Ring expansion means rearrangement

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NOTE: Important charges and non-bonding electrons are shown throughout the animation except during the transition phase

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'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.