Epoxides

[xatlasb]

When do you attack the more vs the less substituted side?

I remember in chads that he said that since epoxides don't have a partial positive charge like a bromonium, then you can attack the least substituted side since there is less steric hindrance.

However, I keep reading in Destroyer how you attack the more substituted side in acidic conditions and the least substituted side in basic conditions?

Can someone clear this up for me? Bit confused about what to follow.

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[KyoPhan]

Destroyer is correct.

I remember in chads that he said that since epoxides don't have a partial positive charge like a bromonium, then you can attack the least substituted side since there is less steric hindrance.

What you said is correct under basic condition.
Chad covers basic and acidic conditions.

The way I remember it is that if you are under acidic condition, you will get an H attached to the O, which will create that positive charge. Then it will react like the bromonium.

 

[xatlasb]

and what is an example of it being under acidic conditions? I know it is NOT when there is added H30+ as a second step...

 

[KyoPhan]

and what is an example of it being under acidic conditions? I know it is NOT when there is added H30+ as a second step...

http://chemwiki.ucdavis.edu/Organic...philes_in_nucleophilic_substitution_reactions
Scroll down to:
"8.6B: Epoxide ring-opening reactions - SN1 vs. SN2, regioselectivity, and stereoselectivity"

 

[kc1469]

This is very simple. The reason why the epoxide is attacked at the more substituted carbon in acidic conditions is that it is stabilized by the positive charge. Since that carbon is more stable, it is attacked by the nucleophile. In basic conditions, there is no positive charge so the nucleophile attacks the less hindered carbon.

 

[Imazergling]

I had the same question. Chad says that if H30+ is added wit it, because it's in the SECOND step, it doesn't really "count" as being in an acidic condition and so the nucleophile ends up attacking the less substituted carbon.

so when would an acid be included in the first step?

 

[Imazergling]

bumppppppppp

 

[Moka11]

I had the same question. Chad says that if H30+ is added wit it, because it's in the SECOND step, it doesn't really "count" as being in an acidic condition and so the nucleophile ends up attacking the less substituted carbon.

so when would an acid be included in the first step?

Chad has an H+ under the arrow on the review sheet. There is no step 1 or 2. The entire epoxide is sitting in an acidic solution from the get-go.

So first the epoxide gets protonated and then the nuc. comes and attacks the more substituted carbon. kc1469 explained the reason nicely.