Effect of Hybridisation on Carbon Compounds
Hybridisation greatly affects the bond length, bond enthalpy and electronegativity of a carbon compound.
Hybridisation in Carbon Compounds
This chapter is based on the knowledge of hybridisation in carbon. You are supposed to know the answer to the following question :
The hybridisation of carbon atoms numbered 1,2,3 and 4 is :
Find hybridisation of each carbon
Three types of hybridisation − sp, sp2 and sp3, are seen in carbon atoms depending upon how many atoms are linked to the carbon.
An sp hybrid orbital has 50% s-character and 50% p-character; an sp2 hybrid orbital has 33.33% s-character and 66.66% p-character; an sp3 hybrid orbital has 25% s-character and 75% p-character.
Hybridisation − Carbon
Bond Length
Bond length is defined as the equilibrium distance between the nuclei of two bonded atoms in a molecule. Each atom of the bonded pair contributes to the bond length. In case of a covalent bond, the contribution from each atom is called the covalent radius of that atom.
The bond length in a covalent molecule AB
R = rA + rB (R is the bond length and rA and rB are the covalent radii of atoms A and B respectively.)
Fig. 1
Effect of Hybridisation on Single, Double and Triple Bond Lengths of Carbon
Since the sp hybrid orbital contains more s-character (50%), it is closer to its nucleus; therefore, it forms shorter bonds. Because of the same reason sp2 hybrid orbital forms shorter bonds than sp3 hybrid orbitals.
The single, double and triple bond lengths in carbon follow the order :
-C−C > -C=C- > -C≡C-
Bond Enthalpy
The amount of energy required to break one mole of bonds of a particular type between two atoms in a gaseous state is known as bond enthalpy. The stronger the bond, the more energy is required to break it. Because of this reason bond enthalpy is also called bond strength.
The unit of bond enthalpy is KJ mol-1.
H2 → H(g) + H(g); ΔH = 435.8 KJ mol-1
Effect of Hybridisation on Bond Strength (Bond Enthalpy)
The strength of the bond increases as the length of the bond decreases. As a result, bond enthalpy decreases from sp to sp3 i.e., : sp > sp2 > sp3. In terms of C−C bond, bond enthalpy follows the order :
C≡C (strongest) > C=C > C-C
This can also be understood in terms of σ and π bonds. A carbon-carbon π bond is weaker than a carbon-carbon σ bond because the extent of overlap in sideways overlap (overlapping in π bond) is small. Since a carbon-carbon double bond contains a weak π bond in addition to a strong σ bond, it is stronger than a single bond which only consists of a strong σ bond. Similarly, a triple bond is stronger than a double bond because it contains two weak π bonds in addition to a strong σ bond (σ and π bonds are discussed in Overlapping of Orbitals).
Electronegativity
The tendency of an atom to attract bonding electrons towards itself is called its electronegativity.
Effect of hybridisation on electronegativity
The greater the s-character of the hybrid orbitals, the greater is the electronegativity because an s orbital holds electrons more tightly to the nucleus.