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Topic: Why m-xylene?  (Read 14252 times)

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Offline curiouscat

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Re: Why m-xylene?
« Reply #15 on: January 04, 2013, 10:16:25 AM »
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

Offline discodermolide

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Re: Why m-xylene?
« Reply #16 on: January 04, 2013, 10:24:13 AM »
Pages 516-521 in the 4th edition. It's too much to reproduce here.

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Offline Pranav

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Re: Why m-xylene?
« Reply #17 on: January 04, 2013, 11:02:29 AM »
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.


Offline discodermolide

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Re: Why m-xylene?
« Reply #18 on: January 04, 2013, 11:08:47 AM »
Just look under toluene.
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Offline Pranav

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Re: Why m-xylene?
« Reply #19 on: January 04, 2013, 11:22:00 AM »
Just look under toluene.

I am still unable to find it. I have this edition of the book: Google Books
Can you direct me to the page number from the link?
« Last Edit: January 04, 2013, 11:49:35 AM by Pranav »

Offline curiouscat

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Re: Why m-xylene?
« Reply #20 on: January 04, 2013, 11:52:37 AM »
Cresols might be an interesting contrast. Does anyone know the corresponding product distribution there?

Offline Raphael

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Re: Why m-xylene?
« Reply #21 on: January 04, 2013, 12:14:07 PM »
Just look under toluene.

I am still unable to find it. I have this edition of the book: Google Books
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)

Offline Pranav

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Re: Why m-xylene?
« Reply #22 on: January 04, 2013, 12:24:28 PM »
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.

Offline orgopete

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Re: Why m-xylene?
« Reply #23 on: January 04, 2013, 05:50:12 PM »
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. 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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Offline curiouscat

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Re: Why m-xylene?
« Reply #24 on: January 08, 2013, 01:04:31 AM »
Quote
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.

Offline orgopete

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Re: Why m-xylene?
« Reply #25 on: January 08, 2013, 01:22:10 AM »
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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Offline curiouscat

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Re: Why m-xylene?
« Reply #26 on: January 08, 2013, 01:43:30 AM »
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.

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