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Topic: SNR2 Radical Mechanism  (Read 2868 times)

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Offline oscillate_wildly1

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SNR2 Radical Mechanism
« on: March 17, 2014, 12:10:17 PM »
Hi,

So all reaction schemes of the SNR2 mechanism of benzylhalides show an intermediate radial anion being formed before radical benzene and the halide anion. I want to know how this is possible in terms of curly arrow mechanisms. I've attached what I think is going on, but it doesn't go through the radical anion. Any insights into the mechanism would be greatly appreciated!

Here's what I have:


Offline discodermolide

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Re: SNR2 Radical Mechanism
« Reply #1 on: March 17, 2014, 12:15:59 PM »
Effectively your mechanism goes through that intermediate. All it is showing is that an electron has been added to the system giving a radical anion (generic). Now the next step is the formation of that benzene radical and iodide.
Can you draw the resonance forms for the stabilisation of the radical?
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Offline oscillate_wildly1

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Re: SNR2 Radical Mechanism
« Reply #2 on: March 17, 2014, 12:58:00 PM »
Thanks for the reply!

So the intermediate shown in text books is misleading (and possibly wrong?), since a radical anion technically isn't formed whilst benzene is covalently bonded to iodine? Because the iodine-benzene bond breaks in the first step, so a radical anion is never formed, unless another electron is introduced into the system?

Here's what I have for the resonance stabilisation forms:



Offline discodermolide

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Re: SNR2 Radical Mechanism
« Reply #3 on: March 17, 2014, 02:01:24 PM »
No, the intermediate is OK.
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Offline orgopete

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Re: SNR2 Radical Mechanism
« Reply #4 on: March 17, 2014, 05:51:09 PM »
Thanks for the reply!

So the intermediate shown in text books is misleading (and possibly wrong?), since a radical anion technically isn't formed whilst benzene is covalently bonded to iodine? Because the iodine-benzene bond breaks in the first step, so a radical anion is never formed, unless another electron is introduced into the system?

Here's what I have for the resonance stabilisation forms:



I don't agree with these resonance structures. The intermediate radical anion was okay. That radical can shift. I suggest drawing in the hydrogen atoms so you won't be confused with what is and is not present. I think once the iodide leaves, a single electron radical can be drawn at its position. Shifting that radical will leave a linear sp-carbon in the ring. I don't think that will happen.
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