Page 26 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

is perfectly tetrahedral, the bond angles in ammonia and water are somewhat reduced. 5
This suggests that the electron-electron repulsions between unshared pairs are greater
than for electrons in bonds to hydrogen. In other words, the unshared pairs occupy SECTION 1.1
somewhat larger orbitals. This is reasonable, since these electrons are not attracted by Description of Molecular
Structure Using Valence
hydrogen nuclei. Bond Concepts
H

C N O
H H H H H
H H H
109.5° 107.3° 104.5°

The hybridization concept can be readily applied to molecules with double and
triple bonds, such as those shown in Scheme 1.1. Second-row elements are described
2
as having sp or sp orbitals, resulting from hybridization of the s orbital with two
or one p orbitals, respectively. The double and triple bonds are conceived as arising
from the overlap of the unhybridized p orbitals on adjacent atoms. These bonds have
a nodal plane and are called bonds. Because the overlap is not as effective as for
3
sp orbitals, these bonds are somewhat weaker than bonds.

trigonal orientation
2
of sp hybrid orbitals

digonal orientation
of sp hybrid orbitals

2
The prototypical hydrocarbon examples of sp and sp hybridization are ethene
and ethyne, respectively. The total electron density between the carbon atoms in these
molecules is the sum from the and bonds. For ethene, the electron density is
somewhat elliptical, because the component is not cylindrically symmetrical. For
ethyne, the combination of the two bonds restores cylindrical symmetry. The electron
density contours for ethene are depicted in Figure 1.2, which shows the highest density
near the nuclei, but with net accumulation of electron density between the carbon and
hydrogen atoms.

H H H
H
C C C H C C H
H H H
H H ethyne
ethene

The hybridization concept also encompasses empty antibonding orbitals, which
are designated by an asterisk . These orbitals have nodes between the bound atoms.
∗
∗
∗
As discussed in Section 1.1.8, and orbitals can interact with filled orbitals and
contribute to the ground state structure of the molecule. These empty orbitals are also