Hydrogenation of a diene and rearrangement

[gasbait]

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I am having trouble understand the answer for this question.

The question is how would 1,3-butadiene react with an equal amount of HCl.
I answered it would be 3-Chloro-1-butene, but the answer is 3-Chloro-1-butene.

I don't understand why the carbocation would rearrange itself in that fashion. Isn't it more stable in its current position, as a secondary carbocation, than moving to the end and becoming primary?

 

[tdod]

i'm not toally sure, but i believe that it's not a rearrangement, it's just that the molecule has 2 resonance forms, so the + charge is distributed b/w the 1 and 3 carbons. the 1-carbon receives the chloride more quickly b/c sterics impair the chloride's ability to attack the 3 carbon.

 

[quicheduty]

No, remember that an allylic carbocation has more stability than a plain primary carbocation, so if there was no second conjugated double bond, you would be correct. However, the allylic primary carbocation provides enough resonance stability so that you can produce the more stable product -- a disubstituted alkene (1-chloro-2-butene) rather than a monosubstituted alkene (3-chloro-1-butene).

 

[gasbait]

So resonance has a larger stabilization effect on a carbocation than substitution?

Ohhh, I understand now! Thanks everybody!

 

[Lunasly]

No, remember that an allylic carbocation has more stability than a plain primary carbocation, so if there was no second conjugated double bond, you would be correct. However, the allylic primary carbocation provides enough resonance stability so that you can produce the more stable product -- a disubstituted alkene (1-chloro-2-butene) rather than a monosubstituted alkene (3-chloro-1-butene).

While the primary carbocation is resonance stabilized, isn't the secondary carbocation also resonance stabilized (thus being more stable). I might be wrong, but isn't this more of a question of whether the thermodynamic vs. kinetic product is favoured?

 

[quicheduty]

While the primary carbocation is resonance stabilized, isn't the secondary carbocation also resonance stabilized (thus being more stable). I might be wrong, but isn't this more of a question of whether the thermodynamic vs. kinetic product is favoured?

Yes and yes, but I think we're reaching different conclusions. The secondary carbocation is resonance stabilized but that's a little irrelevant because looking at the product stability, 1-chloro-2-butene is more thermodynamically favored (disubstituted trans alkene vs. monosubstituted alkene). Basically I was pointing out that the only reason Cl can react at the primary carbon is because it is resonance stabilized.

 

[Lunasly]

Ah OK fair enough. I didn't realize that this type of addition would be tested on the MCAT.