Question

Why is C4H4S (thiophene) aromatic?

Answer 1

Thiophene is aromatic because it has six π electrons in a planar, cyclic, conjugated system.

Explanation:

The structure of thiophene is

At first glance, it appears that thiophene has only four π electrons in a butadiene system, plus an `sp^3` hybridized sulfur atom with two electron pairs.

However, if the sulfur atom re-hybridizes to `sp^2`, it will have an electron pair in a `3p` orbital that can overlap with the carbon `2p` orbitals on either side.

This gives a planar, cyclic, conjugated system that contains six π electrons.

According to Hückel's rule, then, thiophene is an aromatic compound.

The resonance energy of thiophene is 122 kJ/mol.

This is less than the resonance energy of benzene (152 kJ/mol), because the `3p` orbital of `"S"` is bigger than the `2p` orbitals on `"C"`, so the orbital overlap is less effective.

We can see this in a contour plot representing the electron density of thiophene

(from quantumwise.com)

The red colors show that the higher electron density is in the carbon portion of the molecule.