Chapter 1 - Introduction—Molecular Structure and Reactivity

Abstract

This chapter presents a review of the fundamental principles relating to molecular structure, and of the ways in which structural information is conveyed. A crucial aspect of structure, from the mechanistic viewpoint, is the distribution of electrons. Hence, the chapter outlines how to analyze and notate electron distributions. The ability to construct Lewis structures is fundamental to writing or understanding organic reaction mechanisms. It is particularly important because lone pairs of electrons are frequently crucial to the mechanism, but are often omitted from the structures appearing in the chemical literature. There are two methods that are commonly used to show Lewis structures. One method shows all electrons as dots. The other one shows all bonds as lines, and all unshared electrons as dots. When the distribution of valence electrons in a molecule cannot be represented adequately by a single Lewis structure, the structure can be approximated by a combination of Lewis structures that differ only in the placement of electrons. Lewis structures that differ only in the placement of electrons are called resonance structures. These structures are used to show the delocalization of electrons and to help predict the most likely electron distribution in a molecule.