Hydride/Methyl Shifts: Adjacent Only?

[justadream]

Is there a "limit" to how far a proton/methyl can shift to form the most stable carbocation?

In other words, should we only consider possible rearrangements that involve adjacent protons/methyls?

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[KingAssault]

Nope. I just took Ochem and the hydride or methyl shift can occur across multiple carbons if need be.

 

[lumpyduster]

I would only consider the adjacent ones since they will occur the fastest. Especially if it's just in terms of the MCAT.

 

[Czarcasm]

Nope. I just took Ochem and the hydride or methyl shift can occur across multiple carbons if need be.

The only instance where this is true is if you have say a secondary carbocation and you have a neighboring carbon that could form a tertiary carbocation - a methyl or hydride shift will occur so that we have a tertiary carbocation, but if another shift is possible where you can form an even more stablized carbocation, it will happen; for instance, if a neighboring carbon can form a tertiary carbocation that is also benzylic/allylic (resonance stabilized tertiary carbocations are the most stable). This is not something generally presented on the MCAT, but if you understand the basic principle behind it, you should be able to recognize it when it happens.