Page 29 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

8 This structure minimizes electron-electron repulsions and obeys the Pauli principle by
maximizing the separation of electrons having the same spin.
CHAPTER 1
π-bond bent bond double bond with
Chemical Bonding
and Molecular Structure unpaired electrons
x o x
O O O O o x O x o O x o
x o x

double tetrahedra
2
sp hybridizatio n bent bond sp 3
model; dioxygen O 2
model model is paramagnetic
A similar representation of N with offset of the tetrahedral of electrons correctly
2
describes the molecule as having a triple bond, but it is diamagnetic, since there are
equal numbers of electrons of each spin. For ethane, all the electrons are bonding and
are attracted toward the hydrogen nuclei, and the tetrahedra of electrons of opposite
spin both occupy a region of space directed toward a hydrogen nucleus.
H x x
o x o o H
x o x o
o x x x
o O x O o x x N N o x H o C o C o H
x o x x
x o x o x H x o o H

double quartet model
double quartet model double quartet model
five o electrons seven o electrons hydrogen nuclei promote
seven x electrons seven x electrons coincidence of tetrahedra
For most of the molecules and reactions we want to consider, the Pauling
hybridization scheme provides an effective structural framework, and we use VB theory
to describe most of the reactions and properties of organic compounds. However, we
have to keep in mind that it is neither a unique nor a complete description of electron
density, and we will find cases where we need to invoke additional ideas. In particular,
we discuss molecular orbital theory and density functional theory, which are other
ways of describing molecular structure and electron distribution.

1.1.3. Electronegativity and Polarity

The VB concept of electron-pair bonds recognizes that the sharing of electrons
by the nuclei of two different elements is unequal. Pauling defined the concept of
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unequal sharing in terms of electronegativity, defining the term as “the power of an
atom in a molecule to attract electrons to itself.” Electronegativity depends on the
number of protons in the nucleus and is therefore closely associated with position in the
periodic table. The metals on the left of the periodic table are the least electronegative
elements, whereas the halogens on the right have the highest electronegativity in each
row. Electronegativity decreases going down the periodic table for both metals and
nonmetals.
The physical origin of these electronegativity trends is nuclear screening. As the
atomic number increases from lithium to fluorine, the nuclear charge increases, as does
5
L. Pauling, J. Am. Chem. Soc., 54, 3570 (1932).