# Do there exist aromatic compounds similar to azulene?

So for this question I'm considering a chemical to be "similar to azulene" if it obeys the following conditions:

1. It is aromatic and obeys Hückel's rule,
2. It is a fused bicyclic compound,
3. One component ring is aromatic as an anion,
4. The other ring is aromatic as a cation,
5. It is not azulene, and
6. It has been synthesised before and the synthesis has been confirmed.

For instance, I believe bicyclo[3.1.0]hexa-1,3,5-triene should follow all of the previous conditions except possibly the last (but I don't know that it hasn't). Do any chemicals obey all of these conditions?

• Of course with focus to the aromatic core, right? – Alchimista Apr 15 '18 at 9:03
• – orthocresol Apr 15 '18 at 14:43
• Both rings are aromatic, and charge separation isn't complete. – Mithoron Apr 15 '18 at 16:08
• Also you're not asking about simple derivatives of azulene? There's a lot of them... – Mithoron Apr 15 '18 at 16:11

There is this compound which is I believe the closest to anything which you've asked for. Except the fused part this satisfies all your conditions. In a polar solvent the double bond breaks to form two highly stable aromatic rings(tropylium and dicyclopentene) which act as two separate poles. It also lowers the rotation barrier of this compound. Hence it even becomes soluble. Along with azulene this is as close as an organic compound gets to being ionic.

• As linked by ortho, hydrocarbons can be truly ionic. – Mithoron Apr 15 '18 at 16:12

OK, so we cannot store the $\ce{C_6H_4}$ compound referred to in the question on a shelf in a lab. But apparently it has been identified as an intermediate species, see here and, for the second page of the article, here.

In summary: a nucleophilic substitution and elimination is carried out on a bicyclic compound, and the most plausible mechanism, backed by other experiments, is via a bicyclo[3.1.0]hexatriene intermediate. Computations indicate that the molecule has a bond through the appropriate diagonal of the hexagon.

• It seems your links don't work properly for me. – Mithoron Apr 15 '18 at 23:09
• What is wrong? What happens for me is: The first link shows the first page of the article, the second link shows the first page of the next article which is also the second and last page of the one I targeted. It's a sneaky way around the paywall, they need to be more careful! – Oscar Lanzi Apr 15 '18 at 23:15
• I see huge list and dunno what to find there. – Mithoron Apr 15 '18 at 23:22
• How strange! It should d be on pages 1615 and 1616. I dunno how better to explain this b/c the links work differently for me. – Oscar Lanzi Apr 15 '18 at 23:40
• Link is ok here – Alchimista Apr 16 '18 at 10:45