Cutting cyclic ethers

[pandalove89]

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What exactly cuts cyclic ethers or cyclic compounds altogether.


On question #41 on the 2011 DAT destroyer (Orgo section)

It says that a cyclic ether can be cut by grignard reagent. I dont understand how this is possible if Grignard is a base....

 

[pandalove89]

bump

 

[PooyaH]

One thing you need to know is that ether can only be cleaved by strong acids and not bases. But epoxides could be cleaved by both strong acids and bases. If you have a basic solution, the base will add to the less substituted side, and under acidic conditions, the nucleophile attacks the most substituted side.

 

[toothhornet88]

http://picasion.com/.

 

[toothhornet88]

 

[LetsGo2DSchool]

Pretty cool dude.

 

[pandalove89]

yeah, i know the reaction, but why? There must be some sort of explanation. If grignard is a base, how can it cleave an cyclic ether?

 

[radmazindds]

I feel like that oxygen has to get protonated before it can react with a grignard in an SN1/SN2 fashion. Maybe react with an acid first?
R-MgX is in resonance with R- +MgX, so R- acts as a base or nucleophile and can do a substitution if there were a good leaving group (e.g. ROH+). There needs to be some way to displace the electrons for this reaction to work.

OH WAIT!
if we have O- hanging out after the addition of RMgX, then +MgX can stabilize it. yes that works. forget what I said above.

yeah, i know the reaction, but why? There must be some sort of explanation. If grignard is a base, how can it cleave an cyclic ether?

 

[needzmoar]

What exactly cuts cyclic ethers or cyclic compounds altogether.


On question #41 on the 2011 DAT destroyer (Orgo section)

It says that a cyclic ether can be cut by grignard reagent. I dont understand how this is possible if Grignard is a base....

Unless the cyclic ether is under ring strain, they are stable under basic conditions. Cyclic ethers, large or small, are susceptible to cleavage under acidic conditions. I don't know what #41 is because I don't have the 2011 destroyer so I can't help you there.