The electronic configuration of carbon in excited state is given as:
6C : 1s2 2s1 2px1 2p y1 2p z1
In excited carbon atom there are four half-filled orbitals available for bond formation resulting in the formation of four covalent bonds.
It should be noted, however, that the four half-filled valence orbitals are not equivalent, there being three p-orbitals and one s-orbital. As such the four bonds would not be equivalent, the bond using the s-orbital would be different from the three bonds using the p-orbital. Actually carbon forms many compounds such as carbon tetrachloride in which all the four bonds are equivalent. To account for the equivalence of the four bonds, it is assumed that the four available orbitals of carbon, the 2s and three 2p orbitals are mixed or hybridized in a manner as to result in four equivalent orbitals.
The phenomenon of intermixing of orbitals of the same atom having the slightly different energies to form new orbitals which have identical shapes and equivalent energies is known as hybridisation.
Carbon atom can undergo three types of hybridisation in its compounds.
Sp3 HYBRIDATION ( TETRAHEDRAL HYBRIDISATION)
In this type of hybridisation, four orbitals (one 2s and three 2p) of the excited carbon atom hybridise to form four orbitals of equivalent energy and same shape. Each orbital is called Sp3 -hybrid orbital. The four hybrid orbitals are directed towards the comers of a regular tetrahedron. The hybridisation is called tetrahedral hybridisation and the angle between the hybridised orbitals is 109°-28′. (see Fig. 36.3.)
Structure of methane (CH4)
Carbon in methane is sp3 hybridised Here, one orbital of 2s-sub-shell and three orbitals of 2p-sub-shell of excited carbon atom undergo hybridisation to form four sp’3 hybrid orbitals. The process involving promotion of 2s-electron followed by hybridisation is shown in Fig. 36.4.
The carbon atom lies at the centre of the tetrahedron while the four hydrogens occupy the four comers or vertices of tetrahedron as shown in Fig. 36.5.
Structure o Ethane ( C2H6)
In ethane molecule, both the carbon atoms are sp3 hybridised. The geometry around both the carbon atoms is tetrahedral as shown in Fig. 36.6. C-C bond length is 1.54 A and C-H bond length is 1.09 A.
sp2 HYBRIDISATION (TRIGONAL HYBRIDISATION)
In this type of hybridisation, the 2s orbital and two of three 2p orbitals of excited carbon atom hybridise to form three sp2 hybridised orbitals of equivalent energy and identical shape. Each orbital is called sp2-hybrid orbital.
The three sp2 orbitals lie in one plane making an angle of 120° with each other. A sp2 hybridised carbon is called trigonal carbon atom and the hybridisation is known as trigonal hybridisation. ·
Unhybridised 2pz orbital is oriented in a plane at right angle to the plane of three hybridised orbitals. (Fig. 36.8)
Structure of Ethene (CH2 = CH2)
Both the carbon atoms in ethene are sp2 hybridised. All the atoms in ethene lie in the same plane. The unhybridised p-orbital on each carbon is perpendicular to the plane of these atoms, and overlap sidewise to form a 1t-bond between the two carbon atoms as shown in Fig. 36.9.
sp-HYBRIDISATION OR DIAGONAL HYBRISATION (LINEAR HYBRIDISATION)
In this type of hybridisation, the ·2s and only one of the 2p (say 2p) orbitals hybridise forming two equivalent orbitals called sp or diagonal orbitals.
The remaining two 2p orbitals are left in their original state.
The two sp-hybrid orbitals (each of which consists of Two lobes, one big and one small) lie along a straight line and thus make an angle of 180° with each other. The two 2p orbitals which are left in their original state lie in different planes at right angles to each other and also to the hybridised orbitals (Fig. 36.11 ).
Structure of Ethyne (HC = CH)
Both the carbon atoms in ethyne are sp-hybridized. The overlapping of orbitals has been shown in Fig. 36.12.
The π cloud further merge to form a single cylindrical electron cloud along the internuclear axis (Fig 36.12).
Fig. 36.12. The a and n-bonds in ethyne Cylindrical π – electron cloud in acetylene
It may be noted that by observing the structure of a molecule it is possible to predict the state of hybridisation of carbon.
A carbon atom that is directly linked to four other atoms is sp3-hybridised.
A carbon atom that is directly linked to three other atoms is sp2- hybridised.
A carbon atom is sp-hybridised if it is directly linked to Two other atoms.