Methyl & Hydride Shifts: When applicable?

[justadream]

I'm studying SN1 reactions and I know both are possible (I guess this question also applies to E1 reactions as well then).

But how do you know when a compound undergoes a methyl vs. a hydride shift?

For example, lets say you have 2-bromo-3-methylpentane and attack with a weak nucleophile (SN1).

A carbocation will form at C2 (Br- leaving). How do you know whether to move the methyl group (from C3) or the Hydrogen (also from C3) to make the carbocation tertiary?

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[DrknoSDN]

The simple answer ts that both occur. For that scenario you are going to have a certain percentage of the product be shifted and the rest to be un-shifted.

A lot of factors including temperature and steric forces can change how much of a reactant undergoes rearrangement. This also applies to E1 SN1 comparison. Both E1 and SN1 occur simultaneously but the thermochemistry specifics determine what forms as the major product.
Hard to give any steadfast rules (cliff notes) because it really all comes down to what is most likely to form out of all the things that are actually forming.
Just be familiar with what contributes to the rates of a reaction (solvent etc), understand how faster/slower reactions are altered by temperature, and know about the selectivity of halogens and how they compare as leaving groups. This concept is probably a chapter or two in the OChem prep books and it a pretty broad subject.

For your example I believe if anything, the C3 hydrogen would shift to form a more stable tertiary carbocation at C3 where addition can occur. (Subject to solvent, substituents, induction, nucleophile, rate etc)

 

[justadream]

Thanks for the reply. That's actually what the answer key (this is from a TBR problem) assumes occurs.

Now that you said that, it makes sense that the hydrogen shifts for this example (since it makes it tertiary as you said) whereas a methyl shift would leave a secondary carbocation.

I hope this type of logic will hold up in most MCAT scenarios.