[clif1123]

How do you know when a rearrangement (methyl or hydrogen) will occur in mechanisms? Do they only occur when the mechanism involves a carbocation intermediate?

[Yggdrasil]

Methyl and hydogen rearrangements (also known as methyl and hydride shifts) are common in mechanisms involving carbocation intermediates, but they may may occur elsewhere.  You will see one of these rearrangements occur when they would create a more stable carbocation (for example, changing a primary carbocation to a secondary or tertiary carbocation).