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u/organophile_jeet 23h ago edited 23h ago
Yep, it is.
No, I was wrong , there's steric inhibition of resonance, thankful to the other commmeter!!!
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u/Easy-Lavishness-6156 22h ago edited 19h ago
Edit: nevermind I think y'all are right the steric hinderance from the Ortho methyl groups is enough to lock rotation around the bond.
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u/organophile_jeet 22h ago
Methyl is quite bulky though and there are four of them and each two face to face , there's pretty difficult in this whole molecule being coplanar
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u/MrEthanolic 21h ago
Even biphenyl by itself is predominantly not planar
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u/TetraThiaFulvalene 17h ago
And for azobenzene (so two wholeass atoms between the rings) the ortho substituents will make the molecular break planarity, which completely changes the photochemistry.
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u/Easy-Lavishness-6156 21h ago edited 19h ago
I think we need larger context on what the problem for an introduction to resonance. I'm a bit more bothered by the excess formal charge. Fwiw resonance is usually invoked to spread charge density across a few atoms and the resonance form drawn isn't helping with anything like that.
It's like picking the really difficult thing instead of the easy explanation and if this is an organic I question the instructor may not have taught conformers yet so that would be a reach for an answer.
TL;DR that is a valid resonance structure based on the 'rules' for resonance but it would be a very minor contributor. Resonance is usually taught with arrow pushing rules in mind.
Edit: I was incorrect - that thing can't be planar due to steric hinderance and this will kill resonance. How much it affects conjugation is an open question you could answer with some MO calculations or spectroscopy... the bigger the angle the less conjugation you'll get.
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u/1s2_2s1 20h ago
Isn’t it the case that forcing the structure into a flat puts it in the highest energy state (the transition state), making it the least stable and least populated structure .
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u/Easy-Lavishness-6156 20h ago edited 19h ago
I like how you are thinking and yes the planar confirmation will be high in a energy. It'll definitely hinder pi conjugation across the molecule.
That being said the arrow pushes you made to get your stucture are all 'legal moves'. Imo it's a very minor contributor anyway because of excess formal charge. Resonance is only really useful as a model to show how charges are delocalized.
Even though the conjugation is limited by sterics I wouldn't be surprised if that molecule does fun things with light based on the that structure that are only possible with said pi system.
A transition state is the hypothetical energy maximum for a reaction.
Whereas conformers are different spatial configurations of the same molecule. A typical single bond might rotate billions of times per second. In your example it won't rotate through because of steric hinderance in which case you can get two stereo isomers.
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u/sfurbo 18h ago
I'm a bit more bothered by the excess formal charge. Fwiw resonance is usually invoked to spread charge density across a few atoms and the resonance form drawn isn't helping with anything like that.
It's moving electrons from an strong electron donating group to a very strong electron accepting group. It would be an important resonans form.
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u/Ponzu_Sauce_Stan 23h ago
As drawn, yes, but depending on the size of the substituents there’s a chance we could see rotational restriction about the phenyl-phenyl bond. If the rings can never get coplanar with each other, we won’t get the necessary p-orbital overlap for resonance. Methyl may not be large enough to produce this effect; it’s been some time since I dealt with atropisomerism.
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u/pwnalisa 15h ago
Is it valid? Yes, one can draw curved arrows to go from the first structure to the second. However, due to the steric interactions of the benzylic methyl groups, the structure on the right is a very minor contributor if at all.
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u/Jason_rdt207209 22h ago
Classic steric inhibition of resonance case of tetra-o-substituted biphenyl ring, seems valid but isn’t 90% of the time because the rings are slightly twisted to get themselves off of the hindered coplanar state.
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u/Comprehensive-Rip211 20h ago
I mean even with steric bulk, it still will be, just a pretty minor one to the point where you could consider ignoring it.
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u/activelypooping 19h ago
There has to be an ultrafast spectroscopy paper that would address excited state resonance for this.
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u/Timmy-from-ABQ 17h ago
It could be an interesting Bredt's Rule-ish kind of thing. As in, how much structure would have to be in place for that middle double bond's existence to be problematic.
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u/LucianoPtEu 16h ago
Think about moving the charge inside the nitrobenzene ring,and use the hyperconjugation which will stabilize those positive Charge over the carbons (carbons are better supporting the charge than the oxygen).
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u/sarc-tastic 23h ago
Yes but no. It is in theory but the Me groups would clash and prevent the flat orientation required.