[SVXX]

Looks like I have a well of doubts
I have these three substrates with me. How do I decide the order of ionic character amongst them? I've numbered them as I, II and III.



I figure it has something to do with aromaticity...there is a similar question on deciding which double bond(the one connecting the two rings in all three cases) has the greatest ease of rotation. The answers should be the same, as the double bond with the greatest ease of rotation would be the easiest to break, and the respective charges formed should remain stable. Any ideas?

My try : II > III > I?

[a student]

in these structures  you should consider 4n+2 rule, in all of the structures which you have mentioned both rings can obey this rule with out breaking the double bond which is BTW the rings (I mean there is no need for charge separation) this is my idea but I recommend to take a look at this book, ADVANCED ORGANIC CHEMISTRY by MARCH it has a good chapter about aromaticity, I hope it help

[uvcyclotron]

a student is right about the 4n+2 rule of aromaticity, and you are right about the bond with greatest ease of rotation being easier to break..

Now think, a C-C single bond(SB) (σ-bond) is rotatable, while a C-C double bond(DB) (σ & π bond) is not. But when you talk about a DB's rotation, you can say that you are talking about it's SB character..right? more SB char it has, more ease of rotation of the DB. And more the ionic character in the structure, more the SB char in DB..

Now, when you introduce ionic char in the structures..such that δ⁺ charge appears on the bigger ring while a δ^- charge appears on the lower ring (why?) , try using 4n+2 rule now..
remember
4n+2 π e^- : aromatic
4n π e^-: anti-aromatic..

UV

[uvcyclotron]

Hello?
@SVXX: Have you figured out the correct answer yet? Or are you still stuck on it?

[SVXX]

Oh sorry..didn't notice that this thread came up!
Why does the double bond cleave like that? Why not + charge in the smaller rings? Because of aromatic considerations perhaps?
Yes...if - charge appeared on the bigger ring, it wouldn't resonate, quite clearly. Similarly if + charge appeared on the smaller rings, it wouldn't resonate. Resonance(and planarity) are required for aromaticity.
So what do I have to decide? Huckel's rule?

[uvcyclotron]

Similarly if + charge appeared on the smaller rings, it wouldn't resonate. Resonance(and planarity) are required for aromaticity

No, that depends as the size of smaller ring is different in each case, consider each one separately.

And yes,you can use Huckel's rule to show that if - charge appeared on the bigger rings, then the no of pi e^- would be 6+2=8 which would make in anti-aromatic, but if a + charge appears on it, the number of pi e^- would be 6 ! So according to the 4n+2 rule, it would be aromatic then.

Now you have to decide what change, the appearance of - charge on smaller rings, brings into it. like does it make it aromatic(A) or anti A? or neither?
Now try to think and decide the order..