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I have some questions about the mechanism above:

  1. why does ethanol attack the tertiary C before O-C bond breaks? Shouldn't the mechanism proceed via an $S_{N}1$ mechanism?

  2. Why is ethanol, as a weak nucleophile, able to attack tertiary carbon? In a lecture it's said that a partial tertiary carbocation is involved, but left secondary carbon is also losing its electrons to oxygen, therefore becoming a partial carbocation; and if two carbon are both partial carbocation, then the tertiary carbon would be more stable and ethanol should attack the secondary carbon.

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    $\begingroup$ The reaction proceeds via a transition state, where the more stable one is the one to go for. As you said, the tertiary carbon would form a more stable carbocation than the secondary carbon; therefore, if ethanol were to attack the secondary carbon, the transition state form would be relatively unstable compared to an attack at the tertiary carbon. $\endgroup$
    – L3ul
    Commented Mar 17, 2016 at 5:28
  • $\begingroup$ Thank you! I'm not sure if I fully understood it, but mentioning the transition state really helped. $\endgroup$
    – NK Yu
    Commented Mar 17, 2016 at 7:49

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Ring-opening reactions can proceed by either SN2 or SN1 mechanisms, depending on the nature of the epoxide and on the reaction conditions.

If the epoxide is asymmetric, the structure of the product will vary according to which mechanism dominates.

When an asymmetric epoxide undergoes solvolysis in basic methanol, ring-opening occurs by an SN2 mechanism, and the less substituted carbon is the site of nucleophilic attack.

In the case of acid catalysed ring opening, the mechanism is not exactly SN1 or SN2. It is a hybrid of both.

In the first step, the oxygen gets protonated.

Second step the C-O bond starts to break ( the C with more alkyl substituents is preferred for cleavage of C-O bond. In your case the tertiary carbon atom is preferred to the secondary C atom)

Third step the nucleophile ( or alcohol in this case) attacks the electrophilic carbon (the C with which C-O bond is still breaking). Unlike in SN1 mechanism the reaction does not wait for the carbocation to be formed completely. The incoming nucleophile attacks the carbon from a direction opposite to that of direction of C-O bond because C and O are still bonded to some extent.

So hence the mechanism proceeds similar to what you have shown. In acid catalysed ring opening the nucleophile attacks the more substituted carbon because this carbon has more fractional positive charge then the secondary one.

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