Cleavage of epoxides: acid-based vs. base-induced

[SaintJude]

Can someone please offer a rationale as to why acid-catalyzed rx favors more substituted product, while base-induced favors least substitute carbon substitution?

Pls forgive me for all these orgo question, but judging from April test-takers orgo is here to stay. And I'm not trying to see orgo burn me...😳

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[chiddler]

think about the mechanism. how does each proceed? if acid catalyzed, the O is protonated and it takes an electron pair forming a carbocation.

the base catalyzed is just a nucleophilic reaction so it would just attack the least sterically hindered carbon.

 

[syoung]

doesn't explain why Br attacks the C2 after O is protonated. (meaning why does O open in that direction?)

 

[chiddler]

doesn't explain why Br attacks the C2 after O is protonated. (meaning why does O open in that direction?)

i'm not sure which you are referring to. can you please be more specific.

 

[syoung]

After O is protonated, why does the Br- attack C2 instead of C1. to form the primary ROH, instead of a secondary ROH.
This is still in the acid cat. rxn, not the base.

 

[chiddler]

when thinking about epoxides, a useful thing to do is always assume a carbocation forms. regardless of whether this is true or not, it always works.

like i wrote, the oxygen is protonated and then takes the electron pair from the carbon. which carbon? the carbon that can exist more easily as a carbocation. then the Br- attacks this carbon.

 

[syoung]

fair enough.
actually i think we already said that and i just couldn't read and understand... brain fart

 

[SaintJude]

think about the mechanism. how does each proceed? if acid catalyzed, the O is protonated and it takes an electron pair forming a carbocation.

the base catalyzed is just a nucleophilic reaction so it would just attack the least sterically hindered carbon.

👍👍