A rearrangement reaction changes the connectivity of a molecule by moving atoms or groups within its framework. Its product is a constitutional isomer when the overall molecular formula is unchanged, although rearrangements can also occur as steps within a larger reaction.
A carbocation may undergo a hydride or alkyl shift that relocates its positive charge and changes the carbon skeleton. Rearrangements can also be concerted, without a free carbocation intermediate. Identifying which bonds break and form is therefore more informative than treating every rearrangement as the same mechanism.
An acid-promoted skeletal migration of a cyclohexadienone followed by rearomatization to a phenol. Carbonyl protonation activates the conjugated system; migration from a suitably placed quaternary carbon and subsequent proton loss can change ring connectivity while restoring aromaticity.
A para-tethered phenoxide can react through its carbon-nucleophilic resonance contributor. Intramolecular displacement of a tether leaving group at the para carbon creates a spiro ring and a cyclohexadienone. Subsequent acid-induced bond migration to a neighbouring carbon can expand the attached ring and restore the phenol. The two-stage sequence can therefore give a final substitution position different from the initial carbon attack.
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