[pulseultra]

Hey guys.

So my teacher's notes say that "epoxide opening is irreversible when using a nucleophile."
I was wondering if this statement is true even if you're using a weak nucleophile (I was thinking like an iodine anion).
As well, if your cyclic ether is a 5, 6 or 7 membered-ring, we're talking about less ring strain being relieved...

Any thoughts?

[MissPhosgene]

While you can make alpha-iodo alcohols by reacting I2 with an epoxide, it is really easy to reform the epoxide again. Reversibility would depend on reaction conditions.

Basic conditions would definitely result in reversibility. (non-nucleophilic base or else you would get a mess).

If you open an epoxide with PhS-, for example, it is not reversible.

[Doc Oc]

I think you may be misinterpreting what he said.  Look at an aldol reaction.  The reaction is totally reversible unless you manipulate the conditions to favor product formation (either adding a drying agent to scoop the water out or distilling out the product).

On the other hand, epoxide opening relieves a lot of angle strain and once it's open it won't spontaneously close again, you need to add something to force the closing.  That's what is meant by "irreversible."

FYI, iodide is a very good nucleophile.

[MissPhosgene]

I forgot about your comment about larger cyclic ethers.

They are harder to cleave... you usually need something strongly electrophilic such as TMSI, TMS(I)2, BBr3, etc.

[pulseultra]

Thanks for the perspective guys.

J-bone, thanks for reminding that I- is a good nucleophile. I have a hard time getting it in my head that the halides, save for F-, can be both good nucleophiles and good leaving groups. I promise to keep repeating "big electron cloud good for SN2" until it sticks.