Gas Chromatography of Alkanes: Introducing Quantitative Structure-Retention Relationship (QSRR) Models in the Instrumental Analysis Laboratory

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

Table of Contents

Abstract Volume 22 (2017) pp 154-162

Gas Chromatography of Alkanes: Introducing Quantitative Structure-Retention Relationship (QSRR) Models in the Instrumental Analysis Laboratory

Steven C. Petrovic

Department of Chemistry, Southern Oregon University, 1250 Siskiyou Blvd., Ashland, OR 97520, petrovis@sou.edu
Received July 27, 2017. Accepted September 5, 2017.

Published: 29 September 2017

Abstract. The generation of a quantitative structure retention relationship (QSRR) model used to identifiy acyclic alkanes containing three to seven carbons is presented here as a complement to calibration-based experiments in instrumental analysis. Students are introduced to the concept of a topological index (TI) and to two TIs in particular, the Wiener index and the Randić molecular connectivity index, which relate the gas chromatographic (GC) retention of alkanes to their molecular structure in a QSRR model. A training set for each QSRR model is based on the retention of three normal alkanes (pentane, hexane, and heptane) on a thick film, non-polar, capillary GC column. Linear regression is applied to the logarithm of the adjusted retention time vs. calculated TI values for the training set in order to build each QSRR model. Of the two QSRR models examined, the Randić molecular connectivity index performed best by correctly identifying all normal alkanes and branched alkanes with either a single alkyl substituent, or with two methyl substituents located on the second carbon (i.e. 2,2-dimethylbutane and 2,2-dimethylpentane). Limitations in the Randić QSRR model in predicting the identity of highly branched alkanes is discussed with respect to the solvent accessible surface area-to-molecular volume (SASA/MV) ratio of branched vs. normal alkanes for each set of structural isomers. Branched alkanes with a SASA/MV ratio closest to that of the normal alkane isomer were successfully identified using the Randić QSRR model. Students use the Randić QSRR model in this experiment to identify normal and branched alkanes in refill canisters for butane torches.

Key Words: Laboratories and Demonstrations; instrumental analysis

(*) Corresponding author. (E-mail: petrovis@sou.edu )

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Supporting Materials:

In order to support the implementation of this experiment, either as a gas chromatography experiment or as a dry lab to introduce the concept of QSRR models to instrumental analysis students, two files have been included. The first file is the alkane GC-QSRR experimental handout (GC_QSRR_Experimental _Handout.pdf), which includes the group recitation exercise that introduces students to the relationship among topological indexes, molecular branching, and physicochemical properties (e.g. boiling point). The second file is an Excel spreadsheet (GC-QSRR Alkane Data Set.xlsx) containing gas chromatographic data for a calibration set, three branched alkane mixtures, and a butane refill. The second file is helpful for any instructor who wishes to introduce QSRR models into the instrumental analysis curriculum as a dry lab. (3870 KB)