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sp 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 gives the excited state.

Excited state

This time we follow a different course than that used for sp2 hybridization, in which case one 2s and two 2p orbitals were mixed. Instead, the 2s electron and just one of the three 2p orbitals are mixed or hybridized to form two new equivalent orbitals. These two new orbitals are known as sp orbital because they are formed by interaction of one s and one p orbital. The other two 2p orbitals (py and pz) are left unhybridized. The electronic configuration of carbon in its sp hybridized state is:

Hybridized state

Each sp orbital contains an unpaired electron. The shape of an sp orbital is similar to that of an sp2 orbital.

Formation of equivalent sp orbitals

They sp orbitals obtained are identical, that is, they have same the same energy and shape. They differ only in their orientation in space with respect to each other. They lie in a straight line, that is, the angle between the two sp orbitals is 180°. 

Orientation of two sp orbitals

The linear arrangement is favoured because it allows the sp orbitals to stay as far apart from each other as possible and thereby reducing the electron-electron repulsions. The unhybridized py and pz orbitals are at right angles to the line of the sp orbitals. 

Orientation of the unhybridized orbitals

Whenever carbon is bonded to two other atoms or groups it always uses sp hybrid orbitals and two 2p (py and pz) orbitals to form its bonds. For example, acetylene.

Bonding in acetylene

BONDING IN ACETYLENE

In acetylene (H-C≡C-H) each carbon atom is attached to one hydrogen atom by a single covalent bond and to another carbon atom by a triple bond. Since each carbon is attached to two other atoms it uses sp hybrid orbitals and two unhybridized 2p orbitals (py and pz) to form its bonds.

In acetylene there are two C-H single covalent bonds and one C-C triple bond. Each C-H bond is a sigma bond and results from the overlap of an sp orbital from carbon and 1s orbital from hydrogen.

One of the three bonds in C-C triple bond is also a sigma bond and results from the linear overlap of the two sp orbitals, one from each carbon.

The other two bonds in the triple bond are pi bonds and results from the lateral overlap of the unhybridized p orbitals on each carbon.

Formation of the two pi bonds in acetylene

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

Triple bond in acetylene