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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: cabaal on December 09, 2010, 03:51:09 PM

Title: Epoxide ring-opening under acidic conditions
Post by: cabaal on December 09, 2010, 03:51:09 PM
Under acidic conditions the nucleophile attacks the more hindered carbon. Under basic, the less hindered.

(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg135.imageshack.us%2Fimg135%2F6648%2F29894987.png&hash=a3f420cce3c7bdc1983ccf306cd751992b574578)

Why in this problem (acidic conditions) does the reaction give a major product characteristic of the nucleophile attacking the less hindered carbon?
Title: Re: Epoxide ring-opening under acidic conditions
Post by: willug on December 09, 2010, 05:37:06 PM
Why would you not expect the major product to be the one where the EtOH attacks the least hindered carbon?? What is the reasoning behind that rule?

I am confused!
Title: Re: Epoxide ring-opening under acidic conditions
Post by: MissPhosgene on December 09, 2010, 05:40:05 PM
This could be because the sulfuric acid is catalytic and allows the reaction to proceed by an Sn2 mechanism.

I didn't see the full scheme origionally. Sorry.
Title: Re: Epoxide ring-opening under acidic conditions
Post by: cabaal on December 09, 2010, 05:48:43 PM
Quote from: willug on December 09, 2010, 05:37:06 PM
Why would you not expect the major product to be the one where the EtOH attacks the least hindered carbon?? What is the reasoning behind that rule?

I am confused!

My notes say that under basic conditions the nucleophile attacks the least hindered, and under acidic conditions it attacks the most hindered. Is this incorrect?
Title: Re: Epoxide ring-opening under acidic conditions
Post by: MissPhosgene on December 09, 2010, 08:55:26 PM
No, but in order to rationalize the discrepancy I would say the the epoxide is activated by catalytic acid in a way similar to how a carbonyl can be activated towards nucleophilic substitution by catalytic acid.
Title: Re: Epoxide ring-opening under acidic conditions
Post by: orgopete on December 09, 2010, 10:10:31 PM
It looks like an error.
Title: Re: Epoxide ring-opening under acidic conditions
Post by: uvcyclotron on December 10, 2010, 08:04:09 AM
It definitely looks like an error.
As far as I know, epoxide ring opening in guided by stability of C+(Carbocation) formed under Acidic conditions,
and under Basic/Neutral conditions, it is guided by steric hindrance.

So according to that, I think the image itself, provided by you might have error.
Title: Re: Epoxide ring-opening under acidic conditions
Post by: AC Prabakar on December 14, 2010, 07:27:41 AM
Quote from: uvcyclotron on December 10, 2010, 08:04:09 AM
It definitely looks like an error.
As far as I know, epoxide ring opening in guided by stability of C+(Carbocation) formed under Acidic conditions,
and under Basic/Neutral conditions, it is guided by steric hindrance.

So according to that, I think the image itself, provided by you might have error.

Yes! this is correct.

Pls find the detailed of the same in the below page:
http://www.chem.ucalgary.ca/courses/350/Carey/Ch16/ch16-6-3.html
Title: Re: Epoxide ring-opening under acidic conditions
Post by: cupid.callin on December 15, 2010, 09:22:42 AM
@prabakar
I saw your link which says that mechanizm follow carbocation stability.
But in all examples i came across while studying (many as i am studying for my exams these days) i observed that till both carbons forming epoxide. if both are 2 or 1 degree ... SN2 mechanism follows, but if any1 is 3 degree ... SN1 mechanism follows. Though i have no surety about this, its just my observation. But yes this is valid for ether (non cyclic) cleavage surely.

Please any cross check this.