Question

Determine why, the $\mathrm{\pi }$ molecular orbitals and energy levels of pyridine are different from benzene.

Short answer

In benzene, the six valence electrons are placed in the three lowest energies $\mathrm{\pi }$orbitals the electron distribution is the same in all six carbon-carbon bonds. As a result, benzene has carbon-carbon bonds of equal length. Hence, the energy level of $\mathrm{\pi }$molecular orbitals in pyridine is less than the$\mathrm{\pi }$ molecular orbitals in benzene.

In pyridine, the nitrogen atom is more electronegative than carbon-hydrogen bond. Therefore, $\mathrm{\pi }$molecular orbital in the pyridine is different and slightly distorted than $\mathrm{\pi }$ molecular orbital in benzene as in pyridine the electron density is displaced toward the nitrogen atom.

The coefficient that multiplies to $2p_z$orbital of nitrogen atom is larger for the linear combination of atomic orbitals as atomic orbital is used to construct the lowest $\mathrm{\pi }$ molecular orbital and contributes less to highest $\mathrm{\pi }$ molecular orbital.

Step-by-step solution

Breaking the degeneracy in the middle two sets of π  molecular orbitals

N atom breaks the degeneracy in the middle two sets of $\mathrm{\pi }$ molecular orbitals with the orbitals from $2p_z$ orbital on the nitrogen atom

Types of energies in the  π orbitals of pyridine

The $\mathrm{\pi }$ orbitals of pyridine have six different energies and the lowest three are filled with electrons.