Page 24 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

carboxy CO H groups each had characteristic reactivity that was largely independent 3
2
of the hydrocarbon portion of the molecule.
SECTION 1.1
Description of Molecular
Structure Using Valence
H
Bond Concepts
H H

H H
H
Kekule structure
for benzene

These structural formulas were developed without detailed understanding of the
nature of the chemical bond that is represented by the lines in the formulas. There was
a key advance in the understanding of the origin of chemical bonds in 1916, when
G.N. Lewis introduced the concept of electron-pair bonds and the “rule of 8” or octet
rule, as we now know it. Lewis postulated that chemical bonds were the result of
sharing of electron pairs by nuclei and that for the second-row atoms, boron through
1
neon, the most stable structures have eight valence shell electrons. Molecules with
more than eight electrons at any atom are very unstable and usually dissociate, while
those with fewer than eight electrons at any atom are usually highly reactive toward
electron donors. The concept of bonds as electron pairs gave a fuller meaning to the
traditional structural formulas, since the lines then specifically represent single, double,
and triple bonds. The dots represent unshared electrons. Facility with Lewis structures
as a tool for accounting for electrons, bonds, and charges is one of the fundamental
skills developed in introductory organic chemistry.
Lewis structures, however, convey relatively little information about the details
of molecular structure. We need other concepts to deduce information about relative
atomic positions and, especially, electron distribution. Valence bond theory provides
one approach to deeper understanding of molecular structure. Valence bond (VB)
theory has its theoretical foundation in quantum mechanics calculations that demon-
strated that electrons hold nuclei together, that is, form bonds, when shared by two
nuclei. This fact was established in 1927 by calculations on the hydrogen molecule. 2
The results showed that an energy minimum occurs at a certain internuclear distance
if the electrons are free to associate with either nucleus. Electron density accumulates
between the two nuclei. This can be depicted as an electron density map for the
hydrogen molecule, as shown in Figure 1.1a. The area of space occupied by electrons
is referred to as an orbital. A fundamental concept of VB theory is that there is
a concentration of electron density between atoms that are bonded to one another.
Figure 1.1b shows that there is electron density depletion relative to spherical atoms
outside of the hydrogen nuclei. Nonbonding electrons are also described by orbitals,
which are typically more diffuse than bonding ones. The mathematical formulation
of molecular structure by VB theory is also possible. Here, we emphasize qualitative
concepts that provide insight into the relationship between molecular structure and
properties and reactivity.

1 G. N. Lewis, J. Am. Chem. Soc., 38, 762 (1916).
2
W. Heitler and F. London, Z. Phys., 44, 455 (1927).