Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Pranav on January 03, 2013, 08:27:41 AM
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Here's the question: http://i48.tinypic.com/30wutle.png
This looks to me a simple Friedel–Crafts alkylation and the product should be p- and o-xylene. But the answer says its m-xylene. Is their some kind of rearrangement I am unaware of?
Any help is appreciated. Thanks!
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Nope. I would say answer C.
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I myself marked C but as I said before, its D. I want to confirm whether my answer is correct or not. Its likely that my answer is wrong because I have seen a few rearrangements while dealing with the aromatic rings but never came across this one.
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This appears to be temperature dependant.
I found this quote "At 0°C, substituting methyl groups into methylbenzene, you get a mixture of the 2-.3- and 4- isomers in the proportion 54% / 17% / 29%. " and "At 25°C, the proportions change to 3% / 69% / 28%. In other words the proportion of the 3- isomer has increased even more. Raise the temperature some more and the trend continues".
At higher temp. the proportion of the meta apparently increases.
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This appears to be temperature dependant.
I found this quote "At 0°C, substituting methyl groups into methylbenzene, you get a mixture of the 2-.3- and 4- isomers in the proportion 54% / 17% / 29%. " and "At 25°C, the proportions change to 3% / 69% / 28%. In other words the proportion of the 3- isomer has increased even more. Raise the temperature some more and the trend continues".
At higher temp. the proportion of the meta apparently increases.
That'd mean the meta isomer is the thermodynamic product, right? How'd you explain it?
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No idea! ???
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No idea! ???
This might make sense. From Wikipedia:
Std enthalpy change of formation, (gas) (kJ/mol)
ortho 19.0
para 17.96
meta 17.2
That'd say that meta is indeed most thermodynamically stable, eh? So we'd indeed expect meta to dominate at high Temperatures?
I don't know how accurate those numbers are!
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Does anyone have access to Gaussian or another QM / DFT code? I wonder how the energies stack up.
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Thanks everyone for the *delete me*
Isn't their any reason for m-xylene to be formed at higher temperature? The values curiouscat posted show that it is a thermodynamic product. Why m-xylene is stable when the temperature is raised?
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Thanks everyone for the *delete me*
Isn't their any reason for m-xylene to be formed at higher temperature? The values curiouscat posted show that it is a thermodynamic product. Why m-xylene is stable when the temperature is raised?
One mistake I made is that these are ΔH values. I ought to use ΔG.
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It seems that o and p are formed but there is migration of methyl groups. The most thermodynamic stable compound however is m-xylene. Therefore, prolonged heating provides m-xylene. Have never heard it before I must admit.
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It seems that o and p are formed but there is migration of methyl groups. The most thermodynamic stable compound however is m-xylene. Therefore, prolonged heating provides m-xylene. Have never heard it before I must admit.
Yes, I feel a bit let down too. This sounds like a fairly mainstream reaction. How come this was never discussed in any of my OChem classes? Is meta the dominant product in other non-xylene reactions too, I now wonder.
If I may rant a bit against the way OChem was (is?) taught was that it emphasized mechanisms and elegant generalizations a bit too much. But I rarely remember much use of actual experimental / calculated ΔG, Kact values etc. at least at the pedagogic stage.
It makes reactions seem neater and more consistent than they really are in practice. Is this just my idle rant, or do I have sympathizers?
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Symphatizer here. Although I have to thank my Professor who really clarified every mechanism also from the thermodynamic point of view. It was a nice feeling when one day it made "click" in my brain loops making the understanding of every mechanism very easy later on. There are only a few things to now: - reacts with +; acid-base chemistry (deprotonation/protonation) and some general issues.
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Thanks everyone for the *delete me*
Isn't their any reason for m-xylene to be formed at higher temperature? The values curiouscat posted show that it is a thermodynamic product. Why m-xylene is stable when the temperature is raised?
Here are entropies (also from Wikipedia):
Standard molar entropy, J/(mol K)
ortho 353.6
para ?
meta 358.2
I think you ought to do some of the number crunching too now. Over a range of Temps. find the ΔG and see if stability reverses.
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I admit I was not aware of this little fact. But found it in March's book after a long search.
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I admit I was not aware of this little fact. But found it in March's book after a long search.
This book?
March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure
Can you post the snippet / context / section? I'm trying to find it too. The books on Google, free to preview BTW
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Pages 516-521 in the 4th edition. It's too much to reproduce here.
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Pages 516-521 in the 4th edition. It's too much to reproduce here.
I do have the 4th edition of the book but I think there are differences in the copies we both have. In my country, Wiley sells a reduced price edition of the books, so the page numbers won't match. Can you tell me what's present on those pages, like what you searched for in the book or the name of the reaction? Anything would help.
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Just look under toluene.
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Just look under toluene.
I am still unable to find it. I have this edition of the book: Google Books (http://books.google.co.in/books?id=4zrovKbIxOIC&pg=PP1&dq=March%27s+advanced+organic+chemistry+4th+edition&hl=en&sa=X&ei=6wfnUNSkI4TDkQWL2oGQAg&ved=0CE0QuwUwBQ#v=onepage&q&f=false)
Can you direct me to the page number from the link?
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Cresols might be an interesting contrast. Does anyone know the corresponding product distribution there?
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Just look under toluene.
I am still unable to find it. I have this edition of the book: Google Books (http://books.google.co.in/books?id=4zrovKbIxOIC&pg=PP1&dq=March%27s+advanced+organic+chemistry+4th+edition&hl=en&sa=X&ei=6wfnUNSkI4TDkQWL2oGQAg&ved=0CE0QuwUwBQ#v=onepage&q&f=false)
Can you direct me to the page number from the link?
Try looking in the "Aromatic Substitution, Electrophilic" chapter ( or whatever chapter has a name closest to this which is the name from 6th edition)
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I think I have found it, its under "Carbon Leaving Groups" on page 561. The book also states that the meta isomer is preferred because it has the highest thermodynamic stability.
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I was not surprised by this reaction. Alkyl migration is noted in Williamson, Macroscale and Microscale Organic Experiments.
I am not going to be a sympathizer on the mechanism issue. Although I too favor teaching reaction mechanisms, I am reluctant to place too much truth to what may be written. I expect that between initially learning how electrons move to form a product, the description(s) are not all inclusive. That is the case for this reaction. The ortho/para products do form initially, but rearrange to the meta-isomer. If there was a failure, it would have been a failure to rationalize this reaction. It is useful to attempt to rationalize formation of unexpected products, not necessarily to learn an unusual path, but rather to recognize competitive elements present in reactions. A simple example is a Claisen condensation with an alkoxide. Many students forget that extensive attack is also occurring upon the carbonyl group because it is a non-productive reaction. If a different alcohol were used, ester exchange would be much faster than condensation.
I too was schooled in a rearrangement reaction related to a pinacol rearrangement (http://www.chemicalforums.com/index.php?topic=64473.0). I had not expected any rearrangement to occur as I was unaware of by-products in carbonyl group protection reactions. However, it seems that rearrangements can occur for some compounds when an alcohol is not present to trap the intermediate protonated carbonyl group. It isn't necessarily a failure of the mechanism, but a failure to recognize how the intermediates might react in the presence of other potential reacting groups. Did you know benzene-d6 is made with benzene and D2SO4 (I think that is how it is done)?
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Did you know benzene-d6 is made with benzene and D2SO4 (I think that is how it is done)?
Sorry, I didn't understand that part.
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If you protonated benzene with D2SO4 and deprotonate it, you can exchange the hydrogen atoms. If a protonation or Lewis acid complexes with the aromatic ring, migration, dealkylation, or alkyl transfer can occur.
Perhaps someone could report on the fate of doubly labeled and unlabeled xylenes to tell us whether it is an intramolecular migration or whether the labels become scrambled.
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If you protonated benzene with D2SO4 and deprotonate it, you can exchange the hydrogen atoms. If a protonation or Lewis acid complexes with the aromatic ring, migration, dealkylation, or alkyl transfer can occur.
Perhaps someone could report on the fate of doubly labeled and unlabeled xylenes to tell us whether it is an intramolecular migration or whether the labels become scrambled.
Gotcha! Thanks.