2.2.15 Sigma bonds and Pi bonds

pairs of electrons occupying atomic orbitals on neighbouring atoms are share using valence bond theory

• Its a given that bond strength increases with greater bond order.

  • The average bond entralphy of a C-C bond is $346kJmol^{-1}$

  • The average bond enthalpy of a C=C bond is $614kJmol^{-1}$

• Note that the enthalpy of the double bond is significantly higher but not double.

  • $\because$two bonds in the double bond is not the same:

    • One is a sigma bond ( $\sigma$)

    • One is a pi bond ( $\pi$)

    • $\pi$-bonds are weaker than $\sigma$-bonds

• Triple bonds are made from one sigma and two pi bonds.

Sigma bonds

• Single bonds are always sigma bonds

• Imagine two Hydrogen atoms coming towards each other, their $1s^1$orbitals will overlap, and when they do so the electrons form a covalent bond.

• The $1s$orbitals will overlap along an invisible line between the neclei of the two bonding atoms, known as the bond axis or internuclear axis.

  • There is a high electron density along this line

Pi bonds

• If Pi bonds are present in two neighbouring atoms, they can overlap sideways, above, and below the bond axis

• One pi bond has two lobes on opposite sides of the axis as shown in the picture above A pi bond contains two electrons which occupy both lobes