Page 730 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

8

Aromaticity

Introduction

The meaning of the word aromaticity has evolved as understanding of the special
1
properties of benzene and other aromatic molecules has deepened. Originally,
aromaticity was associated with a specific chemical reactivity. The aromatic hydro-
carbons undergo substitution reactions in preference to addition. Later, the idea
of special stability became more important. Benzene can be shown to be much
lower in enthalpy than predicted by summation of the normal bond energies for the
C=C, C−C, and C−H bonds in the Kekule representation of benzene (see p. 265).
Aromaticity is now generally associated with this property of special stability of
certain completely conjugated cyclic molecules. A major contribution to the stability
of aromatic systems comes from the delocalization of electrons in these molecules,
which also imparts other properties that are characteristic of aromaticity, especially a
diamagnetic ring current.
Aromaticity is usually described in MO terminology. Cyclic structures that have
a particularly stable arrangement of occupied molecular orbitals are called aromatic.
Hückel’s rule, a familiar expression of the relationship between an MO description of
structure and aromaticity, is derived from Hückel molecular orbital (HMO) theory and
states that planar monocyclic completely conjugated hydrocarbons will be aromatic
when the ring contains 4n+2 electrons. HMO calculations assign the -orbital
energies of the cyclic unsaturated systems of ring size three to nine as shown in
Figure 8.1. (See Section 1.5, p. 27 to review HMO theory.)
Orbitals below the dotted reference line in the figure are bonding orbitals; when
they are filled, the molecule is stabilized. The orbitals that fall on the reference
line are nonbonding; electrons in these orbitals are neither stabilizing nor destabi-
lizing. The orbitals above the reference line are antibonding; electrons in these orbitals

1
M. Glukhovtsev, J. Chem. Ed., 74, 132 (1997); D. Lloyd, J. Chem. Inf. Comput. Sci., 36, 442 (1996),
Z. Zhou, Int. Rev. Phys. Chem., 11, 243 (1992); J. P. Snyder, Nonbenzenoid Aromatics, Vol. 1, Academic
Press, New York, 1969, Chap. 1. A series of reviews on many aspects of aromaticity is published in
Chem. Rev., 101, 1115–1566 (2001).
713