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sp2 HYBRIDIZATION OF CARBON

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The electronic configuration of carbon in its ground state is:

Ground state

The promotion of one of the two 2s electrons to the empty pz orbital give the excited state.

Excited state

At this point we follow a different course than that used for sp3 hybridization, in which case one 2s and three 2p orbitals were mixed. Instead, the 2s electron and just two of three 2p orbitals are mixed or hybridized to give three new equivalent orbitals. These new orbitals are referred to as sp2 orbitals, because they are formed by interaction of one s and two p orbitals. Third 2pz orbital is left unhybridized. The electronic configuration of the carbon atom in its sp2 hybridized state is:

Hybridized state

Each sp2 orbital has an unpaired electron. The shape of sp2 orbital is similar to that of an sp3 orbital.

Formation of three equivalent sp2 orbitals

The sp2 orbitals obtained are identical, that is, they have the same energy and shape. They differ only in their orientation in space with respect to each other. The three sp2 orbitals lie in the same plane with their axes directed towards the corners of an equilateral triangle. The angle between any pair of orbitals is 120°.

Orientation of three sp2 orbitals

The smaller lobes are not indicated because they do not extend sufficiently far from nucleus to participate in bond formation. The trigonal arrangement is favoured because it allows the sp2 orbitals to stay as far apart from each other as possible and thereby reducing the electron-electron repulsion.

The unhybridized pz orbital is oriented along an axis perpendicular to the plane of sp2 orbitals, with each lobe above and below the plane of the sp2 orbitals. 

Orientation of the pz orbital

Whenever carbon is bonded to three other atoms or groups it always uses sp2 orbitals and a pz orbital to form its bonds. For example, ethylene. 

BONDING IN ETHYLENE

Each carbon atom in ethylene (H2C=CH2), is attached to two hydrogen atoms by single covalent bonds and to another carbon atom by a double bond. Since each carbon is attached to three other atoms, it uses sp2 hybrid orbitals and an unhybridized pz orbital to form its bonds. 

Bonding in ethylene

In ethylene there are four C-H single covalent bonds and one C-C double bond. Each C-H bond is a sigma bond and results from the overlap of one sp2 orbital from carbon and 1s orbital from hydrogen.

One of the two bonds in the double bond is also a sigma bond and results from end-to-end overlap of the sp2 orbitals, one from each carbon atom.

The second bond in the double bond is a pi bond results from the lateral overlap of two unhybridized pz orbitals, one from each carbon atom. pz orbitals can overlap only when all the six atoms lie in the same plane, that is, the plane of sigma bonds. Like the p orbitals from which it is formed, a pi bond consists of two equal parts. One part lies above the plane of the carbons and hydrogens and the other part lies below this plane. These two parts together make up one pi bond. 

Formation of the p bond in ethylene

 Although the C-C double bond is represented by two equivalent lines, remember that one line represents a sigma bond and the other the pi bond.

Double bond in ethylene