Aromaticity Question

[wti130030]

Quiz question regarding a 3,3-dimethyl-bicyclo[4.0.3]nonatetraene:

 

*Picture attached

 

My teacher is insisting that this is an aromatic compound, due to the fact that the pi bond between 1 and 2 can break and form an anion, giving the ring to the left 6 pi electrons to resonate with

 

I simply disagree, since using the same logic would imply a pentalene, for example, would also be aromatic which is commonly held to be antiaromatic.

 

This would contradict the requirement that the delocalization of electrons decreases the energy of the molecule.

 

Any help, explanation, or literature would be appreciated.

[hypervalent_iodine]

The compound as written doesn't satisfy the conditions for aromaticity, so I would tend to agree with you.

 

Regardless of that however, it wouldn't just form an anion; it would have to form a cation at position 2 also and it doesn't seem like it would be a particularly stable one. I'm not convinced that the added stability of the aromatic component would be enough to offset the instability of having two charges floating around like that I would question how spontaneous such a process would be anyway.

[John Cuthber]

According to wiki

"An aromatic (or aryl) compound contains a set of covalently bound atoms with specific characteristics:

• A delocalized conjugated π system, most commonly an arrangement of alternating single and double bonds
• Coplanar structure, with all the contributing atoms in the same plane
• Contributing atoms arranged in one or more rings
• A number of π delocalized electrons that is even, but not a multiple of 4. That is, 4n + 2 number of π electrons, where n=0, 1, 2, 3, and so on. This is known as Hückel's Rule."

and as far as I can see, that molecule breaks the 4th rule by having 8 pi electrons

[rktpro]

John, the 8 electrons don't make a ring current as an sp3 carbon is there too.Cyclohexe-1-ene has 2 pi electrons but is non-aromatic.It can, I feel, be determined experimentally only if the compound is quassi aromatic or not,i.e, the gain in aromatic stabalization compensates the separation of charge-where the carbocation is highly unstable with no hyperconjugation.

Edited March 11, 2014 by rktpro

[John Cuthber]

....Cyclohexe-1-ene ...

pardon?

 

(Though, for the record, biphenyl is clearly aromatic with 12 electrons; but they are in 2 rings.

[rktpro]

pardon?

 

(Though, for the record, biphenyl is clearly aromatic with 12 electrons; but they are in 2 rings.

Sorry, I use a mobile device. I meant to say cyclohex-1-ene or simply cyclohexene. I think my point is clear? English is not my first language.

The 12 electron make two separate aromatic rings.

By saying that 8 pi electrons, that they make ring current; you challenge the very existence of the compound. You make it anti-aromatic not non-aromatic.

Edited March 12, 2014 by rktpro

[hypervalent_iodine]

John, I think what rktpro is saying is that you don't even need to take it as far as the fourth rule outlined in your initial post. The pi electrons are not fully delocalised around the ring system, so you don't even need to consider how many electrons are in it. Simply stating that it has 8 pi electrons and therefore does not satisfy Hückel's rule implies that it is antiaromatic, since Hückel's rule doesn't apply to compounds that don't contain conjugation around the ring. Kind if a semantic issue I guess, but I think that's where rktpro's comments stem from.