Why is 15,16 dihydropyrene aromatic?


Here the middle two carbons are $\ce{sp^3}$ hybridized. Then how is it aromatic? Do we only consider the conditions for aromaticity to apply only at periphery in case of fused rings?


1 Answer 1


While doing calculations for Huckel's rule, in polycyclic compound only peripheral pi electrons are considered for aromaticity. So, 7 double bonds in the periphery i.e. 14 pi electrons. 14=4(3)+2 Huckel's rule satisfied. Aromaticity achieved.

Why are only peripheral pi electrons considered? The reason is that only these elections are participating in delocalization for conjugation. If multiple bonds occur in the centre like in pyrene these central bonds will not be localized. Thus, only peripheral pi electrons are considered in conjugation.

enter image description here

Therefore, first 2 compounds in this image are aromatic and last one is weakly Anti aromatic or Non aromatic.

We can't commit compound to be aromatic on ONLY Huckels rule with this compound must be

  1. Cyclic
  2. Planar
  3. Complete cyclic conjuction
  4. Huckel's rule

Here in 3rd compound, Due to crowding of Interior Hydrogen compound loses its planer nature. Thus, not called to be Aromatic.

(I have attached 3d structure of cyclotetradeca-1,3,5,7,9,11,13-heptaene (3rd compound) Please refer it.)

enter image description here

  • 2
    $\begingroup$ @AniruddhaDeb I would disagree there, AFAIK it is not so clear cut. [14]Annulene has structures which are considered to be aromatic. "... the C2h structure has an aromaticity index of 1.71 and the D2 structure an index of 1.70. Since benzene with an aromaticity index of 1.75 is the prototype of an aromatic system, these two structures must be called aromatic". The other structures are usually taken to be weakly aromatic. Thus, I wouldn't go out and say that [14]Annulene is known to be non-aromatic, it's probably somewhere in the middle. $\endgroup$ Commented Nov 6, 2020 at 12:17

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