CarbocationRearrangements

Carbocation rearrangements are processes in which a carbocation intermediate undergoes structural changes to form a more stable carbocation or a different, often more stable, product. These rearrangements frequently occur during reactions that generate carbocations in situ, such as solvolysis, dehydration, or hydration steps.

The driving forces are stabilization and relief of strain. A rearrangement can transfer a positive charge to a more substituted or resonance-stabilized center, or relieve ring strain. Migrating groups move with their bonding electrons in a 1,2-shift, and the relative migratory aptitude often follows a pattern where hydride or resonance-stabilized groups migrate more readily than primary alkyl groups. The outcome depends on the stability of the resulting cation, subsequent capture by solvent or nucleophile, and, in cyclic systems, relief of geometric tension.

Common types include 1,2-hydride shifts and 1,2-alkyl shifts (Wagner-Meerwein rearrangements), ring expansions or contractions, and rearrangements of cyclopropylcarbinyl or related systems that generate homoallyl or nonclassical cations. Ring systems may open or rearrange to transfer the positive charge to a more favorable position. Some rearrangements proceed via concerted pathways, others via discrete carbocation intermediates, and neighboring group participation can influence the course.

Notable named processes include the Wagner-Meerwein rearrangement and the Pinacol rearrangement, where adjacent group migration accompanies loss of a leaving group to furnish carbonyl compounds. Semipinacol variants involve migration adjacent to a hydroxyl-bearing center. These rearrangements are central to synthetic planning and biosynthetic pathways, affecting product structure, stereochemistry, and the overall efficiency of many organic transformations.