In Chapter 3, we identified an aromatic compound as being a compound which contains a benzene ring (or phenyl group). It is now time to define aromaticity in a more sophisticated manner. In this chapter, we discuss the stability of benzene and other aromatic compounds, explaining it in terms of resonance and molecular orbital theory. You will study the nomenclature of aromatic compounds and the Hückel (4n + 2) rule for predicting aromaticity. The chapter concludes with a brief summary of the spectroscopic properties of arenes.
After you have completed Chapter 15, you should be able to
- fulfil all of the detailed objectives listed under each individual section.
- use the information presented in this chapter, along with material from earlier chapters, to solve problems, particularly road-map problems and those requiring an understanding of spectroscopy.
- explain the concept of aromaticity and the stability of aromatic compounds.
- define, and use in context, the key terms introduced.
- 15.1: Introduction
- Aromatic hydrocarbons contain ring structures with delocalized pi electron systems and are more stable than their saturated analogs.
- 15.4: Aromaticity and the Hückel 4n + 2 Rule
- In 1931, German chemist and physicist Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties. His rule states that if a cyclic, planar molecule has 4n+2 π electrons, it is considered aromatic. This rule would come to be known as Hückel's Rule.
- 15.5: Aromatic Ions
- Carbanions and carbocations may also show aromatic stabilization.