Comments on: Evaluating Resonance Forms (1) – The Rule of Least Charges

By: James Ashenhurst

James Ashenhurst — Wed, 03 May 2023 13:33:31 +0000

In reply to Megaklis Asteris. Yes, this is insignificant because it does not show aa pi bond between any of the carbons. The bond length for a C-C single bond is about 1.50 angstrom, whereas that for a C-C double bond is about 1.40 angstrom. The meaasured bond length is more consistent with the presence of a C-C pi bond.

By: Megaklis Asteris

Megaklis Asteris — Mon, 01 May 2023 14:17:50 +0000

In the allyl anion resonance form C, we too have equal bond lengths and even distribution of negative charge over the end carbon atoms aw in the hybrid of forms A and A’. So there must be other experimental observations based on which this form is rejected as insignificant.

By: James Ashenhurst

James Ashenhurst — Tue, 09 Apr 2019 17:50:09 +0000

In reply to Dadi. For practical purposes, full octets is implied in the rule of "least charges" , simply because the only commonly encountered atom which will ever have less than a full octet is positively charged carbon.

By: Dadi

Dadi — Sun, 24 Feb 2019 17:01:54 +0000

Thanks for a great post (and website).
I wonder about ranking resonance contributors stability.
You treat the rule of least charges as superior.
However, many textbooks state that the demand of full octet prevails.
For example, see ‘Organic chemistry’ by Bruice or here:
http://www.chem.ucla.edu/~harding/tutorials/resonance/imp_res_str_key.html
Could you please refer me to any relevant text (article?) which resolves the issue?

By: Matthew

Matthew — Wed, 16 May 2018 15:35:30 +0000

For example two, I created 5 resonance structures. I believe I’m wrong though because my 5th resonance structure had a lone pair of electrons on the 3rd carbon in allyl cation would imply a resonance structure of A’?

By: james

james — Sun, 11 Dec 2011 18:44:58 +0000

In reply to jess. thanks Jess... you're right re: the dipole. there's probably a better way to word this. Will edit!

By: jess

jess — Fri, 09 Dec 2011 21:20:15 +0000

Hi James! I’ve really been enjoying this series (even though I’m working on one very similar — just don’t have the time!)

Your ethylene example confuses me a bit… wouldn’t there also be a possible B’ resonance form where the pi bond electrons move to the left carbon as a lone pair? And if so, wouldn’t these two resonance forms (insignificant though they may be) essentially “cancel” each other out, so we wouldn’t expect there to be a measurable dipole anyway?

Thanks!

By: azmanam

azmanam — Thu, 08 Dec 2011 18:04:35 +0000

In reply to james.

nonono, you’re supposed to say ‘Feelin’ good, Lewis’
:)

By: james

james — Thu, 08 Dec 2011 17:58:49 +0000

In reply to azmanam. MANY thanks, I'm an idiot for not figuring out how to do this earlier. Very happy with the result.

By: azmanam

azmanam — Thu, 08 Dec 2011 17:57:32 +0000

I noticed the new curved arrows :) Lookin’ good, Billy Ray!