The polarization of a carbonyl group can be represented by a pair of resonance structures

Cyclopropenone and cycloheptatrienone are more stable than anticipated. Cyclopentadienone, however, is relatively unstable and rapidly undergoes a Diels–Alder dimerization. Explain.

Cyclopropenone CycloheptatrienoneCyclopentadienone

The resonance of any molecule depends upon the number of pi electrons in it and whether it is a conjugated cyclic or acyclic system. This can be applied by usingHuckel’s rule

When cyclopropenone polarizes then it becomes an aromatic ring due to the presence of 2 pi electrons. It follows Huckel’s rule of 4n+2 pi electrons where n=0. Thus, cyclopropenone is stable.

2π electrons and ring is aromatic

In the case of cycloheptatrienone, the polarization of the C=O bond takes place. The ring becomes aromatic. It follows 4n+2 pi electrons where n=1, and has 6 pi electrons. Thus, cycloheptatrienone is stable.

6π electrons and ring is aromatic

In the case of cyclopheptadienone if there is polarization then it gives a 4 pi-electron system. It is antiaromatic and is not stable.

4π electrons and the ring is antiaromatic