Benzene & Resonance, What?

[MedPR]

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Answer is D.

I'm pretty sure this question was posted a few months ago, but I can't find the thread so here it is again.

Anyone care to explain?

 

[STxHorn]

D. cannot be right. All the atoms are in exactly the same positions. The correct answer is C.

At least that's what makes sense to me.

 

[gettheleadout]

D is correct. Resonance by definition involves only a delocalization of electrons with no literal movement of either them or the atoms themselves. Consider benzene, where the bond lengths are all equal because of the pi system being spread across all 6 carbons equally. (think of a Robinson representation)

In the 1,3,5-cyclohexatriene molecule, the bond lengths are not equal, so in the two isomers shown the carbons occupy different positions relative to each other. The interchange shown is an isomerization, not resonance.

C cannot be correct because in benzene all bond lengths are equal, and as stated in the question the bond lengths are unequal in 1,3,5-cyclohexatriene.

If that didn't help let me know, because I'm positive D is correct. Interestingly, A and B are also true statements, though not correct answers to the question asked.

 

[MedPR]

D is correct. Resonance by definition involves only a delocalization of electrons with no literal movement of either them or the atoms themselves. Consider benzene, where the bond lengths are all equal because of the pi system being spread across all 6 carbons equally. (think of a Robinson representation)

In the 1,3,5-cyclohexatriene molecule, the bond lengths are not equal, so in the two isomers shown the carbons occupy different positions relative to each other. The interchange shown is an isomerization, not resonance.

C cannot be correct because in benzene all bond lengths are equal, and as stated in the question the bond lengths are unequal in 1,3,5-cyclohexatriene.

If that didn't help let me know, because I'm positive D is correct. Interestingly, A and B are also true statements, though not correct answers to the question asked.

I can see why A and B are not correct answers (though true) and I can also see why C is not true, but I don't understand why D is true. Aren't the two rings the same thing just rotated in space? Doesn't that make them the same compound?

 

[gettheleadout]

I can see why A and B are not correct answers (though true) and I can also see why C is not true, but I don't understand why D is true. Aren't the two rings the same thing just rotated in space? Doesn't that make them the same compound?

Number the carbons and then push electrons using arrows. This should make it more apparent that the carbons would be physically moving, which is impossible in resonance. Yes, the two structures are the same if you rotate them, but that would be comparing two individual molecules of 1,3,5-cyclohexatriene, not a single one interconverting.

 

[STxHorn]

Well, I see all your points as to why D is correct, but 1,3,5-cyclohexatriene is benzene...isn't it? I mean this seems like splitting hairs with the question. My understanding is one name is by international convention the other is trivial.

So...the reason I hold C as correct is because it IS benzene and therefore only exists as a resonance form.


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[MedPR]

Well, I see all your points as to why D is correct, but 1,3,5-cyclohexatriene is benzene...isn't it? I mean this seems like splitting hairs with the question. My understanding is one name is by international convention the other is trivial.

So...the reason I hold C as correct is because it IS benzene and therefore only exists as a resonance form.


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C is not correct. Benzene does not "resonate between those structures." It exists in a stable form of a combination of all the resonance structures. C is actually a trick answer meant to see if you know what resonance really is.

Also, Benzene is not 1,3,5-cyclohexatriene. Benzene does not have 3 double bonds, it has a what you can think of as 1.5 bonds. Its bonds are longer and weaker than the double bonds of a true alkene.

 

[gettheleadout]

Well, I see all your points as to why D is correct, but 1,3,5-cyclohexatriene is benzene...isn't it? I mean this seems like splitting hairs with the question. My understanding is one name is by international convention the other is trivial.

So...the reason I hold C as correct is because it IS benzene and therefore only exists as a resonance form.


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1,3,5-cyclohexatriene is fictional. In such a molecule one has unequal bond lengths as shown. When benzene was first discovered it was thought to be 1,3,5-cyclohexatriene, but was found not to have individual conjugated double bonds as the triene would. Remember that benzene does not have any discrete double or single bonds, so the Kekulé structure that "looks" like a triene is an inadequate representation. The triene does not exist because the bonding electrons are delocalized, forming benzene, but were it to exist and the electrons move as they are shown to in the question, it would not be resonance but isomerization.

 

[STxHorn]

C is not correct. Benzene does not "resonate between those structures." It exists in a stable form of a combination of all the resonance structures. C is actually a trick answer meant to see if you know what resonance really is.

I know what resonance is and it's neither of those structures. I get that, thank you very much. That is my point. The cyclohexatriene is benzene which only exists as one resonance structure. However, using structural formulas, those two representations are how you draw benzene. ...if its benzene then I assume the undrawn resonance. I get by some rules that may be incorrect, but my understanding of what is really happening is not. 1,3,5-cyclohexatriene never exists as either of those molecules drawn.


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EDIT: fixed a couple of confusing typos.

 

[STxHorn]

1,3,5-cyclohexatriene is fictional. In such a molecule one has unequal bond lengths as shown. When benzene was first discovered it was thought to be 1,3,5-cyclohexatriene, but was found not to have individual conjugated double bonds as the triene would. Remember that benzene does not have any discrete double or single bonds, so the Kekulé structure that "looks" like a triene is an inadequate representation. The triene does not exist because the bonding electrons are delocalized, forming benzene, but were it to exist and the electrons move as they are shown to in the question, it would not be resonance but isomerization.

Understood. And that's my problem with the answer. It's asking me for an answer to a fictional molecule when I know what actually exists, benzene, and it is one resonance form (not shown in the picture). So, my mind tells me the "best" answer is C. I get that the ACS exam may indeed say I'm wrong. I disagree.


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[gettheleadout]

I know what resonance is and it's neither of those structures. I get that, thank you very much. That is my point. The cyclohexatriene is benzene which only exists as one resonance structure. However, using structural formulas, those two representations are how you draw benzene. ...if its benzene then I assume the indrawn resonance. I get by done rules that may be incorrect, but my understanding of what is really happening is not. 1,3,5-cyclohexatriene never exists as either of those molecules drawn.


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I assume you replied to MedPR before seeing my post, but just to clarify, benzene is NOT a triene. The question distinguishes this further by illustrating the non-hexagonal shape of the triene.

 

[STxHorn]

I assume you replied to MedPR before seeing my post, but just to clarify, benzene is NOT a triene. The question distinguishes this further by illustrating the non-hexagonal shape of the triene.

I understand. So, are we meant to answer purely hypothetical questions as if we haven't learned what's really occurring? Because I didn't do that in my reasoning. It seems pointless to do that.


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[gettheleadout]

I understand. So, are we meant to answer purely hypothetical questions as if we haven't learned what's really occurring? Because I didn't do that in my reasoning. It seems pointless to do that.


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And the I make the same mistake and respond too quickly haha 😛

You're going about the question the wrong way. It is not necessary to know that the triene doesn't exist, because C is false on its own; look at the two forms shown. Unequal bond length. Benzene has all bond lengths equal. There is no way either of the two structures *shown* contribute at all to the resonance occurring in benzene.

That is, benzene does not resonate between those two structures (which is what C says verbatim). It instead resonates between two hexagonal structures.

 

[STxHorn]

And the I make the same mistake and respond too quickly haha 😛

You're going about the question the wrong way. It is not necessary to know that the triene doesn't exist, because C is false on its own; look at the two forms shown. Unequal bond length. Benzene has all bond lengths equal. There is no way either of the two structures *shown* contribute at all to the resonance occurring in benzene.

I see the reasoning. It just seems like a pointless exercise for molecule that cannot exist as drawn. But I get it.

EDIT: I loved my OChem professor. He taught me both semesters and always taught us to thinking what was really occurring and didn't focus on rules per se. I think that's why I'm so bent toward thinking of the problem in terms of what's really going on.


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