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Topic: Huckel Rule (benzene aromaticity)  (Read 10053 times)

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Offline zerokun

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Huckel Rule (benzene aromaticity)
« on: May 29, 2008, 02:52:34 AM »
A question in Murray text book --p.p524 /Q 15.30

3-chlorocyclopentene , on treatment with AgBF4 , gives a precipitate of AgCl and a stable solution of a product that shows a single 1HNMR absorption at 11.04 chemical shift.

-What is it likely structure of the product ?
-What is its relation to Huckel rule ?

I go and search for chemical shift , the 11.04 accounted for a carboxylic acid functional group
And since it is single signal, it may due to the exchange of OH disrupt coupling.
However, i cant come out with the likely structure, because in the reaction there is nothing seems can form an -OH group. I know about Huckel rule but i cant explain before i can work out the structure


Does anyone has any idea what happens when we add AgBF4 to 3-chlorocyclopropene ?


Offline Rico

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Re: Huckel Rule (benzene aromaticity)
« Reply #1 on: May 29, 2008, 05:09:58 AM »
Hey zerokun

Is your question about 3-chlorocyclopentene or 3-chlorocyclopropene?

Rico

Offline zerokun

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Re: Huckel Rule (benzene aromaticity)
« Reply #2 on: May 29, 2008, 07:24:53 AM »
sorry, is 3-chlorocyclopropene


I try to think and come out another idea that BF4 in this case acts as base. but i not sure...and i get confused with the 1HNMR signal  ??? ??? ???

Offline Rico

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Re: Huckel Rule (benzene aromaticity)
« Reply #3 on: May 29, 2008, 09:08:52 AM »
Hey zerokun

Ok what happens is that the silver-cation takes of the chloride as a chloride-anion, an gives AgCl which precipitates. This leaves a special cation, with BF4- as the counteranion. What is the structure of this cation? An what is so special about it? (hint: this is where the Hückel rule of aromaticity comes in)

Rico

Offline zerokun

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Re: Huckel Rule (benzene aromaticity)
« Reply #4 on: May 31, 2008, 12:31:35 AM »
Hi Rico,

I think i am able to figure out
is that the 3-chlorocyclopropene becomes a special cation in this reaction? because the Cl leaves .

there is 4 pi-electron can delocalise into the ring, and the signal is due to the aromaticity of the compound (delocalization of pi electron) and not COOH as what i supposed.

Thank you for your help !

Offline macman104

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Re: Huckel Rule (benzene aromaticity)
« Reply #5 on: May 31, 2008, 01:20:49 AM »
I think i am able to figure out
is that the 3-chlorocyclopropene becomes a special cation in this reaction? because the Cl leaves
Yes.
Quote
there is 4 pi-electron can delocalise into the ring, and the signal is due to the aromaticity of the compound (delocalization of pi electron) and not COOH as what i supposed.
Correct about it not being a COOH, but count again, there are not 4 pi-electrons.  What number of electrons provide you with an aromatic system according to the huckel rule (_n+_)?  How many does the cyclopropene cation have?

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