We learned in detail how HMO theory explains the structure and aromaticity of benzene in previous posts. In this post let us compare benzene to another organic species and see how it differs from it.
Let us compare cyclobutadiene and benzene-
As seen in the above figure, cyclobutadiene has total four π electrons (as it has two double bonds).By Hund’s rule (refer post # 28) , two electrons will pair up in the lowest energy level (α+2β) but the next two electrons will remain unpaired as there are two non-bonding degenerate energy levels(α).Thus, the system will exist as a diradical. The electrons in the non bonding levels DONOT contribute to the stabilization of the molecule. Furthermore, because these electrons occupy a high energy orbital, they are particularly available for chemical reactions. Thus, cyclobutadiene is extremely unstable. It is antiaromatic.
[Note – Experimental evidence shows that cyclobutadiene does not have a square geometry but is rectangular. In the rectangular geometry, 2 non bonding levels are no longer degenerate- they are of slightly different energies. Thus, one level is doubly occupied and one empty. So, cyclobutadiene does not have unpaired electrons.
The above figure shows the two bond lengths in cyclobutadiene.]
In contrast as we know , benzene is aromatic i.e it has extra stability. This is because all its electrons are paired and in bonding MOs.
In the next post we will study antiaromaticity in greater detail. Till then ,
Be a perpetual student of life and keep learning ….
Good day !
Madoverchemistry 