127.Aromaticity(20)-Annulenes (2).

Is [12]annulene aromatic?

1271.jpg

[12]annulene has 4n π electrons and so is anti-aromatic. It is highly unstable compound and exists only at very low temperatures. At high temperatures it undergoes cyclization to form bicyclic compounds.

1272.jpg


 

[14]annulene.

1273[14]annulene was first synthesized. It has two isomers as shown above which are in equilibrium. This molecule has 4n+2 π (n=3) electrons. In this case, as the size of the ring is large(14 carbon atoms), the internal H‘s are not too close to each other to cause steric strain. Thus, this molecule can maintain its planarity and be aromatic.

 

1274.jpg
Internal hydrogen’s are far apart to cause steric strain.

The NMR spectrum of this compound shows a diamagnetic ring current which is characteristic of aromatic compounds.


[18] annulene and higher annulenes.

This molecule,with 18 carbon atoms, is aromatic too. A similar explanation as that for [14]annulenes holds true for this annulene too-

  • Large ring size ensures that the internal hydrogen’s are far apart from each other. Thus, the structure remains planar.
  • It has 4n+2 π (n=4) electrons.
  • Double and single bonds are in conjugation in this cyclic structure.

1275.jpgThe synthesis of annulenes has been carried to higher annulenes as well.[20]annulene , [22]annulene and [24]annulene have been synthesized. Only [22]annulene amongst these are aromatic.


 

More and more attempts to synthesize aromatic compounds with more number of carbon atoms led to synthesis of a very special class of molecules.What are these compounds? Why are they special? We shall find that out in our next post.Till then,

Be a perpetual student of life and keep learning …..

Good Day !

 

 

References and further reading –

1.Advanced Organic Chemistry. PART A- Structure and mechanisms by Carey, Francis A., Sundberg, Richard J.

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