Q42E On reaction with acid, 4-pyrone ... [FREE SOLUTION]

Chapter 15: Q42E (page 477)

On reaction with acid, 4-pyrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the H眉ckel鈥檚 $4 n + 2$ rule, explain why the protonated product is so stable.
4-Pyrone

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Identical resonance forms

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Resonance structures

Resonance structures or canonical structures are the sets of Lewis structures that are arranged in a way better manner to explain the conjugation of bonds that are not explained by a single Lewis structure.

Resonance structure of 4-pyrone

The resonance structures of protonated 4-pyrone are as follows:

Identical resonance forms

From the above resonating structures of protonated 4-pyrone, three double bonds in the ring are in the conjugation. Double bonds in the conjugation compounds are more stable than compounds that do not have conjugation. Thus, protonated 4- pyrone is more stable than 4-Pyron.

According to Huckel鈥檚 $4 n + 2$ rule protonated 4-pyrone has 6 pi electrons and the molecule is planar and conjugated. So, it is more stable than 4-pyrone.