BC Question on Epoxide Opening

[Mufasha]

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If you haven't taken BC OC test 5 yet, I'd recommend not looking at this. I'm having trouble understanding #20. I get that with acid catalyzation you'd see addition to the more substituted side and with base you'd see addition to the less hindered side. What I don't understand with this question is why the isopropyl group isn't inverted. I thought that ring opening in acid and base both went by SN2, so you'd see inversion regardless. Any help with this would be greatly appreciated!

 

[Thanhn]

It's an inversion of stereochemistry at the central carbon. The reactant has an (R) configuration and we know that epoxide opening is an SN2 reaction. This means that the answer should be an (S) configuration. Answer B is an (S) whereas D is (R). -- B is correct! You can't just say that the isopropyl group is going to be inverted, you need to go through and check the chirality.

 

[Mufasha]

Ahh, okay gotcha. In the problems I've dealt with, when it "looked" inverted, the stereochemistry was also inverted. I guess I took that for granted, so thanks for bringing it to my attention! I'll make sure I actually check the chirality from now on.

 

[emminent]

I was thinking the same as Musfasha b/c usually you would just flip what's inverted like him picking E. Did you encounter a lot of those when you took yours Thanhn?

 

[Ari Rezaei]

You always have to check the R,S configuration to avoid being tricked. Flipping the wedge to a dash may work a lot of the time, but it isn't 100% foolproof. I want you to answer every question right, so I recommend you check the configuration yourself instead of relying on what the images show.