In the Laboratory edited by
Computer Bulletin Board
Steven D. Gammon University of Idaho Moscow, ID 83844
Gas Chromatography and Molecular Modeling A Correlation Experiment for the Undergraduate Laboratory John M. Simpson* and Oswaldo Rivera Penn State, Beaver Campus, Monaca, PA 15061-2799; *
[email protected] Table 1. Representative Student Data
Toluene
182
1.49
1.47
Ethylbenzene
207
2.40
2.56
p-Xylene
212
2.51
2.69
2-Ethyltoluene
229
4.56
5.00
3-Ethyltoluene
233
4.11
4.63
1,2,4-Trimethylbenzene
228
5.37
5.61
NOTE: Results for column temperature of 122 °C.
Table 2. Student Correlation Results
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bp vs RT
A
B
A
B
132
9
.842
.886
.952
.980
1999
122
7
.887
.921
.963
.979
Note: RT is retention time; A is the Carbowax 20 M column; B is the DC 200 column.
Column A
Column B 5
Ret Time / min
5 4 3 2
0 110
Procedure
120
130
140
150
160
4
3
2
0 110
170
120
Boiling Point / °C
130
140
150
160
170
Boiling Point / °C 6
5
Ret Time / min
The procedure described below was used in the beginning organic chemistry lab in fall 1998 and fall 1999. Students calculated molecular surface area and measured gas chromatographic retention times for the series of six common aromatic hydrocarbons listed in Table 1. To find surface area, students modeled each of the arenes using PCModel (version 6.0, Serena Software) and calculated surface areas using the PCModel dot surface calculation (parameters: points per, 100; iterations, 20). A Gow Mac 400 Thermal Conductivity GC was used to measure retention times on both polar and nonpolar columns (column A: SS, 1.2 m, 20% Carbowax 20 M; column B: SS, 1.2 m, 20% DC 200; flow rate = 150 mL/min).
Correlation Coefficient R Surface Area vs RT
1998
Ret Time / min
We found that both of these goals can be achieved by limiting the number of arenes to six and the descriptors to molecular surface area and boiling point.
Column No. of T/°C Students
Year
1. It should be possible to collect the data and do the modeling in one or two lab periods. 2. It should provide reasonable correlation values.
Av Retention Time/min Av Surface Area/Å2 Carbowax 20 M DC 200
Compound
Ret Time / min
This paper describes a laboratory experiment in which first-year organic chemistry students at Penn State Beaver correlate gas chromatographic retention times for a series of aromatic hydrocarbons to two molecular descriptors (calculated molecular surface area and boiling point). Numerous examples of gas chromatography experiments (1) and molecular modeling exercises (2) for undergraduates can be found in the literature. However, we wished to create an experiment that would not only provide students with an introduction to gas chromatography techniques and molecular modeling but would also integrate gas chromatography data and molecular modeling results. The development of the experiment was initiated by a report of Woloszyn and Jurs (3) that gas chromatographic retention indices for aromatic hydrocarbons correlate well with various molecular size descriptors (such as partial positive charged surface area, partial negative charged surface area, molecular thickness, and molecular weight) as well as boiling point. Their results are based upon a large number (up to 67 in one case) of molecules, many molecular descriptors, and work being done by experienced experimenters. Clearly such an extensive investigation is not feasible in a beginning organic chemistry lab with its inherent time and resource constraints. We have found, however, that it is possible to simplify the Woloszyn and Jurs procedure and still obtain acceptable correlation results in the hands of undergraduates. We identified two important goals for the experiment:
4 3 2 1 180
190
200
210
220
230
240
5 4 3 2 1 180
Surface Area / Å2
190
200
210
220
230
240
Surface Area / Å2
Figure 1. Plots of retention time versus boiling point and surface area for polar (column A) and nonpolar (column B) columns.
Journal of Chemical Education • Vol. 78 No. 7 July 2001 • JChemEd.chem.wisc.edu
In the Laboratory
One section (1998) used a column temperature of 132 °C and the other (1999) used a column temperature of 122 °C. The GC was interfaced to an NEC CS500 computer (200 MHz) equipped with EZCHROM software to collect, display, and store the data. Boiling point data were obtained from the CRC Handbook (4 ). EZSTAT (version 1.0.0, Trinity Software) was used to determine four correlation coefficient (R) values: retention time vs boiling point on column A, retention time vs boiling point on column B, retention time vs surface area on column A, and retention time vs surface area on column B. Hazards All chemicals used in this experiment are used in very small (10-µL) quantities and should present minimal exposure hazard. All of the aromatic hydrocarbons are flammable and should be stored in a cool, well-ventilated area. In addition, they may be harmful or toxic if inhaled, ingested, or absorbed through the skin. Safety goggles and gloves should be used. Refer to MSDSs. Results Student correlation results from the two sections are summarized in Table 2 and the data are plotted in Figure 1. The correlation coefficients using boiling point and retention time are quite reasonable in both polar and nonpolar columns. They compare favorably to the Woloszyn and Jurs values in which a model using boiling points as the most significant descriptor is correlated to retention indices (SE-30, R = .998; Carbowax 20 M, R = .994). The correlation coefficients using molecular surface area and retention time are more dependent on column polarity and temperature than the values for boiling point vs retention time. However, an acceptable result (R = .921) was obtained at 122 °C in the DC 200 column. Woloszyn and Jurs report better values (SE-30, R = .993; Carbowax 20 M, R = .987) for a model in which molecular size is correlated to retention indices. However, their model is more complex (more than one size descriptor is used) and more samples (N = 65–67) are run.
Conclusion We believe that this experiment provides useful experience in gas chromatography techniques, the use of molecular modeling software, and performing correlation studies. Furthermore, it satisfies our goal of providing students with acceptable correlation results in a time frame suitable for an undergraduate lab course. The experiment also provides an opportunity to enhance student understanding of intermolecular forces by linking the interaction of arene molecules and stationary phases in a GC column to an experimental result (retention time). We asked students to comment on their correlation results by specifically addressing questions such as, Why does the nonpolar column give better R values for surface area vs retention time for this series of aromatic molecules? Why does the boiling point data give better R values than the surface area for this series of aromatic molecules? Why are the literature R values better than the student values (i.e., what could be done to improve the student values? Finally, although this experiment was done in an organic laboratory course, we are confident that it would also be appropriate for use in a general chemistry lab. Acknowledgment We wish to thank The Commonwealth College of the Pennsylvania State University for a grant to purchase the gas chromatograph and computer hardware and software needed to complete this project. Literature Cited 1. Wedvik, J. C.; McManaman, C.; Anderson, J. S.; Carroll, M. K. J. Chem. Educ. 1998, 75, 885 and references within. 2. Mebane, R. C.; Schanley, S. A.; Rybolt, T. R.; Bruce, C. D. J. Chem. Educ. 1999, 76, 688 and references within. 3. Woloszyn, T. F.; Jurs, P. C. Anal. Chem. 1993, 65, 582. 4. CRC Handbook of Chemistry and Physics, 65th ed.; CRC Press: Boca Raton, FL, 1984.
JChemEd.chem.wisc.edu • Vol. 78 No. 7 July 2001 • Journal of Chemical Education
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