# Is 2-pyrone an aromatic compound?

I was told by my professor that α-pyrone is not an aromatic compound, but I count two π-electrons as well as one of the lone pairs on oxygen giving six total which would be one of Hückel's $$4n + 2$$ numbers. Does the carbonyl contribute to aromaticity?

• Not everything that follows the 4n+2 rule is aromatic ... not to mention this rule is not so hot when we have a pendant pi-bonding atom. Jul 26 at 20:40
• Jul 26 at 21:25
• chemistry.stackexchange.com/questions/101265/… Jul 26 at 21:27
• The second one particularity for this link pubs.acs.org/doi/abs/10.1021/ja00750a055 Jul 26 at 21:30

This answer discusses this situation in connection with the closely related 2-pyridone. The $$4n+2$$ rule does not apply at all when there are pendant pi-bonding atoms, but in the case of a carbonyl oxygen atom (with its high electronegativity) we can consider a contributing structure where the carbonyl pi bond is polarized towards oxygen: $$\ce{\overset{+}{C} -\overset{-}{O}}$$. If this polarization leaves a ring satisfying the $$4n+2$$ rule, this zwitterion becomes a significant contributing structure and the molecule will show aromatic characteristics.

In the case of $$2$$-pyrone the zwitterion structure leaves six pi electrons in the conjugated ring and thus an aromatic contribution is expected. The proton nmr spectrum[1] shows proton NMR shifts greater than that in ethylene and closer to that in benzene, indicating aromatic character. At the same time Wikipedia reports that 2-pyrone

may participate in a variety of cycloaddition reactions to form bicyclic lactones. For example, it readily undergoes Diels-Alder reactions with alkynes producing, upon loss of carbon dioxide, substituted benzenes.[2]

Thus the compound has mixed aromatic and aliphatic characteristics, a common occurrence with molecules having an aromatic-zwitterion contribution from a pendant carbonyl oxygen.

References

1. Pirkle,W.H. & Dines,M. (1069). _J.Heterocyl. Chem. 6, 1 (1969).