Rearrangement reaction (organic chemistry)

Characteristics and common types

Rearrangements are identified by which fragment moves and how the electronic structure evolves. Important, frequently encountered types include 1,2-rearrangements (1,2-hydride and 1,2-alkyl shifts), pericyclic rearrangements (such as sigmatropic shifts), allylic rearrangements and skeletal reorganizations like the Wagner–Meerwein rearrangement. Olefin metathesis is sometimes grouped with rearrangements because it exchanges alkylidene fragments between alkenes, changing carbon connectivity.

Mechanisms and how they differ

Mechanistic pathways vary. In many ionic 1,2-shifts, a cationic center forms and a neighboring group migrates to stabilize it, often described by curved-arrow electron-flow diagrams in textbooks. Pericyclic rearrangements proceed through cyclic transition states where orbital symmetry and concerted electron reorganization govern stereochemistry and feasibility. In some migrations—especially alkyl or alkoxy shifts—the moving group may glide along a bonding framework in a way that cannot be fully captured by a simple sequence of discrete bond-breaking and bond-forming steps. Metal catalysts can open alternative, lower-energy paths for migration or exchange.

Examples and applications

Classic examples include the Wagner–Meerwein rearrangement, common in terpene and steroid chemistry, and the Claisen and Cope rearrangements among pericyclic reactions. Allylic rearrangements often appear in substitution reactions and in rearrangements catalyzed by acids or metals. Olefin metathesis, used extensively in polymer and materials synthesis, reconfigures carbon–carbon double bonds to build complex architectures. These reactions are exploited in natural product synthesis, pharmaceutical modifications and industrial processes where altering carbon frameworks is essential.

Historical context and practical notes

Many rearrangements were uncovered during the 19th and 20th centuries as chemists sought mechanisms for unexpected product distributions. Understanding advanced when physical methods allowed detection of intermediates and when molecular orbital theory clarified pericyclic selectivity. In practice, solvent, temperature, catalysts and substituent effects strongly influence whether a rearrangement proceeds and which isomer predominates. Reaction diagrams with curved arrows remain valuable teaching tools but are simplified representations of more complex, often concerted electronic reorganizations.

Distinctions and further reading

Distinguish rearrangements from simple substitutions (where connectivity to the central atom changes without skeleton reorganization) and from fragmentations or additions that alter atom counts or add new bonds. When evaluating a reaction, consider whether the transformation is intramolecular (within one molecule) or intermolecular (between molecules), whether it is concerted or stepwise, and whether catalysis is involved. For additional summaries and mechanisms consult dedicated resources and reviews listed below.

• General overview of rearrangement reactions
• Carbon skeleton rearrangements
• Structural isomerism and rearrangements
• Examples of substituent migrations
• Alkyl and aryl group migrations
• Atom-to-atom migration cases
• Mechanistic arrow-pushing notation
• Curved-arrow formalism in organic mechanisms
• Electron flow concepts
• Bond reorganization during rearrangements
• Textbook treatments of organic rearrangements
• Mechanistic studies and intermediates
• Non-ionic migration examples
• Pericyclic rearrangements and orbital symmetry
• Orbital interactions in concerted processes
• Sequences and formal electron-transfer depictions