BR2+ trans-2-pentene AND Br2+ cis-2-pentene

[Rolling]

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http://greenchem.uoregon.edu/ACSGoingGreenSite/PDFs/20050315TuesPM/Exp2Dragojlovic.pdf

If you scroll down to page 3 you can see the image. I just googled this to find a picture so it was easier to show.

I don't quite understand how Br2 can add to the trans-2-pentene and it LOOKS like it adds SYN. Clearly that doesn't make sense, because the bromonium ion intermediate that forms requires that the Bromine ion that will add in the 2nd step attacks from the BACKside yielding anti addition. I'm sure it has something to do with the trans geometry, but I can't really put my finger on it. Any help would be great.

 

[Gogogirl]

I dont know if this answers your question..

from "cis-syn-cis"
tran reactant + anti addition = cis product

it looks like a syn addition but it's not...
hope it helps cuz I am not that great at exlpaining stuf...and I know it's kinda hard to gaspe..😵

 

[wanderer]

Sawhorse projections are the key here.

 

[Rolling]

dude...explain

 

[refenestration]

it seems like for cis-2-pentene the Br addition happens within the same plane as the methyl and ethyl groups. for the trans molecule the addition happens at 90 degrees to the plane of the molecule. Br's come from "above" and "below". so the addition is still anti. but after you rotate the bonds so that methyl and ethyl correspond with the fisher orientation, the two bromine groups end up on the same side. at leas that's a way to do it.

 

[wanderer]

dude...explain

Draw the molecules in some sort of 3-d conformation, like the sawhorse. The H's should be eclipsing, and the methyl group should be eclipsing the ethyl group (for addition to the cis isomer). Form the Bromonium bridge. Now add the Br to one methyl and invert the carbon (as if it's an internal sN2 reaction), convert back to fisher, it should look like the molecule posted