sp3 HYBRIDIZATION OF CARBON
In terms of energy level diagram, the electronic configuration of the carbon atom in its ground state may be represented as:
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| Ground state |
Since
there are only two unpaired electrons or half-filled orbitals, it might
be expected that only two single covalent bonds will be formed.
On this basis, carbon would combine with two hydrogen atoms (H=1s1) to form a molecule CH2. The two C-H bonds would be formed by overlap of the 2p orbitals (px and py)
with the 1s orbital of each hydrogen atom. Since the angle separating
the p orbitals is 90°, the C-H bonds would be at right angles to each
other. But from chemical analysis we know that the simple stable
compound that carbon forms with hydrogen is methane (CH4) and this compound contains four identical C-H bonds.
Now let’s assume that one of the 2s electrons in the ground state is moved to the empty pz orbital. Since 2pz
orbital is at a higher energy level than the 2s orbital, this promotion
process would require some input of energy. This energy is supplied in
the form of heat or light. This new state of carbon is referred to as
the excited state. In terms of energy level diagram, the electronic configuration of the carbon atom in its excited state is:
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| Excited state |
Since
there are four unpaired electrons or half-filled orbitals in the
valence shell of the carbon atom in its excited state, it might be
expected that four covalent bonds will be formed. On this basis, carbon
would combine with four hydrogen atoms to form a CH4 molecule. The three C-H bonds would be formed by the overlap of three 2p orbitals (px, py and pz) with the 1s orbital of each hydrogen atom. The 4th
C-H bond will be formed by the overlap of the 2s orbital of carbon with
the 1s orbital of a hydrogen atom. Since the angle separating the p
orbitals in an atom is 90°, the three C-H bonds may be expected to be at
right angles to each other. The 4th
C-H bond involving the overlap of s orbitals will not have any
directional characteristics because s orbitals are spherically
symmetrical. This implies that two different types of C-H bonds are
involved in the formation of methane molecule, right? Wrong!
Experimentally, methane has been shown to contain four identical C-H
bonds that are directed towards the corners of a regular tetrahedron.
To
form four identical bonds, carbon must contribute a set of four
equivalent orbitals. This can be achieved if the 2s and the three 2p
orbitals in the excited state are mixed or hybridized to give four new equivalent orbitals. These new orbitals are known as sp3 orbitals.
Mixing of a pure s orbital and three p orbitals is like mixing of a
gallon of pure red paint and three gallons of white paint to give four
gallons of pink paint. This process of mixing of pure orbitals to give a
set of new equivalent orbitals is termed as hybridization and the carbon is said to be in hybridized state. The electronic configuration of the carbon atom in sp3 hybridized state is:
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| Hybridized state |
In terms of energy level diagram, the above electron configuration may be represented as:
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| Four equivalent sp3 hybrid orbitals |
Each sp3 contains one electron. Since each sp3 orbital is obtained from one s and three p orbitals, it has 25% s-character and 75% p-character. Each sp3 orbital has a large lobe and a small lobe.
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| Shape of an sp3 hybrid orbital |
The four new sp3
orbitals obtained are identical (same energy and shape) but differ only
in their orientation in space with respect to each other. The four sp3
orbitals are arranged in such a way that their axes are directed
towards the corners of a regular tetrahedron with carbon located at the
centre. The angle between any two orbitals is therefore, 109°28’. The
smaller lobes are not indicated because they do not extend sufficiently
far from nucleus to participate in bond formation.
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| Orientation of four sp3 hybrid orbitals |
The tetrahedral arrangement is favoured because it allows the sp3
orbitals to stay as far away from each other as possible and thereby
reducing the electron-electron repulsion. This is in keeping with the
fact that each sp3 orbital contains an electron, and electrons stay as far apart as possible because they have the same charge.
BONDING IN METHANE
In
methane carbon forms single covalent bonds with four hydrogen atoms.
Since the carbon atom is attached to four other atoms it uses sp3 orbitals to form these bonds. Each C-H bond is the result of an overlap one sp3 orbital from carbon and 1s orbital from hydrogen.
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| Bonding in methane |
Since the four sp3
orbitals are oriented in such as a way that their axes are directed
towards the corners of a regular tetrahedron with carbon located at the
centre, the resulting C-H bonds are also directed towards the vertices
of a tetrahedron with carbon at the centre. Thus, the bonding angles in
methane are the same as the angles between the axes of the sp3 orbitals, that is, 109°28’.
The covalent bonds formed by the overlap of sp3
orbitals and s orbitals are sigma bonds because the electron density in
each bond is symmetrical about the line joining the centre of two
bonded atoms. Thus, all C-H bonds in methane are sigma bonds.
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