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Here I have an epoxide opening reaction, but I was wondering that when a nucleophile attacks, How would the stereochemistry work? I have 2 products I predicted and I'm not sure which one would be correct. It's either the first one predicted in the reaction of the second picture I have attached. Or are they actually the same thing?
enter image description here enter image description here

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  • $\begingroup$ Where did you get the second one from? $\endgroup$ Commented Dec 3, 2020 at 18:00
  • $\begingroup$ @orthocresol It's just another prediction I had, so I wasn't sure which one is right $\endgroup$
    – chemochem
    Commented Dec 3, 2020 at 18:10
  • $\begingroup$ Yes, and why did you predict that? For example in your first one, you have a (very good!) mechanism which explains why you’re coming up with that particular stereochemistry. $\endgroup$ Commented Dec 3, 2020 at 18:19
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    $\begingroup$ Hmm, yes, you could put it that way. If you're not fully convinced, my suggestion would be to make a model of it. That's the ultimate source of truth. $\endgroup$ Commented Dec 3, 2020 at 18:55
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    $\begingroup$ A few points to ponder. The stereochemistry of the epoxide implies a single enantiomer. If the reaction is indeed SN2, then there is inversion of stereochemistry. If you know how to assign R/S configurations, compare your two bromohydrin structures. Are they the same? What if the reaction is SN1? Lastly, what is the significance of the bold wedge in the bromohydrins? $\endgroup$
    – user55119
    Commented Dec 3, 2020 at 20:07

1 Answer 1

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In these three-membered ring reactions, it's an SN2 substitution reaction that happens. Hence, there is an inversion of the configuration associated.

Edit: Perhaps the mechanism is more complicated than SN2, contrary to Klein's book which states that it's SN2, this online source says that the actual mechanism is between sn1 and SN2 (leaning to SN2 side) to explain the preference of nucleophile to attack tertiary carbon though more sterically hindered.


Reference: See page-431 of Organic Chemistry David-R-Klein

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  • $\begingroup$ Well, it could have some SN1, given the relative stability of a benzylic cation. I think that is what user55119's comment hints at. I'm not sure about the relative extents. $\endgroup$ Commented Dec 4, 2020 at 9:28
  • $\begingroup$ @orthocresol: I was not pushing for an SN1 mechanism but rather for the OP to ponder the consequences of an SN1 mechanism vs. an SN2 route. $\endgroup$
    – user55119
    Commented Dec 4, 2020 at 15:40
  • $\begingroup$ Why the down vote? If there is any mistake in my answer I'd be more than happy to correct it. $\endgroup$
    – Babu
    Commented May 3, 2021 at 14:37

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