Huckel’s Rule: What Does 4n+2 Mean?
Last updated: January 27th, 2020 |
Hückel’s Rule: What Does 4n+2 Mean?
“4n+2 is not a formula that you apply to see if your molecule is aromatic. It is a formula that tells you what numbers are in the magic series. If your pi electron value matches any number in this series then you have the capacity for aromaticity.” – Claire
Table of Contents
- If You’re Looking For “n” In A Molecule, You’re Looking For The Wrong Thing
- “n” Is Not A Characteristic Of The Molecule!
- In Huckel’s Rule, The Formula (4n+2) Is An Algebraic Expression Of The Series 2, 6, 10, 14… Where ‘n’ Is A Natural Number
- Summary: “n” Comes From Algebra, NOT From Chemistry
- (Advanced) References and Further Reading
The other night a student came to me with a question about aromaticity.
“There’s one thing I don’t get”, she said. “They say a molecule has to have 4n+2 electrons to be aromatic. How do you find ‘n’ ?”
“n” is not a characteristic of the molecule! Let me explain.
In order for a molecule to be aromatic, it has to have the following characteristics:
- It must be cyclic
- It must be conjugated (i.e. all atoms around the ring must be able to participate in π-bonding through resonance)
- It must be flat
3. In Huckel’s Rule, The Formula (4n+2) Is An Algebraic Expression Of The Series 2, 6, 10, 14… Where ‘n’ Is A Natural Number
- CYCLOHEPTATRIENYLIUM OXIDE
W. von E. Doering and Francis L. Detert
Journal of the American Chemical Society 1951 73 (2), 876-877
In this paper, esteemed Harvard chemist William Eggers von Doering succinctly summarized the Huckel rule as 4n + 2 pi electrons (although writes it (2n+4 here) in his synthesis of cycloheptatrienylium oxide (“tropone”).