Ring expansion means rearrangement
NOTE: Important charges and non-bonding electrons are shown throughout the animation except during the transition phase
Click the structures and reaction arrows in sequence to view the 3D models and animations respectively'More stable' usually means 'more substituted', but cations can also be made more stable if they become less strained. Four-membered rings adjacent to cations readily rearrange to five-membered rings in order to relieve ring strain. The cation is formed by protonation of an alkene and the rearrangement step transforms a stable tertiary cation into a less stable secondary cation. Relief of strain from a four- to a five-membered ring makes the alkyl migration favourable.