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Topic: Why m-xylene?  (Read 14324 times)

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Offline Pranav

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Why m-xylene?
« on: January 03, 2013, 08:27:41 AM »
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!
« Last Edit: January 03, 2013, 09:19:51 AM by Arkcon »

Offline OC pro

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Re: Why m-xylene?
« Reply #1 on: January 03, 2013, 10:05:47 AM »
Nope. I would say answer C.

Offline Pranav

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Re: Why m-xylene?
« Reply #2 on: January 03, 2013, 11:31:51 AM »
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.

Offline discodermolide

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Re: Why m-xylene?
« Reply #3 on: January 03, 2013, 11:37:21 AM »
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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Offline curiouscat

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Re: Why m-xylene?
« Reply #4 on: January 03, 2013, 11:54:02 AM »
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?

Offline discodermolide

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Re: Why m-xylene?
« Reply #5 on: January 03, 2013, 12:07:02 PM »
No idea! ???
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Offline curiouscat

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Re: Why m-xylene?
« Reply #6 on: January 03, 2013, 12:17:37 PM »
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!

Offline curiouscat

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Re: Why m-xylene?
« Reply #7 on: January 03, 2013, 01:26:49 PM »
Does anyone have access to Gaussian or another QM / DFT code? I wonder how the energies stack up.

Offline Pranav

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Re: Why m-xylene?
« Reply #8 on: January 04, 2013, 03:48:34 AM »
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?

Offline curiouscat

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Re: Why m-xylene?
« Reply #9 on: January 04, 2013, 07:10:53 AM »
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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Re: Why m-xylene?
« Reply #10 on: January 04, 2013, 07:48:30 AM »
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. 

Offline curiouscat

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Re: Why m-xylene?
« Reply #11 on: January 04, 2013, 07:56:58 AM »
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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Re: Why m-xylene?
« Reply #12 on: January 04, 2013, 08:28:03 AM »
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.

Offline curiouscat

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Re: Why m-xylene?
« Reply #13 on: January 04, 2013, 10:07:16 AM »
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.

Offline discodermolide

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Re: Why m-xylene?
« Reply #14 on: January 04, 2013, 10:08:03 AM »
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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