Alkene reaction with Br2 and NaCl (saturated)

In summary, Br and Cl are different in their reactivity and selectivity, and Cl will be added first and Br will go to the substituted carbon.
  • #1
Snoop06
20
0
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
 
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  • #2
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"
 
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  • #3
Himanshu said:
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.
 
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  • #4
the electrophile Br(d+) will first attach to the pi bond in the alkene, then there will be a selection between Cl- and Br-.
 
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  • #5
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?
 
  • #6
Spirochete said:
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.
 

Related to Alkene reaction with Br2 and NaCl (saturated)

1. What is the purpose of adding Br2 and NaCl to an alkene reaction?

The addition of Br2 and NaCl to an alkene reaction is known as halogenation. This reaction is used to introduce a halogen (in this case, bromine) to the alkene molecule, resulting in the formation of a dihalide product. The addition of NaCl helps to ensure that the reaction is carried out in a controlled and saturated manner.

2. How does the presence of NaCl affect the reaction between Br2 and an alkene?

NaCl serves as a source of chloride ions, which act as a catalyst for the reaction between Br2 and the alkene. The chloride ions help to polarize the Br2 molecule, making it more reactive and allowing it to attack the alkene more easily. This results in a more efficient and controlled halogenation reaction.

3. What is the difference between a saturated and unsaturated alkene reaction with Br2 and NaCl?

In an unsaturated alkene reaction, the alkene molecule contains carbon-carbon double bonds and is therefore able to undergo additional reactions. This makes the reaction more difficult to control and can result in the formation of multiple products. In a saturated alkene reaction, the alkene molecule has been treated with an additional reagent (such as NaCl) to ensure that only one halogen is added to each alkene molecule, resulting in a more controlled and predictable reaction.

4. Can other halogens be used in place of Br2 for the alkene reaction with NaCl?

Yes, other halogens such as chlorine (Cl2) or iodine (I2) can be used in place of Br2 for the alkene reaction with NaCl. However, each halogen will result in a different product, as they have different reactivity and selectivity towards the alkene molecule.

5. What is the overall chemical equation for the alkene reaction with Br2 and NaCl?

The overall chemical equation for the alkene reaction with Br2 and NaCl is: alkene + Br2 + NaCl → alkyl dihalide + NaBr. This equation shows the addition of a bromine molecule (Br2) to the alkene, followed by the addition of a chloride ion (from NaCl) to form the final product, an alkyl dihalide.

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