[tizrhf]

Hello! I'm new on this forum. Last week i came across an explanation for the known stability of halonium ions. It said that these kind of ions have aromatic character (2 pi electrons). Chlorine, being the most electronegative of the halogens involved in forming halonium ions would destabilize the aromaticity with it's strong inductive effect, so this would explain why the chloronium ions are the most unstable from the series. Now the thing that bothers me is that I don't understand how this halonium ions are aromatic (those 2 electrons come from the halogen) because I can't see any pi-pi or n-pi conjugation there. The halonium ions derived from alkynes have a double bond in it, which creates a great tension and instability. But their instability is also related to the fact that they are antiaromatic  (that is, they have 4 pi electrons). But that confuses me a lot because that ion looks just like a 2-electron cyclopropenilium ion. Of course there, the number of pi electrons (2) and the conjugation is very obvious. I would be very grateful if someone could clarify this situation. Thank you very much.

[tizrhf]

nobody? after a week ? what kind of forum do you have here ??

[Dan]

I too am sceptical of this explanation. Can you ost a specific example.

[movies]

Halonium ions of alkenes can't be aromatic because, as you pointed out, there aren't any pi orbitals on the carbons.

As for the alkyne derived halonium ions, I think you can think of those as antiaromatic.  The positive charge isn't residing in the aromatic system, it's in the plane of the three membered ring.  Remember that to make the halonium ion the pi bond (2 electrons) add to the vacant orbital of the cationic halogen.  The result is a three-center structure with only two electrons, the so-called 3-center, 2 electron bond that is common in organometallic chemistry.  If you think in terms of electronegativity, most of the electron density will be close to the halogen and the carbon will bear most of the positive charge, and it clearly can't put the positive charge into the aromatic system because you already have a filled pi orbital there.

[tizrhf]

I guess there's no pi conjugation involved here. You are right about the resemblance with the three-center-2 electron-structures from organometallic compounds, but you miss an important pioint here. The hypervalent halogen has an empty orbital which resembles very much a p orbital. The orbitals on carbon also have very high p character (the carbocation, with the ring opened would be a resonance structure of the halonium ion), and i think there could be some interaction between these three p orbitals in the ring. As there are two electrons involved, Huckel's rule is obeyed. Of course, we're not talking about the extensive conjugation found in molecules as benzene, naphtalene etc, but i'm talking about a pi interaction, which can be regarded as aromatic.

What do you think ??

[Dan]

The orbitals on carbon also have very high p character (the carbocation, with the ring opened would be a resonance structure of the halonium ion), and i think there could be some interaction between these three p orbitals in the ring.

If I understand you correctly, the symmetry is wrong, that p orbital would have a lobe pointing at the halogen ie. it's 90 degrees out.
Also, what is the empty orbital on the halogen that resembles a p orbital?

At the moment, I think this aromaticity argument is absolute rubbish, but post some diagrams, try to convince me.

[tizrhf]

Why would it be pointing the way you mentioned ? What makes you say that ? I think those p orbitals are parallel.

[Dan]

This is what I mean.