[Mel22]

hello, can anyone give me a hint on what I do wrong in this mechanism? That would be amazing - thanks!

[Mel22]

this is my attempt and what I think is wrong with it

[Mel22]

sorry for the pic to be the wrong way round. On my computer it is actually the right way round and if you click on the attachment in the post, you will be able to see it properly too. I also, tried to attach it the reverse way, but that didn't help either. I don't really understand why it does that.

If  someone could give me a hint anyway, that would be amazing!

[EDIT]: it now seems to work

[Winga]

1st step is addition, 2nd step is elimination, how come there is a SN2 reaction?

[Mel22]

But then I still don't understand why it would only eliminate one Bromide and why the inversion occurs.

[Winga]

1st step:
Bromonium is formed as intermediate which the Br+ ion binds with 2 vinylic carbons forming a tricyclic ring. This hinders the Br- ion to attack at that side (the ring), so that, Br- must attack from the back.

2nd step:
H and Br must be aligned anti-periplanar in order to undergo elimination. There are 2 H and 2 Br, any combination with give you the same product.

Two reactions have the same mechanism, the starting materials are stereoisomeric to each other.
These two stereoisomers undergo bromination, the first one gives a new product A and the second one gives B, which are also stereisomers. Check the R,S configuration.

[plu]

Only one of the bromide ions is eliminated due to the difficultly associated with removing a vinyl hydrogen  (this is needed for the E2 elimination).  You would need a much stronger base than OH^- to accomplish that.

[Mel22]

cool! Thanks so much - it does make more sense now!

I have one more question/insecurity, though: If I try to think of it three dimensionally, the intermediates A and B are still in their original cis/trans configuration - this seems to first change when elimination occurs as Br and H must be antiperiplanar. Is that correct??

Again, thanks a lot  !

[Winga]

You mean the product A & B?
We can't use cis/trans configuration to describe them anymore, we need to use R/S configuration.
You can rotate the C-C single bond (change the conformation) until H & Br becomes anti-periplanar to each other.

[Mel22]

Coooool!!! I have never heard of R/S configuration, but I have used the rotation in my corrected mechanism anyway!  

Again, thanks everyone - that was very helpful!