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Magazine Chapter 16: Q15P (page 818)
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鈥揂lder dimerization. Explain.
Cyclopropenone CycloheptatrienoneCyclopentadienone
Short Answer
2蟺 electrons and ring is aromatic
6蟺 electrons and ring is aromatic
4蟺 electrons and the ring is antiaromatic
Step by step solution
Resonance
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鈥檚 rule
Classification of the molecules as stable or unstable
When cyclopropenone polarizes then it becomes an aromatic ring due to the presence of 2 pi electrons. It follows Huckel鈥檚 rule of 4n+2 pi electrons where n=0. Thus, cyclopropenone is stable.
2蟺 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.
6蟺 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.
4蟺 electrons and the ring is antiaromatic
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