markovnikov w/ carbocation

[skylark]

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if if i have a markovnikov reaction and the halide now attaches to the most subsituted part of the alkene, what happens if the adjacent C NEXT to the most substituted alkene was tertiary (which makes it even more stable than the already substituted alkene)..would that mean the halide would move again? thus a carbocation/halide shift?

does carbocation/halide shift have to be an adjacent carbon or can it "jump"?

i hope my explanations were clear..THANKS!

 

[Extirpator]

if if i have a markovnikov reaction and the halide now attaches to the most subsituted part of the alkene, what happens if the adjacent C NEXT to the most substituted alkene was tertiary (which makes it even more stable than the already substituted alkene)..would that mean the halide would move again? thus a carbocation/halide shift?

does carbocation/halide shift have to be an adjacent carbon or can it "jump"?

i hope my explanations were clear..THANKS!

A methyl shift can occur prior to the boning of the halide, thus creating the more stable carbocation. The halide will then bind to this more stable carbocation, forming the major product. A methyl shift, although favorable, will not always occur before the halide binds. If the halide binds prior to the shift (binding to the less substituted as you describe), then it form the minor product. So to answer your question: no, a carbocation/halide shift does not occur... just major and minor products, caused by the methyl shift which occurs prior to the halide binding the carboocation.

Edit: yes, the shift (methyl shift, not "carbocation/halide" shift) only occurs with carbons that are dirrectly adjacent to the carbocation

and technically (although I don't think it is what you are talking about) the halide can come off the minor product, allowing a methyl shift and then reatach after the shift forming the major product.... so if that's what you were talking about, then I misunderstood the question and yes it can occur ha

 

[UCB05]

Assuming I have the right kind of structure in my mind based on your description, electrophilic addition will give you a secondary carbocation intermediate adjacent to a tertiary carbon, correct? If so, it is possible to get a hydride shift, but probably not a methyl shift. Transfer of a hydride ion would give you a tertiary carbocation intermediate next to a secondary carbon, whereas a methyl shift would just create a secondary carbocation on a new carbon adjacent to the old, now-tertiary carbon, which basically changed nothing. an alkene adjacent to a quaternary carbon would definitely undergo methyl shift though. Anyone know the likelihood of a secondary to tertiary hydride shift, though? seems to me secondary carbocations are usually stable enough to not have to undergo rearrangement to a tertiary cation.

Also, as Daveyk said above, the halide never shifts. the shifting happens to the carbocation after the alkene picks up a proton but BEFORE the halide attaches

 

[Extirpator]

Yeah, you're right. I wasn't paying attention to the part where he said next to a "tertiary" carbon... In that case, a hydride shift would definately be occuring instead of the methide. In my post I was thinking the initial carbocation was adjacent to a quaternary carbon 😛