Bonding in Organic Molecules. Atomic-orbital Models

I n previous chapters, we have shown how you can use ball-and-stick models to predict the general arrangements in space of organic molecules. The sticks correspond to chemical bonds, which we represent in structural formulas as lines, or in Lewis struc'tures as pairs of dots denoting shared pairs of electrons. Remembering that electrons and nuclei are charged particles, and that it is electrical forces of attraction and repulsion between the electrons and nuclei that determine the bonding, perhaps we should be surprised that such simple mechanical models provide so much useful information. What we will try to do in this chapter is to show you how the modern electronic theory of chemical bonding provides strong support for the use of ball-and-stick models for many organic molecules, and also where it indicates that the models need to be modified or cannot properly represent the structural arrangement. There are several qualitative approaches to bonding in polyatomic molecules, but we shall discuss here the most widely used and currently popular approach. This approach involves setting up appropriate atomic orbitals for the atoms and considering that each bond arises from the attractive electrical forces of two or more nuclei for a pair of electrons in overlapping atomic orbitals, with each orbital on a different atom. The geometry of the bonds is assumed to be determined by the geometry of the orbitals and by the repulsive forces between the electrons. In the course of showing how this approach