I'm sorry about the 'urgent' plug. My exam is tomorrow.
I would like to understand precisely why the ethenylcyclobutane doesn't yield a tertiary carbocation at the third step by hydride shift and instead shifts to a cyclopentane with a secondary carbocation before rearranging to a tertiary carbocation.
If it is due to ring strain, does this type of mechanism apply to all rings of this format?
Thank you a thousand times.