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Topic: aromatic REACTIONS  (Read 4847 times)

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Offline helper

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aromatic REACTIONS
« on: December 31, 2013, 01:11:55 PM »
hello,
my wuestion is attached. thank you ahead:)

Offline orgopete

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Re: aromatic REACTIONS
« Reply #1 on: December 31, 2013, 05:19:44 PM »
I don't know the answer, but I would guess you should start by drawing the intermediate that could form. Can it do any chemistry besides reverse back to starting material? Were similar examples taught in class or your textbook?
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Offline Rutherford

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Re: aromatic REACTIONS
« Reply #2 on: January 01, 2014, 08:11:33 AM »
The electrophilic aromatic substitution you wrote shouldn't happen without a Lewis acid, because the amino residue activates by resonance ortho and para positions but they are occupied by alkyl moieties while the positive and negative inductive effect of the alkyl and amino moiety cancel each other almost out completely.
I think that substitution on another position would be more probable. Which position is stabilized by the aromatic benzene nucleus?

Offline orgopete

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Re: aromatic REACTIONS
« Reply #3 on: January 01, 2014, 08:50:36 AM »
Anilines are sufficiently electron rich to react without a Lewis acid.

Hint, draw the expected intermediate (and it's resonance structures), and think rearrangement.
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Offline AlphaScent

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Re: aromatic REACTIONS
« Reply #4 on: January 02, 2014, 05:00:40 PM »
When I see this I assume allylic bromination.  There is not enough information in my opinion.  How many equivalents of bromine? Solvent?

I assume 1 eq. in DCM?
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Offline orgopete

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Re: aromatic REACTIONS
« Reply #5 on: January 02, 2014, 07:46:41 PM »
Hint, draw the expected intermediate (and it's resonance structures), and think rearrangement.

Follow the hint. Look at the intermediate. Is there a quaternary center next to a secondary center? Do you need more?
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Offline Vidya

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Re: aromatic REACTIONS
« Reply #6 on: January 02, 2014, 07:59:46 PM »
Molecule is sterically very hindered

Offline helper

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Re: aromatic REACTIONS
« Reply #7 on: January 03, 2014, 12:33:55 PM »
Thank you all, I have tried to draw the intermediate and I can not understand why the reaction could not occure the only reason that is reasoable is the steric inderence.

Offline AlphaScent

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If you're not part of the solution, then you're part of the precipitate

Offline orgopete

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Re: aromatic REACTIONS
« Reply #9 on: January 03, 2014, 05:44:10 PM »
Thank you all, I have tried to draw the intermediate and I can not understand why the reaction could not occure the only reason that is reasoable is the steric inderence.

That is not the right intermediate. The nitrogen has the greatest directing power. Donation leads to an ortho attack. Try that intermediate (and it's resonance structures).

There had been a question about the synthesis of 1,3,5-tri-t-butylbenzene from 1,4-di-t-butylbenzene several years ago on this forum or, for example see post-lab question 1 here, http://courses.chem.psu.edu/chem36/Experiments/Exp61.pdf. You can also see that mesitylene can be produced from a mixture of xylene with an acid catalyst, Wikipedia. No mechanism has been provided, but alkyl rearrangements have precedent. The reaction in question appears to follow a very plausible route (to me).
« Last Edit: January 03, 2014, 08:00:21 PM by orgopete »
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Re: aromatic REACTIONS
« Reply #10 on: January 04, 2014, 01:08:10 AM »
Now I got confused. May you please draw the intermediate?

Offline orgopete

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Re: aromatic REACTIONS
« Reply #11 on: January 04, 2014, 01:47:24 AM »
If bromination occurs on the ortho carbon, then it will have an Et and Br on the ortho carbon. One of the resonance structures will have an adjacent carbocation, and it is secondary. It rearranges.
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Offline orgopete

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Rearrangement of 1,2,4-trimethylbenzene to 1,3,5-trimethylbenzene (mesitylene)
« Reply #12 on: January 04, 2014, 05:24:33 PM »
Okay, without giving the answer, let me suggest how 1,2,4-trimethylbenzene might rearrange to mesitylene with an acid catalyst. Although I could have written this as a simple equilibrium, as obviously the starting material on the left is simply a different resonance form than that on the right, I wanted to show how one of the resonance structures might rearrange. I don't know this is exactly what happens in the bromination reaction, isomerization of xylenes to mesitylene and toluene, rearrangement of tri-t-butylbenzene, and others, but I believe it is similar to other rearrangement reactions. Deuterobenzene is produced by protonation of benzene with D2SO4, so while this may look unusual, it is not.

Perhaps someone will be able to draw the product of the bromination reaction.
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Offline helper

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Re: aromatic REACTIONS
« Reply #13 on: January 07, 2014, 05:18:57 PM »
thank you veymuch, you are very helpfull!

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