Question

when should hydride, alkyl, benzyl groups be shifted in a reaction?

Answer 1

If you are asking about migratory aptitude of Hydride, alkyl and phenyl groups, the order is: hydride > Phenyl = Tert-Alkyl > Sec-Alkyl > Primary Alkyl > Methyl

There are two ways to understand / remember it:

(1) The group which has maximum charge-density (smaller size) has higher migratory aptitude. Hence, we can further predict order of migratory aptitude among these to be: p-hydroxy phenyl > p-methyl phenyl > phenyl > p-nitro phenyl

(2) The migration which produces most stable carbocation, is preferred.