The Benzene Molecule

[Ronald Hyde]

The problem with guessing what someone else means is that you can easily guess wrongly, as you have done here.

You have guessed wrongly because you have absolutely no idea what I mean and instead of the asking the simple and sensible question

'Please explain further?' or 'what do you mean?' or somesuch

You have the appalling arrogance to dimiss my comments as 'derivative' - whatever that means.

 

Now you're the one who's misunderstanding. I did not say your comments were 'derivative',

I was refering to the concept of resonance as explained by the other posters.

 

Do you understand the term 'delocalisation energy', which for benzene is 155 kjoules per mole.

How would you measure this ?

 

Do you know what the LCAO method is or what a basis means?

 

That's just regular quantum mechanics applied to chemistry, finding the ground state

of a system etc..

Do you know what the

[math]{\psi _B}[/math]

wavefunctions represent or what their contribution is to the resonance mechanism?

No, I don't know about that.

 

Finally do you know why acetoacetic acid is not a resonance structure, but a tuatomeric compound?

If you like what is the essential difference between a tautomeric substance and a resonance substance?

 

[math]C{H_3}.CO.C{H_2}.COO{C_2}{H_5}\; \Leftrightarrow \;C{H_3}.C(OH):CH.COO{C_2}{H_5}[/math]

It involves the displacement of a proton, in this case, and exhibits different properties.

 

Oh and by the way the correct term for what happens in ammonia or the ammonium ion is hybridisation not resonance.

 

Hybridisation and resonance are independent concepts, they can happen together, or not. Feynman describes the

resonance.

[juanrga]

You caused me to reread the first two pages of the Feynman reference, and he's describing two states which differ in time,

 

As said to you "The inversion of the Ammonia molecule is a real process". Feynman is just describing that.

 

The simple fact that they are very long lived states compared to the benzene states

 

What part of the statement "the Benzene molecule is not given by any of the resonant forms but that both resonant forms together describe the molecule" you do not still understand? Take a look to the image in #23. That is a representation of the Benzene molecule beyond the limitations of Lewis structures.

 

people will still tell you the W boson is 'real'.

 

Nobody, except you, mentioned a W boson. Evidently the W boson is real. What is your misunderstanding here?

Edited August 8, 2012 by juanrga

[studiot]

studiot, on 7 August 2012 - 05:12 PM, said:

 

So when offered further more detailed information about a subject why are you not interested?

 

 

Ronald Hyde : Your direct answer to my question:-

 

No, it's not that I'm not interested, not at all. I would not in the least deny their truth. It's that they're all derivative.

 

 

Yet when challenged about this you deny it and state quite the opposite

 

I did not say your comments were 'derivative',

 

Because you were so busy avoiding hearing what others say you missed the most important point of what I wrote in my post.

 

It is well established that an oscillation or vibration occurs by exchanging energy back and fore within the vibrating medium.

So with a pendulum for instance the energy is constantly swopped back and fore between kinetic and potential energy, but the total is constant.

Further more there has to be enough energy in the system for the system to reach max velocity and max stored potential energy.

 

Now I mentioned that the delocalisation energy of benzene is 155 kJ/mole.

 

You did not respond.

 

This means that the sum of the bond energies of three carbon - carbon double bonds, three carbon-carbon single bonds and six carbon-hydrogen bonds exceeds the measured total bond energy of benzene by 155kJ/mole.

 

If the delocalisation (also called resonance) was achieved by switching back and fore between states then the system would have to have the energy to exist in those states, however briefly.

 

But it does not possess enough energy to actually exist in a Kekule state.

 

Ergo it must exist in some other state.

 

As to the lesser point that you also initially chose to ignore, although were finally pushed to admit you did not understand.

 

No, I don't know about that.

 

 

The Kekule formula is not the only conceivable arrangement using classical single/double bond configurations. These other lesser ones also contribute to the delocalisation (resonance) effect.

 

I am, however glad you understood my point about acetoacetic acid. This can be expanded in a discussion of resonance (which affects only electrons) and other processes which affect the nuclear part of the molecule.

Edited August 8, 2012 by studiot

[Ronald Hyde]

 

It is well established that an oscillation or vibration occurs by exchanging energy back and fore within the vibrating medium.

So with a pendulum for instance the energy is constantly swopped back and fore between kinetic and potential energy, but the total is constant.

Further more there has to be enough energy in the system for the system to reach max velocity and max stored potential energy.

 

Now I mentioned that the delocalisation energy of benzene is 155 kJ/mole.

 

You did not respond.

 

This means that the sum of the bond energies of three carbon - carbon double bonds, three carbon-carbon single bonds and six carbon-hydrogen bonds exceeds the measured total bond energy of benzene by 155kJ/mole.

 

If the delocalisation (also called resonance) was achieved by switching back and fore between states then the system would have to have the energy to exist in those states, however briefly.

 

But it does not possess enough energy to actually exist in a Kekule state.

 

Ergo it must exist in some other state.

 

As to the lesser point that you also initially chose to ignore, although were finally pushed to admit you did not understand.

 

 

You do realize, do you not, that if you could supply enough energy to make one of these states, and it lasted even for

a microsecond, that the state could be said to 'exist', in a physical sense, and all of this elaborate explanation would be

moot.

 

I know that I'm being very stubborn, and that this bothers you no end, but it's not for the purpose of bothering you,

it's the principle of the thing, in this case a physical principal, that may apply throughout Nature.

[mississippichem]

You do realize, do you not, that if you could supply enough energy to make one of these states, and it lasted even for

a microsecond, that the state could be said to 'exist', in a physical sense, and all of this elaborate explanation would be

moot.

 

I know that I'm being very stubborn, and that this bothers you no end, but it's not for the purpose of bothering you,

it's the principle of the thing, in this case a physical principal, that may apply throughout Nature.

 

Nope. As I said before, the electronic structure of benzene is such that no excited state will yield a Kekule structure!

 

Do the Huckel calculation for the groundstate and it will be obvious. It's not a question of what might happen, we already know. The book is closed on aromaticity.

[studiot]

I know that I'm being very stubborn, and that this bothers you no end, but it's not for the purpose of bothering you,

it's the principle of the thing, in this case a physical principal, that may apply throughout Nature.

 

Stubborn is not the word or phrase I'd employ.

 

But then I'm just the only one here trying to work through a real practical explanation for what others have told you without detailed justification.

 

As to the universal application of a principle there are many things that no application of energy will enable.

 

For example take a shovel of calcium carbonate in equilibrium with a saturated solution of the same. No amount of energy input will turn your plaster into plaster solution because calcium carbonate is basically insoluble.

Edited August 8, 2012 by studiot

[juanrga]

You do realize, do you not, that if you could supply enough energy to make one of these states, and it lasted even for

a microsecond, that the state could be said to 'exist', in a physical sense

 

Both Physics and Nature claim otherwise.

[Ronald Hyde]

Nope. As I said before, the electronic structure of benzene is such that no excited state will yield a Kekule structure!

Do the Huckel calculation for the groundstate and it will be obvious. It's not a question of what might happen, we already know. The book is closed on aromaticity.

Then I shall admit that I'm wrong, and benzene was a bad choice for an example.

[mississippichem]

Then I shall admit that I'm wrong, and benzene was a bad choice for an example.

 

Thank you for your polite concession.

 

I've enjoyed the discussion thoroughly and will be glad to continue if you have further questions or comments.

[Ronald Hyde]

Thank you for your polite concession.

 

I've enjoyed the discussion thoroughly and will be glad to continue if you have further questions or comments.

 

Well, I'm certainly glad that someone enjoyed it, I've gained from it, but I do have one more question in the same vein. In #1

I did not use the word Kekule, but I think my description did fit the Kekule representation of benzene. However my description

of RB did not require a particular representation, it only required two time varying states that were mirror images of each other.

Are there no time varying states that are mirror images of each other, under any circumstances?

 

Thank you for you answer.

[juanrga]

However my description of RB did not require a particular representation, it only required two time varying states that were mirror images of each other.

 

As has been said to you several times now, quantum resonance in benzene molecule does not involve "two time varying states".

 

First, each one of the resonant forms is not a quantum state for benzene, but that the quantum state of benzene molecule is given by the two resonant forms. Second, each resonant form and, as a consequence, the resulting quantum state are all time independent.

Edited August 9, 2012 by juanrga

[studiot]

Just to add to what juanrga has told you,

 

You seem confused between a state and a configuration.

 

Configurations are geometrical arrangements in 1, 2 or 3 dimensions that can have mirror images.

 

States are not. They are a point on a graph, representing the value of some chosen function or set of functions at some chosen point in time or space or even some generalised variable.

 

The participating forms in benzene resonance are configurations.

Edited August 9, 2012 by studiot

[Ronald Hyde]

As has been said to you several times now, quantum resonance in benzene molecule does not involve "two time varying states".

 

First, each one of the resonant forms is not a quantum state for benzene, but that the quantum state of benzene molecule is given by the two resonant forms. Second, each resonant form and, as a consequence, the resulting quantum state are all time independent.

This is helpful.

 

Just to add to what juanrga has told you,

 

You seem confused between a state and a configuration.

 

Configurations are geometrical arrangements in 1, 2 or 3 dimensions that can have mirror images.

 

States are not. They are a point on a graph, representing the value of some chosen function or set of functions at some chosen point in time or space or even some generalised variable.

 

The participating forms in benzene resonance are configurations.

 

These last two posts have greatly clarified the situation in my own mind. In the question in #35 substitute the word 'configuration' for 'state' and it will be correctly phrased.

That is the question I seek the answer for. I will modify the OP in due course.

 

I can see I really should have payed the syntax on this one.

 

I thank you all for being ( sort of ) patient with me. I know what I want, but not always the words to put it in.