[oscillate_wildly1]

Hi,

So all reaction schemes of the S_NR² 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:

[discodermolide]

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?

[oscillate_wildly1]

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:

[discodermolide]

No, the intermediate is OK.

[orgopete]

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.