20.4 Aromatic, antiaromatic, nonaromatic

20.4 Aromatic, antiaromatic, nonaromatic

20.4.1 4N+2, Hückel’s rule

In these flat, completely conjugated ring systems, we find that there is a pattern for which types of molecules are aromatic and which are antiaromatic.

2 e^- = aromatic 4 e^- = antiaromatic 6 e^- = aromatic 8 e^- = antiaromatic

We find that is the number of electrons in the pi system = 4N+2, where N is an integer, then the system is aromatic. If the number of electrons in the pi system = 4N, then the system is antiaromatic. This is Hückel’s rule.

2 e^- = 4N+2 = 4(0)+2 = aromatic 4 e^- = 4N = 4(1) = antiaromatic 6 e^- = 4N+2 = 4(1)+2 = aromatic 8 e^- = 4N = 4(2) = antiaromatic