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Question description

I was solving this question and I thought the answer was B>C>A as more the resonance, more the pi electrons are delocalized so more free movement.

However the answer mentions it as B>A>C.

What is wrong with my reasoning?

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    $\begingroup$ There is no question in the screenshot. How can there be an answer then? $\endgroup$ – imalipusram May 3 at 11:44
  • $\begingroup$ Um.... There is Actually. Maybe you are unable to load it $\endgroup$ – Vamsi Krishna May 3 at 11:56
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    $\begingroup$ I only can read "compare c-c bond rotation across A,B,C" and then some answers to choose from. I'd expect a question like "sort the structures in respect of increasing energy barriers for the rotation around the double bond which is connecting the rings" or sth like that. $\endgroup$ – imalipusram May 3 at 12:07
  • $\begingroup$ They have mentioned the rotation around the bond that they have shown with a curved arrow. And compare how freely each can rotate about it. Compare means arrange in either descending or ascending order $\endgroup$ – Vamsi Krishna May 3 at 12:17
  • $\begingroup$ bro go for Bayer strain theory! I can see hybridization of all the surrounded atom is mostly same $\endgroup$ – Jack Rod May 3 at 12:37
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Here you need to consider two factors.

  1. The one which follows $4n+2$ rule.
  2. The stability of the ring.

When we see the IIIrd one, the structure don't follow the $4n+2$ rule when the double bond breaks. That means, that the double bond will be reluctant towards breaking. Hence its stability will be least.

COMING TO NEXT ONE

The order of stability of the ring is in the order: 6−member > 5−member > 7−member > 4−member > 3−member.

So when we combine these facts we find the correct order is B>A>C.

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  • $\begingroup$ Why won't it be aromatic? The electron pair can go to either ring right? $\endgroup$ – Vamsi Krishna May 3 at 11:41
  • $\begingroup$ if the electron shifts to 7 membered ring, it will follow 4n rule and if the electron shifts to 3 membered ring it will follow 4n rule. I hope you got the answer else draw all possible structure and and check whether it follows 4n+2 rule or 4n rule $\endgroup$ – Prem Pandit May 3 at 12:16
  • $\begingroup$ Oh..... Yes thanks a lot I get it now $\endgroup$ – Vamsi Krishna May 3 at 12:18
  • $\begingroup$ Your stability order is irrelevant here, as it pertains saturated rings. $\endgroup$ – Mithoron May 5 at 15:53

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