Question

Pyridine and pyrimidine are both aromatic compounds, how many electrons does each nitrogen atom contribute to the electron cloud of each compound?

Answer 1

Okay, let's see.

This is asking you to consider which of nitrogen's valence electrons count towards the `2n+2` rule (Hückel's Rule) when it comes to aromaticity.

The only ones that are delocalized within the `pi` electron cloud are the ones in the ring, i.e. not in an orbital perpendicular to the `2p_z` (vertical) orbitals of each carbon in the ring.

There are only three electron groups around each nitrogen, indicating a trigonal planar electron geometry.

Since the `sp^2` orbitals are in the same plane as the rest of the molecule, and since the `z` axis is vertical and the `xy`-plane is horizontal, the `xy`-plane is perpendicular to the `z` axis; so, the `sp^2` orbital of nitrogen is perpendicular to the `2p_z` of each carbon.

Hence, the `sp^2` orbital does not contribute its lone pair into the ring, and nitrogen only contributes electrons into the ring that are already bonding.

Each nitrogen therefore contributes approximately three `pi` electrons to the aromatic ring. If we assume the electronegativities of carbon and nitrogen are similar enough, then there would be three `pi` electrons per nitrogen.