129.Aromaticity(22) -Antiaromaticity.

We saw earlier that aromaticity refers to the extra stability that some cyclic compounds have, owing to their structure. Antiaromaticity means exactly the opposite of that.

An aromatic compound is more stable than an analogous cyclic compound with localized
electrons. In contrast, an antiaromatic compound is less stable than its analogous
cyclic compound with localized electrons. Aromaticity is characterized by stability,
whereas antiaromaticity is characterized by instability.

Compounds that are comparatively destabilised by a closed loop of electrons are antiaromatic. The delocalization of π electrons in these species, leads to conspicuous decrease or even loss in thermodynamic stability of these compounds. These compounds have very low or negative values of resonance energy.

e.g.1,3-cyclobutadiene

1192This compound has 4π electrons. Hückel’s rule predicts no aromatic character in 1,3-cyclobutadiene. However, this compound is found to be much less stable than a normal conjugated cyclic compound! 

After years of attempts to prepare cyclobutadiene, the goal was finally achieved by Criegee and Schroder in 1959. It is clear that cyclobutadiene and its derivatives are extremely unstable compounds with very short lifetimes. This exists only as a matrix-isolated species. They can be stabilized –

I) At room temperature, by embedding in the cavity of hemicarcerand (host).
II)At low temperature, in matrices.

In either case, the cyclobutadiene molecules are forced to remain apart from each other. If two molecules come in contact with each other , they dimerize by Diel’s Alder reaction  as follows –

1292.jpg


Conditions for antiaromaticity.

For a compound to be antiaromatic, it must fulfill the following conditions –

1)It must be a planar. 

2)It should have a ring structure.

3)It should have an uninterrupted π electron cloud system.

2)It should have 4n π electrons, where n is any whole number. In short, the π electron cloud should have even number of pairs of π electrons.

So basically antiaromaticity demands the same conditions as aromaticity except for the last condition i.e presence of 4n π electrons.


We will study ring current in detail in the upcoming posts. For now just remember –

0
Aromatic compounds ⇒ have a diamagnetic ring current and
Antiaromatic compounds ⇒ have a paramagnetic ring current.


Other examples of antiaromatic compounds are –

Antiaromaticity_examples.jpgIn our next post we will continue our discourse on aromaticity. Till then ,

Be a perpetual student of life and keep learning….

Good day !

 

 

References and further reading –

  1. https://www.sciencedirect.com/science/article/pii/B9780123957528500071
  2. Advanced Organic Chemistry: Part A: Structure and Mechanisms,by Francis A. Carey, Richard J. Sundberg

Image source –

1.By Physorg 2013 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=29249827

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