# Why does the opening of an epoxide occur via an Sn2 like mechanism when using a methanoate ion as a nucleophile and methanol as a solvent?

Below is a picture of the mechanism:

I understand the mechanism but what I don't understand is why it's $\mathrm{S_N2}$. The solvent used is polar and protic, there is a tertiary carbon that would be suitable for $\mathrm{S_N1}$ as well. For what reason does this reaction occur exclusively via $\mathrm{S_N2}$?

• unfavourable leaving group Feb 4 '15 at 14:12
• Its exactly the same with either mechanism Feb 4 '15 at 14:13
• unfavorable for Sn1 Feb 4 '15 at 14:14