Quantitative Thermodynamic Descriptions of Aromaticity. A Computational Exercise for the Organic Chemistry Laboratory

Abstract

This article describes an exercise that enables students to establish a quantitative scale of aromaticity via computer-driven quantum mechanical calculations using Spartan software. The method utilizes a group of analogous isodesmic reactions from which the energy difference between two isomeric cyclic polyenes is calculated from their optimized geometries. The energy differences found are used to characterize structures as aromatic, nonaromatic, or antiaromatic depending on the value obtained. A representative group of structures, including hydrocarbons, hydrocarbon ions, and heterocycles are studied. The exercise is designed for students in their first course in organic chemistry and can be completed in approximately 3–4 hours making it suitable for the organic laboratory. Huckel’s rule is illustrated using classic examples of aromatic and antiaromatic compounds. In addition, the computational model used is examined by comparison with other potentially useful models.