Is resonance bonding superposition?

[Hari Seldon]

I was explaining this fascinating site: http://pipeline.corante.com/archives...lar_images.php to my wife and I was trying to explain resonance to her when it occurred to me that perhaps resonance is just an example of superposition. For example, in a benzene ring, is it just the case that each of the six bonds is a superposition of being double and single. Or perhaps each electron is in a superposition of being here and there. Is this the case?

[Busy Scissors]

It is the case, yes, at least as far as the everyday meaning of superposition is concerned. The bond order of benzene, for example, is 1.5 - midway between that of a single bond and a double bond.

When undergrads first learn resonance they draw out one resonance form, and then use curly arrows to show the pi bonds moving to another form. This can be confusing, as it suggests the resonance forms are in some sort of chemical equilibrium through the movement of electrons. This is not the case, and it should be emphasised that each resonance structure merely contributes to an overall resonance hybrid - as the wiki page puts it the molecule does not oscillate back and forth between the contributing structures. So in benzene each bond is a superposition of a single and double bond.

Does this answer your question, or did you have a more specific definition of superposition in mind?

[Derleth]

So I suppose triple bonds in a benzene ring are Right Out?

[Szlater]

Originally Posted by Derleth

So I suppose triple bonds in a benzene ring are Right Out?

Wouldn't be a benzene ring then, t'would be something else... A cyclohexyne.

However, I don't think you can make a 6 carbon cycloalkyne... I think it's eight carbons minimum (cyclooctyne).

[Busy Scissors]

A triple bond in benzene is called benzyne and does actually exist, if only transiently - its hideous strain means it has a short lifetime, termed a reactive intermediate. You can, however, trap it many different ways to make functionalised benzenes - benzyne chemistry is quite a rich field as a result (my research group does quite a bit of benzyne chemistry as it happens).
You can do the same sort of thing with cyclohexyne - generate it as a reactive intermediate and trap it in various bond-forming events.

[Szlater]

Originally Posted by Busy Scissors

A triple bond in benzene is called benzyne and does actually exist, if only transiently - its hideous strain means it has a short lifetime, termed a reactive intermediate. You can, however, trap it many different ways to make functionalised benzenes - benzyne chemistry is quite a rich field as a result (my research group does quite a bit of benzyne chemistry as it happens).
You can do the same sort of thing with cyclohexyne - generate it as a reactive intermediate and trap it in various bond-forming events.

I thought I did quite well considering my under-grad organic chemistry is 15 years out of date

Actually, I don't think I did organic chemistry in my final year....I think I only did analytical and pharmacological/medicinal units only...so 16 years out of date.

Now I teach 7-11 year olds to dissolve salt in water and to separate sand and marbles.

[robby]

Originally Posted by Busy Scissors

It is the case, yes, at least as far as the everyday meaning of superposition is concerned. The bond order of benzene, for example, is 1.5 - midway between that of a single bond and a double bond.

When undergrads first learn resonance they draw out one resonance form, and then use curly arrows to show the pi bonds moving to another form. This can be confusing, as it suggests the resonance forms are in some sort of chemical equilibrium through the movement of electrons. This is not the case, and it should be emphasised that each resonance structure merely contributes to an overall resonance hybrid - as the wiki page puts it the molecule does not oscillate back and forth between the contributing structures. So in benzene each bond is a superposition of a single and double bond.

Does this answer your question, or did you have a more specific definition of superposition in mind?

Back when I was teaching this (over a decade ago now), I used the following analogy. Resonance structures are like representing the color "purple" in the following manner:

RED <----> BLUE

This does NOT mean that the color in question is oscillating between "red" and "blue"; instead it is something in between (i.e. "purple").

Similarly, a molecule with resonance structures does not oscillate between the different contributing structure; instead, it is something in between.

[JWT Kottekoe]

I never did understand why the word resonance is used. It makes it sound like the bonds are oscillating. The reality is actually much simpler to visualize.

[Hari Seldon]

Originally Posted by Busy Scissors

It is the case, yes, at least as far as the everyday meaning of superposition is concerned. The bond order of benzene, for example, is 1.5 - midway between that of a single bond and a double bond.

When undergrads first learn resonance they draw out one resonance form, and then use curly arrows to show the pi bonds moving to another form. This can be confusing, as it suggests the resonance forms are in some sort of chemical equilibrium through the movement of electrons. This is not the case, and it should be emphasised that each resonance structure merely contributes to an overall resonance hybrid - as the wiki page puts it the molecule does not oscillate back and forth between the contributing structures. So in benzene each bond is a superposition of a single and double bond.

Does this answer your question, or did you have a more specific definition of superposition in mind?

This exactly answers my question. Did you look at that web site, BTW? It is a stunning photo using an atomic force microscope of something called hexabenzocoronene which consists of a benzene ring, joined by more rings to 6 more benzene rings. I was showing this to my wife and explaining to her about resonance bonds (which this was full of and not just 50-50 resonance; see photo D in the link) and I was suddenly struck by the idea, apparently true, that perhaps all these bonds were simply in states of quantum superposition. I googled resonance bonds and there were several sites that described them, but the magic word superposition never appeared. Of course, I turned to TSD for the answer. Many thanks.

[KarlGauss]

Originally Posted by Hari Seldon

Did you look at that web site, BTW? It is a stunning photo using an atomic force microscope. . .

The site links to an earlier image from the same group. I was interested to see the skepticism expressed about that image by many commenters who used words to the effect of (and sometimes explicitly) "It's Photoshopped".

I don't know enough to comment but it does remind us that (much like those fabulous NASA, and other, astronomy pics), the image gets a lot of "processing", with such processing necessarily based on the investigator's expectations.

[Hari Seldon]

I read the comments and while there were a couple that claimed they were photoshopped, they were unsupported by evidence and there was an explicit denial. So the authors are being accused of scientific fraud. I would want more evidence than that.

[astro]

Originally Posted by Szlater

I thought I did quite well considering my under-grad organic chemistry is 15 years out of date

Actually, I don't think I did organic chemistry in my final year....I think I only did analytical and pharmacological/medicinal units only...so 16 years out of date.

Now I teach 7-11 year olds to dissolve salt in water and to separate sand and marbles.

There's this show called "Breaking Bad".... eh...never mind.