Question

How do you count pi electrons in aromatic compounds?

Answer 1

`pi` bonds are simply the second bond made in a double bond. Any pure double bond is one sigma/`sigma` and one pi/`pi` bond.

• Since any one chemical bond (meaning only one line in bond line notation) contains at most two electrons, you can count two `pi` electrons per double bond, and ignore the `sigma` electrons.
• If you see lone pairs, consider the molecular geometry, and only the `pi` electrons that are in the ring count towards aromaticity.

Here are some examples of rings that may or may not be aromatic:

Note that the only `pi` electrons I've counted are in the ring. The others are either outside of the ring or `sigma` electrons.

Counting from top to bottom, column-wise:

1. Aromatic, because `4n + 2 = 6` `pi` electrons in the ring (with `n = 1`), planar, fully conjugated all around, and cyclic.

2. Aromatic, because `4n + 2 = 6` `pi` electrons in the ring (with `n = 1`), planar, fully conjugated all around, and cyclic. The `pi` electrons in the double bond outside of the ring do not count towards the `pi` electrons one considers for aromaticity.

3. Nonaromatic, because `4n + 2 ne 4` `pi` electrons, where `n` must be an integer. It's also not conjugated all around, so it's not antiaromatic. The `pi` electrons in the double bond outside of the ring do not count towards the `pi` electrons one considers for aromaticity.

4. Aromatic, because `4n + 2 = 6` `pi` electrons in the ring (with `n = 1`), planar, fully conjugated all around, and cyclic. The lone pair is actually in a pure `2p` orbital perpendicular to the ring. Don't be fooled, as the alkyl carbon has an implicit hydrogen.

5. Aromatic, because `4n + 2 = 6` `pi` electrons in the ring (with `n = 1`), planar, fully conjugated all around, and cyclic. The lone pair is actually in a pure `2p` orbital perpendicular to the ring, which means they count as `pi` electrons.

6. Aromatic, because `4n + 2 = 6` `pi` electrons in the ring (with `n = 1`), planar, fully conjugated all around, and cyclic. Only one of the lone pairs is actually in a pure `2p` orbital perpendicular to the ring, which means those count as `pi` electrons. The other lone pair is actually in a `sigma` (actually, `sp^2`) orbital, so it doesn't count. Thus furan is not antiaromatic.