[MiuMixi]

When a carbonium ion adjacent to a quaternary carbon, but one of the branches is an ethyl or longer chain, can a 1,2 alkyl shift occur with that ethyl chain? Or is alkyl shift only limited to methyl groups

[sharbeldam]

If for instance you can a look at pinacol rearrangement, sometimes a Phenyl group can rearrange, so it's not limited, But I'd say it's easier/faster for hydrogen/methyl to migrate, Happy to be corrected if I'm wrong.

[hollytara]

Other things being equal, the mass of the group makes a difference - lighter migrates faster.  But the difference in migration may create a mixture.  Phenyl migrates (the old 'phenonium ion' queation)...

If migration is coupled to other structural factors - changes in ring strain, for example - that can also change the preference.

[Babcock_Hall]

There are some examples in which a ring expands.  I recall seeing one in a textbook, in which a 5-membered ring became a six-membered ring.