Extensions of a Basic Laboratory Experiment: [4+2] and [2+2] Cycloadditions

Abstract

We describe an extended third-year undergraduate chemistry laboratory exercise in which a number of techniques and concepts are applied to the same set of chemical reactions. The reactions are the photochemical and thermal cycloadditions of s-nitrostyrene and 2,3-dimethylbutadiene. This can be viewed as a single long lab or a series of connected exercises exploring a number of themes. This serves as an example of the goals and pedagogy used in a course designed to prepare students for a fourth-year undergraduate thesis. The exercises include synthesis, NMR spectroscopy, and computer-based molecular modeling. The same reagents are used, but the photochemical reaction gives the [2+2] product and the thermal reaction produces the [4+2] product. These reactions are used as a basis for discussions of a number of topics: the Woodward�Hoffmann rules, stereochemistry and regiochemistry, kinetic versus thermodynamic control of reactions, analysis of stereochemistry by 1D NMR, introduction of 2D NMR methods, molecular modeling, and use of computer programs for accurate visualization. This set of exercises is an example of an extension of the typical three-hour lab into more detailed investigations and an introduction to research methods