Epoxide Ring Opening

[bluedevil61212]

---

Members don't see this ad.

Wikipremed says that in acidic conditions, the nucleophile attacks the more substituted carbon. In basic conditions, the nucleophile attacks the less substituted carbon because of steric hindrance. Why doesn't the nucleophile attack the less substituted carbon in acid as well?

 

[wiloghby]

Wikipremed says that in acidic conditions, the nucleophile attacks the more substituted carbon. In basic conditions, the nucleophile attacks the less substituted carbon because of steric hindrance. Why doesn't the nucleophile attack the less substituted carbon in acid as well?

The mechanisms are different if I remember correctly. Basic cleavage is Sn2 and concerted and prefers a less crowded attack site, while acdic cleavage, which starts by protonating the epoxide oxygen, is actually more of an Sn1 reaction.

i.e. In acidic cleavage, the protonated oxygen is like an intramolecular leaving groups and "leaves" one of the carbons of the ring leaving behind what can be thought of as a carbocation. Carbocation stability is greater at the more substituted of the epoxide carbons. It is this carbocation that the acid cleaving the ring then adds to.

This explanation is NOT exact but is easiest to think of for the MCAT..acidic cleavage is actually not exactly Sn1 because backside attack is still required, so the stereochemistry is different.

This site goes over why the carbocation Sn1 mechanism is not entirely correct and goes over the stereochemistry...http://ochemstudy.wikidot.com/epoxide-ring-opening

 

[Endure]

Wikipremed says that in acidic conditions, the nucleophile attacks the more substituted carbon. In basic conditions, the nucleophile attacks the less substituted carbon because of steric hindrance. Why doesn't the nucleophile attack the less substituted carbon in acid as well?

Under acidic conditions, the oxygen on the ether is protonated, generating a mild electrophile. The protonated ether pulls electron density away from the carbons on the epoxide ring, forming a partial positive charge (a milder carbocation). The partial positive charge is more stable on the carbon that is more substituted, facilitating ring opening at the more substituted site.

Base-catalyzed epoxide opening is dependent on steric hindrance, not partial positive charge stability.

Hope that helps.