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Topic: Bromination of trans-stilbene  (Read 18170 times)

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Offline chrisbb

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Bromination of trans-stilbene
« on: October 07, 2010, 04:50:31 PM »
Hi everyone.
I'm trying to understand this mechanism and the R/S configurations for my lab.... not trying to just write down the answer given to us.

Here is what I have:



So.
I chose that the bromine attacks from the top, creating a Bromonium ion. The next Br- comes in from the BOTTOM, because the first bromine takes up too much room. The top bromine would be pointing up, and then bottom one point down....

Is that correct?

I understand how to write R/S configurations, but I don't think that the mechanism is correct, due to the fact that I'm getting two S configurations....
I'm a little confused as to the right hand carbon.... what did I do wrong to get 2 R's??

Thanks.

Offline Jzalkm

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Re: Bromination of trans-stilbene
« Reply #1 on: October 07, 2010, 10:19:32 PM »
learnt a lot from your mechanism. nicely written. thanks.

two things i would like to point out: attack of bromine and R/S configuration.


1. If you choose to attack the bromine from the bottom, you will get the other enantiomer i believe.

I find no problems with both carbons being in the same configuration in the final structure.

2. I think the carbon bonded to bromine (i.e. the -CHBrPh group) has got more priority than the phenyl group (1st point of difference); Ph would be number 3 and both would be R instead of S in the structure you have drawn. conversely, both would be S in the other enantiomer.


Offline ooosh

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Re: Bromination of trans-stilbene
« Reply #2 on: October 08, 2010, 03:54:26 AM »
I draw a picture to help chrisbb understand this mechanism . I used Fischer projection in the picture ,if you don't know what is it or how use it ,you may need this :
 http://en.wikipedia.org/wiki/Fischer_projection

Offline chrisbb

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Re: Bromination of trans-stilbene
« Reply #3 on: October 08, 2010, 11:28:16 AM »
amazing diagram!
Thank you! Yes I understand the Fisher Projections! :)

Question still though.
When looking at each chrial center, I list the priorities as:
1) Br
2) Phenyl
3) R-group
4) H

Am I correct in doing so for my diagram?

Offline ooosh

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Re: Bromination of trans-stilbene
« Reply #4 on: October 08, 2010, 08:47:53 PM »
the priorities :

Br

C-Br

Ph

H

I think this is where you confused.

Offline chrisbb

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Re: Bromination of trans-stilbene
« Reply #5 on: October 09, 2010, 07:38:11 PM »
thanks, yes it turns out that I forgot that the right hand carbon was the wrong configuration.
Every time I looking, both hydrogen's came OUT the page, when one should be in and the other out.

Thanks for all your help :)

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