[ducthinh]

Hi...
        I really confused about the mechanism of the reaction of PCl5 and R-OH . In some books , they said that R-OH react with PCl5 via SNi mechanism but the other said that go through SN2 mechanism......plz tell me something about that . thankz

[Albert]

To be frank with you, I wouldn't say that this kind of reaction has an SN2 or SN1 mechanism.

However, final step looks like an SN2 mechanism more than an SN1 one.

This mechanism is the same for primary, secondary and tertiary alcohols and for PCl3, PCl5 and P + I2.

[movies]

I would think that the first step of the mechanism would have to be S_N1 like with attack at phosphorus.  S_N2 would require you to get through a lot of steric bulk first.

[Albert]

So, do you think this reaction to be a 'mix' of S_N-mechanisms?

[movies]

I guess you could call it that.  I think that the alcohol attacking the PCl₅ probably starts with one of the Cls dissociating from P to make PCl₄⁺, and then the alcohol attacks P (so that part is S_N1 like).  The second part, where Cl^- attacks C, is almost certainly S_N2 style, unless it is at a tertiary carbon or something like that.

[plu]

According to McMurry's "Organic Chemistry", both steps of the reaction are S_N2  

[runinwind]

It's neither sn1 or sn2,it's mixed.No extra reaction!