Chemistry Reference
In-Depth Information
that the
p
orbitals are positioned at right angles to
each other, whilst the
s
orbital is spherical and might
bond in any direction. If all the bonds in methane
turn out to be equivalent, they must be some sort of
'hybrid' version of those we have predicted. We can
explain many features of organic chemicals, including
their reactivity and shape, by a mathematical model
in which
hybrid orbitals
for carbon are derived by
mixing the one 2
s
orbital and three 2
p
atomic orbitals
(Figure 2.9). This generates four equivalent hybrid
orbitals, which we designate
sp
3
, since they are
derived from one
s
orbital and three
p
orbitals. The
sp
3
orbitals
will be at an energy level intermediate
between those of the 2
s
and 2
p
orbitals, and will
have properties intermediate between
s
and
p
, though
with greater
p
character. The mathematical model
then provides us with the shape and orientation of
these hybrid orbitals (Figure 2.10). For convenience
of drawing, we tend to omit the small lobes at the
centre of the array.
These new hybrid orbitals are then all equivalent,
and spaced to minimize any interaction; this is a
tetrahedral array, the best way of arranging four
groups around a central point. Each hybrid orbital
can now accommodate one electron.
Now we can consider the bonding in
methane
.
Using orbital overlap as in the hydrogen molecule as
a model, each
sp
3
orbital of carbon can now overlap
with a 1
s
orbital of a hydrogen atom, generating a
bonding molecular orbital, i.e. a
σ
bond. Four such
carbon
sp
3
-hybridized carbon
2
p
2
sp
3
2
s
1
s
1
s
mixing of 2
s
and 2
p
orbitals to
create
sp
3
hybrid orbitals
Figure 2.9
Electronic configuration:
sp
3
-hybridized carbon atom
+
sp
3
¼ s
¾ p
109˚
2
p
x
2
p
z
2
s
2
p
y
tetrahedral
sp
3
(small lobes omitted)
small lobes complicate
picture
Figure 2.10
sp
3
hybrid orbitals
