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Topic: Ortho versus Para  (Read 3324 times)

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Offline daf44

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Ortho versus Para
« on: August 11, 2011, 11:45:58 AM »
Hi,

Why does the bromination of toluene mainly gives para-bromotoluene, while the nitration of toluene mainly gives ortho-nitrotoluene?


Offline Nosterius

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Re: Ortho versus Para
« Reply #1 on: August 11, 2011, 02:00:42 PM »
This is a really good question.

Are you new to electrophilic aromatic substitution? Do you have some basis in frontier orbitals? And in hard-soft acids and bases?

Offline daf44

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Re: Ortho versus Para
« Reply #2 on: August 11, 2011, 02:09:21 PM »
I'm not new to E.A.S. and I have some basis about HOMO/LUMO and soft/hard bases ans acids.

Offline Nosterius

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Re: Ortho versus Para
« Reply #3 on: August 11, 2011, 02:28:31 PM »
You should take a look into Ian Fleming's "Frontier Orbitals and Organic Chemical Reactions".

In short, what I was able to find in there is that the harder the electrophile, the less ortho/para selective it is. That is, nitronium (NO2+) is much harder than bromine (Br2).

I am still trying to find the "why" of this, but I currently do not have the time to reread chapters 1-3 of this book right now.

I hope this helps...

Offline discodermolide

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Re: Ortho versus Para
« Reply #4 on: August 11, 2011, 05:04:39 PM »
This is a really good question.

Are you new to electrophilic aromatic substitution? Do you have some basis in frontier orbitals? And in hard-soft acids and bases?
Perhaps you should take a look at this review in Angewandte chemie
Farewell to the HSAB Treatment of Ambident Reactivity
Herbert Mayr,* Martin Breugst, and Armin R. Ofial
DOI: 10.1002/anie.201007100
Published this year in the ASAP's
Development Chemists do it on Scale, Research Chemists just do it!
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