COVALENT BONDING
33
only six electrons in its outer shell. This arrangement
is best accommodated by sp
2 hybridization and a
vacant p orbital. The alternative of four sp
3 hybrid
orbitals with one unfilled does not minimize repulsion
between the filled orbitals, and is also a higher
energy arrangement. To deduce this, we need to
go back to the energy diagrams for sp
3 and sp
2
hybrid orbitals. The lower p character of sp
2 hybrid
orbitals means they are of lower energy than sp
3
orbitals; this is because the 2p orbitals are of higher
energy than the 2s orbital. Consequently, we can
work out that six electrons in sp
2 orbitals will have
a lower energy than six electrons in sp
3 orbitals.
Hence, the methyl carbocation is of planar sp
2 nature
with an unoccupied p orbital at right angles to this
plane. The consequences of this will be developed in
Section 6.2.
There is also a third type of reactive species that we
shall discuss in detail in Chapter 9, namely radicals.
Briefly, radicals are uncharged entities that carry
an unpaired electron. A methyl radical CH 3
• results
from the fission of a C–H bond in methane so that
each atom retains one of the electrons. In the methyl
radical, carbon is sp
2 hybridized and forms three
σ C–H bonds, whilst a single unpaired electron is
held in a 2p orbital oriented at right angles to the
plane containing the σ bonds. The unpaired electron
is always shown as a dot. The simplest of the radical
species is the other fission product, a hydrogen atom.
C
H
H
H
methyl radical
CH 3
C
H
H
H
H
H
− H
•
cleavage of
bond;
each atom retains one electron
hydrogen atom
C H
2.6.3 Hybrid orbitals in oxygen and nitrogen
Hybridization concepts can also be applied to atoms
other than carbon. Here, we look at how we can
understand the properties of oxygen and nitrogen
compounds by considering hybrid orbitals for these
atoms.
Let us recap on the electronic configurations of
oxygen and nitrogen. Nitrogen has one more electron
than carbon, and oxygen has two more. For each
atom, we can consider hybrid sp
3 orbitals derived
from the 2s and 2p orbitals as we have seen with
carbon (Figure 2.20). We shall then obtain electronic
configurations in which nitrogen has two paired
electrons in one of these orbitals, whilst the remaining
three orbitals each have a single unpaired electron
available for bonding. Oxygen has two sets of paired
electrons and has two unpaired electrons available for
bonding.
1s
2s
2p
1s
2s
2p
1s
2s
2p
carbon
nitrogen
oxygen
Energy
1s
2sp 3
1s
2sp 3
1s
2sp 3
Energy
Figure 2.20 Electronic configurations: sp
3 -hybridized nitrogen and oxygen
33
only six electrons in its outer shell. This arrangement
is best accommodated by sp
2 hybridization and a
vacant p orbital. The alternative of four sp
3 hybrid
orbitals with one unfilled does not minimize repulsion
between the filled orbitals, and is also a higher
energy arrangement. To deduce this, we need to
go back to the energy diagrams for sp
3 and sp
2
hybrid orbitals. The lower p character of sp
2 hybrid
orbitals means they are of lower energy than sp
3
orbitals; this is because the 2p orbitals are of higher
energy than the 2s orbital. Consequently, we can
work out that six electrons in sp
2 orbitals will have
a lower energy than six electrons in sp
3 orbitals.
Hence, the methyl carbocation is of planar sp
2 nature
with an unoccupied p orbital at right angles to this
plane. The consequences of this will be developed in
Section 6.2.
There is also a third type of reactive species that we
shall discuss in detail in Chapter 9, namely radicals.
Briefly, radicals are uncharged entities that carry
an unpaired electron. A methyl radical CH 3
• results
from the fission of a C–H bond in methane so that
each atom retains one of the electrons. In the methyl
radical, carbon is sp
2 hybridized and forms three
σ C–H bonds, whilst a single unpaired electron is
held in a 2p orbital oriented at right angles to the
plane containing the σ bonds. The unpaired electron
is always shown as a dot. The simplest of the radical
species is the other fission product, a hydrogen atom.
C
H
H
H
methyl radical
CH 3
C
H
H
H
H
H
− H
•
cleavage of
bond;
each atom retains one electron
hydrogen atom
C H
2.6.3 Hybrid orbitals in oxygen and nitrogen
Hybridization concepts can also be applied to atoms
other than carbon. Here, we look at how we can
understand the properties of oxygen and nitrogen
compounds by considering hybrid orbitals for these
atoms.
Let us recap on the electronic configurations of
oxygen and nitrogen. Nitrogen has one more electron
than carbon, and oxygen has two more. For each
atom, we can consider hybrid sp
3 orbitals derived
from the 2s and 2p orbitals as we have seen with
carbon (Figure 2.20). We shall then obtain electronic
configurations in which nitrogen has two paired
electrons in one of these orbitals, whilst the remaining
three orbitals each have a single unpaired electron
available for bonding. Oxygen has two sets of paired
electrons and has two unpaired electrons available for
bonding.
1s
2s
2p
1s
2s
2p
1s
2s
2p
carbon
nitrogen
oxygen
Energy
1s
2sp 3
1s
2sp 3
1s
2sp 3
Energy
Figure 2.20 Electronic configurations: sp
3 -hybridized nitrogen and oxygen