riseNshine

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Sep 1, 2010
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Question is as follows:

What is the most stable product that is formed with 1,3-butaidene is reacted with an equal amount of HCl. Answer is 1-Chloro-2-butene, I thought it was 3-Chloro-2-butene.

According to the answer, after a carbocation is formed, the chloride nucleophile will attack the least hindered carbon containing a positive charge. Since the more substituted double bonds are most stable, the most stable product formed has an internal double bond.

However, my confusion is that the internal double bond results in a carbocation that is less substituted than if the double bond was terminal. Is it always true that the internal versus terminal alkene decision trumps the substitution of the carbocation?

Thanks
 

amberkre

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May 7, 2012
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I actually just went over this for my first exam in OChem 2.

When 1,3-butadiene reacts with HCL only 2 products form: 3-chloro-1-butene (when H and Cl add to the first and second carbon=1,2 adduct) and 1-chloro-2-butene (when H and Cl add to the first and 4th carbon=1,4adduct)


3 chloro-2-butene does not form at all. Look at the mechanism behind the Electrophilic reactions on conjugated dienes. I'm a little confused about your question on stability. If you are comparing the two products that form:

3-chloro-1-butene - this product is less stable thermodynamically because the double bond is terminal which is an unstable position since the double bond is not fully substituted

1-chloro-2-butene- this product is more stable thermodynamically because the double bond is in the internal position, you can see its true because when the reaction is done at a high temp this product forms in a higher ratio
 

PlumbLine

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May 23, 2012
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Question is as follows:

What is the most stable product that is formed with 1,3-butaidene is reacted with an equal amount of HCl. Answer is 1-Chloro-2-butene, I thought it was 3-Chloro-2-butene.

According to the answer, after a carbocation is formed, the chloride nucleophile will attack the least hindered carbon containing a positive charge. Since the more substituted double bonds are most stable, the most stable product formed has an internal double bond.

However, my confusion is that the internal double bond results in a carbocation that is less substituted than if the double bond was terminal. Is it always true that the internal versus terminal alkene decision trumps the substitution of the carbocation?

Thanks
It will actually depend on the temperature. At very low temperatures there isn't enough energy for the alkene to "shift", because, as you pointed out, the intermediate carbocation is less substituted. Therefore that transition state has a higher energy level, thus the transition has higher activation energy. 3-Chloro-1-butene will be the major product.

However if there is enough energy, the shift will proceed and the molecule reaches a lower energy state (as you suspected) with an internal alkene rather than a terminal. 1-Chloro-2-butene will be the major product.

Since the question asked "what is the most stable product" it would certainly be the one with the internal alkene. I believe in most cases it is assumed that there is enough energy for the more stable product to form. They would probably point out "at very low temperatures" if they wanted to test you on the other product.

I'm not sure that your proposed product, 3-chloro-2-butene, exists. I can't picture it based on the way it's named.

Hope this helps!