In your organic chemistry class, questions like predict the product or mechanisms might oftentimes involve a step with rearrangements (hydride or methyl shifts).
1. How do we know when rearrangements are possible?
Simple, any mechanisms that involves a carbocation can have rearrangement. Therefore, reactions Sn1, E1, as well as reactions of alkenes with HX and H3O+ could have rearrangements.
2. How do we know when we should do a rerraganement? If there is a more stable position for a carbocation on a neighboring carbon in the molecule, a shift will happen. For example, let's say that we made a secondary carbocation, but there is a tertiary position next to it, a rearrangement will occur to place the carbocation into a more stable position.
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