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What is the composition of misture of these compounds in their keto-enol tautomeric form?

1.Ethyl aceto acetate
[CH₃-CO-CH₂-CO-OCH₃


2.Acetyl acetone
[CH₃-CO-CH₂-CO-CH₃]

Explain the reason for their more or less enol content in the two tautomers[Brief explanation please]

[movies]

Well, what have you come up with so far?

Think about what makes an alpha proton to a carbonyl acidic.  How are the alpha protons different in the two cases you showed?

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Comeon movies.Dont consider me a amateur.I would not have asked it if it was so simple.Any way here is what written in my book.


The second structure i have mentioned will have more enol content because it has conjugation in its enol form[most importantly] and secondly it has intramolecular H bonding.{1st factor is the most important]

Now in 1st structure there is intramolecular H bonding but the most important factor i.e the conjugation is not there.Though the enol seems to be conjugated system.But the C=O group will not interact with C=C in it due to its crossed conjugated structure[draw it to make it clear].Now i dont understand what is a cross conjugated structure and what is its significance?

[My Book-Unabridged editon of Organic chemistry by IL Finar]


So please give a brief of this.
Any help appreciated.

[movies]

Heh, sorry I just like to keep everyone honest.

The explanation from your book is correct, I guess.  I wouldn't call it a cross-conjugated system, but I suppose you could make that argument.

Cross conjugation occurs when you have two electron donating groups on opposite sides of a withdrawing group.  In such a case you can only draw resonance structures that involve one of these donating groups at a time.  Therefore, the competition between the two groups leads to overall a worse stabilization.

The explanation I would go for would be based more on the pK_a difference between the two compounds.  The one with a higher pK_a would have less enol content.  

The pK_a explanation is a lot simpler, but I suppose the cross-conjugation explanation is more complete.  The cross-conjugation effect is accounted for in the pK_a, but not explicitly.

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Yes.This is more like it.

Ok now can you tell about that whether their will be cross conjugation in these compounds.

1.Benzoic acid.
2.C₆H₅-COOCH₃

And if it exist.What will be the nature of Orientation of benzene ring in them.


I myself found that there is crossed conjugation in this structure O=C₆H₄=O.And thats why it is somewhat more unstable than benzene ring though having a conjugated structure.Is it correct?

I hope you will clear my doubt.
Thanks.

[movies]

Benzoic acid and methyl benzoate would both be cross conjugated.  For this to occur, the bezene ring would have to be aligned right to allow for overlap between the p-orbitals of the ring carbon and the carbonyl carbon.

Benzoquinone is also cross-conjugated, but it's an unfair comparison with benzene since they are in a different oxidation state.

You have to think of it more like "divinyl ketone is less stable than would be expected based on the energy of acrolein."

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you mean to say that the -OH with C=O carbonyl is not in plane with with benzene ring in benzoic acid that means only carbonyl group and benzene ring are in plane causing conjugation not the OH group.Hence COOH group is meta group.Am i correct?

Now tell me i am coorect on this or not?
The reason why the OH group is not in plane because O is Sp3 Hybridized.Right or not?Or what is the reason they are not in a plane.

Also Please explain me the Orientation of benzene ring in Cinnamic acid.

Waiting for reply...

[movies]

The OH and the C=O will always be in the same plane or at least close enough.  The key is the rotation around the C-C bond between the ring carbon and the C=O carbon.  The p-orbitals on these two carbons have to line up right for the ring to be conjugated with the carbonyl.  The meta directing ability of such compounds is due to the conjugation between the carbonyl and the ring, but the OH has little effect.  I'm not sure that you can do Freidel-Crafts reactions with carboxylic acids though, they might coordinate the Lewis acid too strongly or else just react with the acid chloride.

Cinnamic acid is pretty much the same as benzoic acid, you just add the two carbons in between.  Again the key to conjugation between the ring and the exocyclic double bond is the the p-orbitals of the ipso ring carbon and the benzylic carbon have to be lined up in a bonding fashion.  In order to have conjugation all the way to the acid then the C=O carbon and the carbon adjacent to it must also have the p-orbitals aligned.

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You mean the p orbital of C is parallel with that of p orbital of key C of the benzene ring.But the p orbital of O is not properly oriented as with rest to show any conjugating effect.[Though OH is in plane with carbonyl]

Now the favourite question is why the same is not with the Ethyl acetoacetate which i mentioned in my 1st post.Is that becuase in that case all the p orbitals are with similar parallel orientation.

But i see the similar Conjugation in the Ethyl acetoacetate.So why it has proper orientation but not that in benzoic acid.I mean how can you tell about the orientation of the p orbital.

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And what is the orientation of C6H5-COOCH3

By the conjugation between carbonyl group with ring, this should also be mata directing.Is it?

Waiting for your reply...

[cbee]

I don't think the ethylacetoacetate case has much to do with p-orbital orientation specifically.  I also believe (although I could be mistaken) that your book is wrong.  There is conjugation to the ester of ethylacetoacetate, just less than for the acetyl acetone.  This is becuase the carbonyl of the ester is less electropositive than that of the  ketone (due to the lone pair donation) of the OEt), and therefore less conjugated with the electron donating enol tautomer.

I think.


Cisco

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My book could be wrong.But i will go with what Movies have posted.He is by far the most eligible posters in Organic chemistry section.Thats why i prefer to discuss with him whenever i have doubt in Basic concepts.

[movies]

The effect in ethyl acetoacetate and benzoic acid is really the same.  There is the same concern about orbital alignment for conjugation to occur.  The rotation about the C-C bond is variable, but only when the orbitals are aligned (so only when in a particular orientation) will there be conjugation between the two groups.  In either case the C-C bond could rotate so that the two p-orbitals in question were at a 90 degree angle to one another, and then conjugation is impossible.

I should say that I think that this is a rare application of the term "cross-conjugated" and if I were confronted with this question I would give an answer like cbee wrote.  However, the "electropositivity" argument actually includes the cross conjugation argument implicitly.  One is an orbital argument, the other is an empirical (or perhaps you could call it an "electrostatic") argument.

Bottom line, to have a cross-conjugated system, you have to have more than one group in the proper orientation for donating electrons.