In the Laboratory
An Integrated Extraction/Crystallization/Distillation Experiment Claude Amsterdamsky Laboratoire de Chimie Organique, ESPCI, 10 Rue Vauquelin, 75231 Paris Cedex 05, France Mastering extraction, crystallization, and distillation is a prerequisite for efficiency in the organic chemistry laboratory. In most laboratory textbooks (1) those operations are described separately or used to isolate a final product. A published procedure in this Journal (2) involves microscale separation of a five-component mixture in three 3-hour laboratory periods. However our aim was a shorter exercise on a standard scale. We therefore propose the separation and purification of a three-component mixture containing benzoic acid, 2,2,4-trimethylpentane (isooctane), and methoxybenzene (anisole) plus a trace of picric acid. The separation can be done during two 4-hour laboratory periods. For the past three years, this experiment has been carried out by students in our laboratory with excellent results. The principal components of the mixture are inexpensive and nontoxic. Benzoic acid is extracted by an aqueous solution of sodium carbonate and precipitated through acidification. Picric acid imparts a yellowish color to the crude acid. By recrystallization from boiling water pure colorless benzoic acid is obtained (mp = 120–122 °C, lit. 122.4 °C [3]). The liquid components are easily separated by distillation, as 2,2,4-trimethylpentane boils at 99 °C and methoxybenzene at 154 °C. A first crude distillation allows the students to observe the course of temperature change and to draw a graph of distillation temperature vs. volume showing the plateau between the two fractions. A careful redistillation yields 2,2,4-trimethylpentane and methoxybenzene with good purity. A flow chart outlining the procedure is
Benzoic acid 2,2,4-trimethylpentane Methoxybenzene Picric acid (trace)
Aqueous Sodium carbonate Extraction
Sodium benzoate Aqueous
2,2,4-trimethylpentane Methoxybenzene Organic
Concentrated HCl
Rough distillation
Benzoic acid Crude Recrystallization Water Benzoic acid Pure
Report weight m.pt.
2,2,4-trimethylpentane Impure Careful Distillation 2,2,4-trimethylpentane Pure
Report weight b.pt. G.C. analysis
Methoxybenzene Impure
given in Figure 1. A bulk solution is prepared from 1 L of 2,2,4-trimethylpentane, 1 L of methoxybenzene, 100 g of benzoic acid, and 0.36 g (0.02%) of picric acid; each student receives 50 mL of the mixture, containing 23.56 g of methoxybenzene, 16.11 g of 2,2,4-trimethylpentane, 2.32 g of benzoic acid, and ca. 8 mg of picric acid. After extraction and distillation the following results are obtained (Table 1). Experimental Procedure Benzoic acid, 2,2,4-trimethylpentane and methoxybenzene are not toxic but may cause slight irritation. Gloves are recommended for mixture handling. Picric acid is explosive in the dry state. However, the amount per student is very small (ca. 8 mg), and it is always in solution; therefore it does not present a significant danger.
Benzoic Acid Extraction Fifty milliliters of mixture is transferred to a separatory funnel and extracted twice with 25 mL of a 10% solution of Na2 CO3 in water, care being taken to vent the separatory funnel. Completeness of the extraction can be checked by extracting a third time and adding concentrated HCl to the solution; a precipitate of benzoic acid indicates an incomplete extraction. The aqueous phases are pooled and acidified to pH 1 by concentrated HCl, and the precipitate is vacuum-filtered on a Buchner funnel and washed with cold water. Recrystallization of the off-yellow acid in boiling water yields white crystals of benzoic acid (mp = 122 °C, Kofler hot stage). Separation of 2,2,4-Trimethylpentane and Methoxybenzene The mixture remaining in the separatory funnel is washed with water, decanted into an Erlenmeyer flask, dried over CaCl2 , and filtered through glass wool. The clear liquid is then transferred to a distillation flask equipped with a Vigreux column and heated with an electric heating mantle. Carborundum chips are added to regulate boiling. A first rough fractional distillation yields two fractions: A, with a boiling point below 105 °C, and B, with a boiling point above 105 °C A is redistilled, the first fraction being taken below 95 °C, the main fraction between 95 and 105 °C. B is then added to the cooled distilling flask. An intermediate fraction is taken between 105 and 155 °C, methoxybenzene being taken between 155 and 160 °C. Fraction purity can be checked by GC on a packed
Careful Distillation Methoxybenzene Pure
Report weight b.pt. G.C. analysis
Figure1. Flow chart for separation of the three-component mixture.
Table 1. Results for 108 Experiments Experimental Theoretical Yield Yield ± SD Yield Purity (g) (g) (%) (%) Benzoic acid 2.32 1.49 ± 0.32 64 – 2,2,4-Trimethylpentane
16.11
11.68 ± 2.58
72.5
90.2
Methoxybenzene
23.56
16.21 ± 3.03
68.6
98.7
JChemEd.chem.wisc.edu • Vol. 75 No. 2 February 1998 • Journal of Chemical Education
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In the Laboratory OV-225 column (5% on Chromosorb W-HMDS 80/100 mesh) at 80 °C. Acknowledgments We thank Delphine Vinclet for her help in testing these procedures and J. F. Lawrence for reviewing the text. Literature Cited 1. Fieser, L. F.; Williamson, K. L. Organic Experiments, 5th ed.; Heath: Lexington, MA, 1983; pp 23–62. Durst, H. D.; Gokel, G. W. Experimental Organic Chemistry, 2nd ed.; McGraw-Hill: New York, 1987; pp 74, 84, 95. Most, C. F., Jr. Experimental
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Organic Chemistry; Wiley: New York, 1988; pp 77–135. Harwood, L. M.; Moody, C. J. Experimental Organic Chemistry; Blackwell Scientific: Oxford, U. K., 1989; pp 114–121. Tietze, L.-F.; Eicher, T. Reactions and Syntheses in the Organic Laboratory; University Science Books: Mill Valley, CA, 1989; pp 23–25. Pasto, D.; Johnson, C. R.; Miller, M. J. Experiments and Techniques in Organic Chemistry; Prentice Hall: Englewood Cliffs, NJ, 1992; pp 43, 47, 56. Mayo, D. W.; Pike, R. M.; Trumper, P. K. Microscale Organic Laboratory, 3rd ed.; Wiley: New York, 1994; pp 61, 71, 90. Roberts, R. M.; Gilbert, J. C.; Martin, S. F. Experimental Organic Chemistry; Harcourt Brace: Fort Worth, TX, 1994; pp 69, 96, 129. 2. O’Hara-Mays, E. P.; Yuen, G. P. J. Chem. Educ. 1989, 66, 961– 964. 3. Handbook of Chemistry and Physics; Weast, R. C., Ed.; CRC: Cleveland, OH, 1975; p C-174.
Journal of Chemical Education • Vol. 75 No. 2 February 1998 • JChemEd.chem.wisc.edu
