Elimination Rxn Question!!!

[deeps005]

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I'm taking my DAT on tuesday and am totally confused about one of the questions on the TopScore exam. The question is from the orgo section and goes like this:

Which of the following products could be formed when butadiene reacts with HCl?

the answer is 3-chlorobutene and 4-chlorobut-2-ene

I don't understand how 4-chlorobut-2-ene could be a product. I think this has something to do with carbocation rearrangement but it seems like to get 4-chlorobut-2-ene the carbocation would have to rearrange itself to something less stable and I didn't think that happens. PLEASE HELP! thanks!

 

[PERFECT3434]

deep,

butadiene can react with HCL in two ways.

1)by electrophillic hydrohalogenation which will give you the 3-chlorobutene.
2) by radical hydrohalogentaion which should give you 4-chlorobut-2-ene.

 

[PERFECT3434]

i am pretty sure that it undergoes the radical hydrohalogentaion mechanism, but i am trying to still find out how it becomes a 2-but-ene instead of 4-chlorobutene. probably due to the stability of the double bond. if i recall correctly, having 4-chlorobut-2-ene is more substituted than 4-chlorobutene and therefore more stable.
so i think you do get 4-chlorobutene 1st but it rearranges to 4-chlorobut-2-ene.

 

[ncalcate]

deeps,

This is a classic question testing your knowledge of 1,2 and 1,4 addition (as well as the difference between the kinetic and thermodynamic products).

The HCl adds not via a radical mechanism, but by an addition to the double bond.

The H+ initially adds to carbon #1 on either end, giving you a carbocation on carbon #2 (and the double bond then going across carbons 3 and 4).

At this point, one of two things happen:

1. The Cl comes in and adds to the carbocation, giving you 3-chloro-1-butene. This is the kinetic product.

2. The intermediate rearranges, moving the double bond to go across the middle carbons, and with the carbocation on the end carbon. Then the Cl adds, giving you 1-chloro-2-butene. This is the thermodynamic product. This is considered more stable, because the resulting alkene is disubstituted, as compared to the kinetic product, which is only monosubstitued.

The 3-chloro-1-butene forms faster because it goes via a more stable carbocation (hence why it is called the kinetic product).

If you have an organic textbook handy, you can look under the chapter that covers conjugated unsaturated systems. This is Chap 13 in Solomons (if you have Solomons).

Hope this helps.

Nick

 

[Mo007]

Bravo! ncalcate.