crysLllshaw formed. If not, scratch therideuf the flask with n glass rod ns bdurr. You should have found one or more solvents from which your unknown can be recrystallized and have determined suitable conditions for recrystallizatibn. If two or more solvents can be used, choose the one whieh seems most satisfactory for use in the next part. d) Weigh 1.03g of your unknown and place in a 125-mlErlenmeyer flask. Add solvent to the flask until the liquid just kovers the solid. Heat to boiling and slowly add solvent to the boiling mixture until the solid completely dissolves. Proceed with the recrystallization as indicated in your preliminary experiments. Suction-filter the crystals, let them dry until the next laboratory period, weigh the crystals, and determine the melting paint. ~
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Results The solvents chosen for the experiment were water, methanol, carbon tetrachloride, toluene, petroleum ether (bp 30'-60°C). acetic acid, and acetone. Each of the 19 compounds listed was tested according to the above procedure. Satisfactory results were obtained for the solvent and compound pairs listed below. Water: salicylic acid, adipic acid, acetanilide, henzoic acid, 2-nitrobenzoic acid, p-benzoquinone, 4-nitroaniline. Methanol: naphthalene.. di~henvl. - ~~. - . methvl3-nitrohenzoate. 4.nitroheneoi(: acid, triphenylmethanol. .1-nitroacetanilide, Carhon tetrachloride: trinhenvlmethanol. . . Toluene: 4-methoxybenzoic acid, anthranilamide, 4-aminobenzoic acid, 4-nitroacetanilide. Petroleum ether: 2-methylhenzoic acid, stilbene, l-nitronaphthalene. Acetic acid: stilbene. Acetone: Not suitable for any of the above compounds. ~
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Comments Solid compmnds used were supplied to thestudents in the form of pure samples, as obtained commercially. The identity of the compound supplied to each student was not disclo.sed. Students were graded upon their writwn report and the purity of their recrvstalli~eds a m ~ l e but . not uoon their choice of solvent for the recrystallizk&.
