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This compound is planar conjugated and has 8 pi electrons on its periphery so it can be anti aromatic but if we polarize the pi bond at the bottom of the right ring the left ring will become aromatic making it stable. are there any theories or data indicating toward stability of this compound at room temperature? any kind of input is appreciated!


1 Answer 1


This is pentalene. It is indeed antiaromatic, and highly unstable tending to dimerize even at -100°C. With smaller rings the antiaromaticity and instability are much stronger than in the 8-pi electron system cyclooctatetraene.

There are two ways to stabilize this molecule. One is to introduce substituents that sterically hinder reactions such as tert-butyl, or that disperse the antiaromatic coupling such as cumulated benzene rings. The second method is to add two electrons, forming a dianion with the correct pi electron count to make an aromatic system.


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