In general, we've learned that alkoxide groups are awful leaving groups, unless we're talking about epoxide rings whose ring strain allow the alkoxide to act as a leaving group in a ring-opening reaction. However, if a structure contained both an epoxide and a relatively good leaving group like Chloride, what would predominate if a strong base/nucleophile was introduced: Epoxide opening or the substitution reaction? An example is provided below of what I'm referring to:

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If I perform, the epoxide ring opening, I get something unusual:

I left the stereochemistry ambigious because I honestly did not know how to assign it since a new stereocenter is formed.

If I perform the Sn2 substitution, the reaction is completed quickly:

enter image description here

Notice that both of my proposed mechanism reform the epoxide ring, but do generate entirely different structures.

  • 1
    $\begingroup$ chemistry.stackexchange.com/questions/73398/… $\endgroup$
    – Mithoron
    Commented Mar 12, 2020 at 19:13
  • $\begingroup$ I did not realize someone had posed a similar question. Thank you for the link. $\endgroup$ Commented Mar 12, 2020 at 19:19
  • $\begingroup$ Chloride isn't really that great a leaving group, either. The summary of this question is that two fast steps are collectively faster than one slow step. $\endgroup$ Commented Mar 12, 2020 at 22:24


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