Organic Chemistry As a Second Language: Second Semester Topics 4th Edition

Book Preface

If you are using this book, then you have likely begun the second half of your organic chemistry course. By now, you have certainly encountered aromatic rings, such as benzene. In this chapter, we will explore the criteria for aromaticity, and we will discover many compounds (other than benzene) that are also classified as aromatic.

This type of drawing (a hexagon with a circle in the center) is not suitable when drawing mechanisms of reactions, because mechanisms require that we keep track of electrons meticulously. But, it is helpful to see this type of drawing, even though wewon’t use it again in this book, because it represents all six π electrons of the ring as a single entity, rather than as three separate π bonds. Indeed, a benzene ring should be viewed as one functional group, rather than as three separate functional groups. This is perhaps most evident when we consider the special stability associated with a benzene ring. To illustrate this stability, we can compare the reactivity of cyclohexene and benzene:

Cyclohexene is an alkene, and it will react with molecular bromine (Br2) via an addition process, as expected for alkenes. In contrast, no reaction occurs when benzene is treated with Br2, because the stability associated with the ring (of six π electrons) would be destroyed by an addition process. That is, the six π electrons of the ring represent a single functional group that does not react with Br2, as alkenes do.

Understanding the source of the stability of benzene requires MO (molecular orbital) theory. You may or may not be responsible for MO theory in your course, so you should consult your textbook and/or lecture notes to see whether MO theory was covered. Derivatives of benzene, called substituted benzenes, also exhibit the stability associated with a ring of six π electrons:

The ring can be monosubstituted, as shown above, or it can be disubstituted, or even polysubstituted (the ring can accommodate up to six different groups). Many derivatives of benzene were originally isolated from the fragrant extracts of trees and plants, so these compounds were described as being aromatic, in reference to their pleasant odors. Over time, it became apparent that many derivatives of benzene are, in fact, odorless. Nevertheless, the term aromatic is still currently used to describe derivatives of benzene, whether those compounds have odors or not.