Alkene reaction with Br2 and NaCl (saturated)

[Snoop06]

Hi all, I am studying for an exam tomorrow and can't seem to find this mechanism anywhere. I know that a Br and a Cl are added in Markovnikov fashion but I don't know which would be added first and which would go on the more substituted carbon. Any help is appreciated.

Thanks

 

[Himanshu]

There is a marked difference of the 'reactivity' and 'selectivity' between Br and Cl. Cl is more reactive than Br and Br is more selective than Cl. Therefore Cl would be added first and Br would go to the substituted carbon.

You may want to visit this site for organic reactions and there mechanisms

http://www.organic-chemistry.org/reactions.htm"

 

[Snoop06]

There is a marked difference of the 'reactivity' and 'selectivity' between Br and Cl. Cl is more reactive than Br and Br is more selective than Cl. Therefore Cl would be added first and Br would go to the substituted carbon.

You may want to visit this site for organic reactions and there mechanisms

http://www.organic-chemistry.org/reactions.htm"

Thank you :)

I bookmarked that page for future use, I'd never been there before but it seems like it has a lot of good info.

 

[Kushal]

the electrophile Br(d+) will first attach to the pi bond in the alkene, then there will be a selection between Cl- and Br-.

 

[Spirochete]

Just wanted to confirm the answer for the OP since there have been two different responses to this question. Kushal is correct, I think Himanshu is confusing the problem with another issue.

Typically only electrophiles can add to a pi bond. Therefore NaCl, which is a source of Cl-, will not add first. Only Br2 can act as an electrophile source, by adding Br+ to the pi bond, forming a bromonium ion intermediate and Br-.

At this point there will be competition between the two nucleophiles present, Br- and Cl-. Either way the nucleophile will add with a specific stereochemistry. The nucleophile will also add in a markovnikov fashion to the more substituted carbon, but this will only be relevant if Cl- adds. I'll leave it to you to explain why it occurs this way. HINT: Think of the structure of the intermediate. Make a model if you need to, it may be helpful to use tubing to form some of the bond angles. Also, draw the resonance structures of a halonium ion and the actual structure, and think about how the charge is concentrated on the two carbons.

I'm not sure which nucleophile would add faster in the second step, does anyone else know?

 

[Snoop06]

Just wanted to confirm the answer for the OP since there have been two different responses to this question. Kushal is correct, I think Himanshu is confusing the problem with another issue.

Typically only electrophiles can add to a pi bond. Therefore NaCl, which is a source of Cl-, will not add first. Only Br2 can act as an electrophile source, by adding Br+ to the pi bond, forming a bromonium ion intermediate and Br-.

At this point there will be competition between the two nucleophiles present, Br- and Cl-. Either way the nucleophile will add with a specific stereochemistry. The nucleophile will also add in a markovnikov fashion to the more substituted carbon, but this will only be relevant if Cl- adds. I'll leave it to you to explain why it occurs this way. HINT: Think of the structure of the intermediate. Make a model if you need to, it may be helpful to use tubing to form some of the bond angles. Also, draw the resonance structures of a halonium ion and the actual structure, and think about how the charge is concentrated on the two carbons.

I'm not sure which nucleophile would add faster in the second step, does anyone else know?

Ah I see now, I didn't even realize the Cl would be a nucleophile (it makes sense that it is, I just didn't look at it that way for some reason).

Thanks for clearing that up.