the microscale laboratory was calculated as an internal check on the precision of the work. The average value was E = 21.61 Llmol-cm. The relative standard deviation was 1.1%.Christian et. al. report a molar absorptivity for cyclopentanone in water of 21.1 Llmol-cm ( I ) . They also report a Kd = 2.64 for cyclopentanone distributed between carbon tetrachloride and water at 25 OC. The Gibb's free energy change for their system is -2.41 kJ. All of these values are in excellent agreement with the values reported in this study There are two observations that may be useful in carrying out this experiment. The lowest concentration of cyclohexanone may require extra shaking. Cyclohexanone appears to have some surfactant properties. The low concentrations of cyclohexanone do not allow as much surfise area between phases to form in the unstable emulsion as will form at the hieher concentrations. Eouilibriurn is attained more slowly &r the lower concentrat;ons. This effect may be responsible for the low values of Kd a t the low end of the concentration range. In addition, the measured absorbance values for the extracted low concentrations are small which makes these solutions sensitive to enor. The lower concentrations also may give some difficulty with air bubbles or bubbles of eyelohexane adhering to the cell wall and thereby interfering with the optical analysis. If this happens tip the cell over so that the bubble at the top of the cell passes over the transparent surface of the cell. If this fails, allow the cell to sit for a longer time. As variations, a class might be divided into groups each of which is assigned a different compound to explore the effect of various substituent groups on the distribution constant. Alternative15 the effect of added salts on the value of Kdcould be determined. The class used approximately 30 drops of cyclohexanone and 35 nL of cyclohexane. The entire class used the same set of cells, pipets, and volumetric flasks. The cost of the chemicals is estimated, using current prices, to be $15.23 for the cyclohexane and $0.62for the cyclohexanone. Most of the waste cyclohexane may be recovered by distillation. The cost of reagents works out to $1.13 per student.
of limonene have somewhat different odors, the students usually are able to determine (by comparison to authentic materials) which enantiomer is present in orange peel, illustrating the different properties of mirror images in biological systems. We have found this experiment to require about 2 hand to be rather popular with the students. Experimental Extraction
The rind of an orange is grated onto a piece of wax paper or aluminum foil (not plain paper; it will absorb the oil!) until approximately 2 g are obtained; the exact weight is recorded. The grated peel is transferred to a 10-mL Erlenmeyer flask and mixed with 2 mL of hexanes for a few minutes, after which the hexanes are pipetted into a filter pipet containing about 1 cm of granular sodium sulfate. The filtrate is collected in a tared 5-mL conical vial. The extraction is repeated a second time using another 2 mL of hexanes. Finally, the transfer pipet and filter are rinsed with a small amount of hexanes (approximately 0.5 mL). The solvent is removed by evaporation until the level of the orange liquid stops decreasing andlor the weight remains nearly constant for a few minutes. The weight and appearance of the crude oil is recorded, and the odor of the crude limonene is compared to that of authentic samples of (+)and (-)-limonene (available from Aldrich) provided by the
Acknowledgment
The author thanks the students of Quantitative Analysis for performing the experiment. Literature Cited I . L W ~ ,R.~.;christl.~.,s.~.:~ffap-g,~. E.J ~ h y cham e im,73,an3a78.
A Microscale Isolation of Limonene from Orange Peels Charles M. Gamer and Chad Garibaldi
Baylor University Waco, TX 76798 As recently observed (11,the teaching of laboratory techniques in the context of natural product isolationslmanipulations makes for especially interesting and relevant experiments. However, many natural products are found a t low concentrations in nature, complicating their isolation at the microscale. Orange peel is a rich and convenient source of the temene limonene. Macroscale isolations of limunene that rely on steam distillation have been reported (1, 2,. We have developed a rapid and reliable extmclive microscale isolation ofiimonene This experiment provides a context in which to teach extraction, preparative GC, IH spectroscopy, and capillary GC. Because the enantiomers A146
Journal of Chemical Education
-
0
1
2
3
4
5
6
7
8
Time (mid Preparative GC chmmatogram of orange peel extract (180 "C)
Purification and Analysis by Gas Chromatography The crude limonene is purified by preparative GC using a 20% Carbowax column (0.25-in. OD x 8-R. long) operated at approximately 180"C. Resolution of the limonene can be achieved with injections of at least 25 pL (and usually 50 pL). (See figure.) Generally, it is not necessary to wait for any late-eluting materials to clear the instrument before re-injecting. The limonene is collected in a tared tube and the process is repeated until all of the crude oil has been purified, which
