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Topic: Sn2 / E2 Question  (Read 2445 times)

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Offline sn1sn2e1e2

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Sn2 / E2 Question
« on: April 15, 2014, 09:19:50 PM »
Which substance will react most rapidly with Br2 (aq)?

(A) Benzene
(B) Chloropropane
(C) Propanone
(D) Propene

I immediately eliminated A and C.

I know that B undergoes Sn2 reaction to form its enantiomer (and it's also very fast)

I know that D undergoes E2 which is also very fast.

I end up choosing B but the correct one is D.

How do I know whether substitution or elimination for this reaction occurs the fastest? Both do not seem to have any sterically hindered methyl groups.. and OH- is a pretty strong nuc/ base


Offline AlphaScent

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Re: Sn2 / E2 Question
« Reply #1 on: April 15, 2014, 10:05:23 PM »
The question is what will react fastest with bromine.  Why would propene, an alkene react fastest?  In what way could a reaction take place? 
If you're not part of the solution, then you're part of the precipitate

Offline AlphaScent

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Re: Sn2 / E2 Question
« Reply #2 on: April 15, 2014, 10:12:17 PM »
It is not an elimination reaction for D.  It is the addition of bromine across the double bond.

Look at this website.

http://www.masterorganicchemistry.com/reaction-guide/bromination-of-alkenes-with-br2-to-give-dibromides/
If you're not part of the solution, then you're part of the precipitate

Offline sn1sn2e1e2

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Re: Sn2 / E2 Question
« Reply #3 on: April 15, 2014, 10:17:28 PM »
Sorry, I meant to put addition reaction  :P

My guess is that propene establishes a more stable carbocation when a bromine atom is added on one side of the double bond?

For B Chloropropane, doesn't Sn2 reaction occur the fastest? (1 step process)
Adding across a double bond is a two step process..

I still can't see how D is the correct answer though

Offline AlphaScent

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Re: Sn2 / E2 Question
« Reply #4 on: April 15, 2014, 10:28:38 PM »
You form a Bromonium ion.  Not a carbocation.  Look at the website.

In order to do the substitution it would have to be a bromide.  Not molecular bromine.  You can do what is called a Finkelstein reaction.  React the chloropropane with sodium bromide in acetone and then reflux.
If you're not part of the solution, then you're part of the precipitate

Offline zsinger

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Re: Sn2 / E2 Question
« Reply #5 on: April 16, 2014, 12:44:17 PM »
D undergoes an elimination?……NOT favorable……
             -Zack
"The answer is of zero significance if one cannot distinctly arrive at said place with an explanation"

Offline Babcock_Hall

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Re: Sn2 / E2 Question
« Reply #6 on: April 16, 2014, 04:00:32 PM »
C would react, but it would need a catalyst.

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