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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: njoyeux on November 07, 2022, 02:54:38 PM

Title: Resonance forms of pyracyclene
Post by: njoyeux on November 07, 2022, 02:54:38 PM
Hello all,

I have a question regarding the resonance of pyracyclene.
<br />C1CC2CCC3CCC4CCC1C4C23<br />

If I’m correct, pyracyclene has four resonance forms:
<br />C1C=C2C=CC3=CC=C4C=CC=1C4=C23<br />

<br />C1C=C2C=CC3C=CC4C=CC=1C=4C2=3<br />

<br />C1=CC2C=CC3=CC=C4C=CC1=C4C=23<br />

<br />C1=CC2=CC=C3C=CC4=CC=C1C4=C23<br />

Now my problem is estimating the dominance of each form. Are they equal or is there one which is more dominant?

This is what I’ve got so far:

Rules for estimating stability of resonance structures

- The greater the number of covalent bonds, the greater the stability since more atoms will have complete octets.
- The structure with the least number of formal charges is more stable.
- The structure with the least separation of formal charge is more stable.
- A structure with a negative charge on the more electronegative atom will be more stable.
- Positive charges on the least electronegative atom (most electropositive) is more stable.
- Resonance forms that are equivalent have no difference in stability and contribute equally (eg. Benzene).

Pyracyclene has the same carbon atoms, the same number of covalent bonds and a formal charge of zero. I don’t see any difference in stability either. So it seems that all forms are equivalent.

Is this correct?
Title: Re: Resonance forms of pyracyclene
Post by: Enthalpy on November 07, 2022, 07:18:23 PM
The second and third forms are symmetric up/down, but the first form has no such symmetric form listed. Did you possibly miss it?
Title: Re: Resonance forms of pyracyclene
Post by: njoyeux on November 08, 2022, 08:19:43 AM
No, I didn't miss it. The symmetry of the first form is the first form itself. Don't be fooled by the double bond in the middle. This bond is symmetric up/down (I know, the double bond symbol is confusing).

The pyracyclene molecule has two symmetries: up/down and left/right. You can see that all symmetries for all forms are present.
Title: Re: Resonance forms of pyracyclene
Post by: Orcio_87 on November 09, 2022, 04:03:39 PM
It looks like it fullfils the Huckel's rule, so all four structures should be equal.
Title: Re: Resonance forms of pyracyclene
Post by: Enthalpy on November 10, 2022, 06:05:26 AM
Quote from: njoyeux on November 08, 2022, 08:19:43 AM
The symmetry of the first form is the first form itself.
My bad.
Title: Re: Resonance forms of pyracyclene
Post by: Enthalpy on November 10, 2022, 06:09:41 AM
Quote from: Orcio_87 on November 09, 2022, 04:03:39 PM
It looks like it fullfils the Huckel's rule, so all four structures should be equal.
I don't grab this logic. Does Hückel's rule tell anything more than "aromatic"? In 1,2-dichlorobenzene, I don't expect both forms to be equally probable.
Title: Re: Resonance forms of pyracyclene
Post by: njoyeux on November 11, 2022, 07:59:29 AM
Quote from: Enthalpy on November 10, 2022, 06:09:41 AM
Quote from: Orcio_87 on November 09, 2022, 04:03:39 PM
It looks like it fullfils the Huckel's rule, so all four structures should be equal.
I don't grab this logic. Does Hückel's rule tell anything more than "aromatic"? In 1,2-dichlorobenzene, I don't expect both forms to be equally probable.

I don't grab the logic either.

Plus, on the following research paper pyracylene is described as a bordeline case of aromaticity.
https://pubs.acs.org/doi/pdf/10.1021/ja012541%2B (https://pubs.acs.org/doi/pdf/10.1021/ja012541%2B)


What does that tell us?