Q42E - Organic Chemistry (Mcmurry)

Question

On reaction with acid, 4-pyrone is protonated on the carbonyl-group oxygen to give a stable cationic product. Using resonance structures and the Hückel’s $\mathbf{4}\mathbf{n}\mathbf{+}\mathbf{2}$ rule, explain why the protonated product is so stable.

4-Pyrone

Step-by-Step Solution

Answer

Identical resonance forms

Step 1: 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.

Step 2: 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’s $4\mathrm{n}+2$ rule protonated 4-pyrone has 6 pi electrons and the molecule is planar and conjugated. So, it is more stable than 4-pyrone.