Rearrangements
What happens when a molecule reorganises its own skeleton — a group migrates with its bonding electrons from one atom to an adjacent (or, in sigmatropic shifts, a remote) atom, giving a constitutionally different product. Two layers on every reaction: a hand-held beginner path (plain “what it does”, arrow-by-arrow mechanism, graded worked examples, trap boxes) and a research-grade advanced/reference path (stereoelectronics, migratory-aptitude series, antiperiplanar migration geometry, chair-like [3,3] transition states, suprafacial/antarafacial analysis, isotope-labelling & stereochemical evidence for concerted vs stepwise pathways, scope & FG tables, and modern asymmetric variants). Carbocation 1,2-shifts: Pinacol–pinacolone, Wagner–Meerwein, semipinacol. Migration to nitrogen: Beckmann, Hofmann, Curtius, Lossen, Schmidt. Carbene / carbanion: Wolff & Arndt–Eistert, Favorskii, benzilic acid. [3,3]-sigmatropic: Claisen & Cope (oxy-Cope, aza-Cope, Ireland– and Johnson–Claisen). Aromatic / ylide: Fries, Stevens & Sommelet–Hauser ([1,2] vs [2,3]), dienone–phenol — closing with a “which rearrangement is this?” decision map.
The 21 sections in Part 5
- 1The 1,2-shift and migratory aptitude (do this first) Free below
- 2The Pinacol–pinacolone rearrangement
- 3The Wagner–Meerwein rearrangement
- 4The Semipinacol rearrangement
- 5The common thread: acyl nitrene / N–LG, and the isocyanate
- 6The Beckmann rearrangement
- 7The Hofmann rearrangement (bromamide degradation)
- 8The Curtius rearrangement
- 9The Lossen rearrangement
- 10The Schmidt reaction
- 11The Wolff rearrangement & Arndt–Eistert homologation
- 12The Favorskii rearrangement
- 13The Benzilic acid rearrangement
- 14Nomenclature, Woodward–Hoffmann rules, and the chair TS
- 15The Claisen rearrangement ([3,3], with O)
- 16The Cope rearrangement ([3,3], all-carbon) & the oxy-Cope
- 17The Fries rearrangement
- 18The Stevens [1,2] & Sommelet–Hauser [2,3] rearrangements
- 19The Dienone–phenol rearrangement
- 20A four-question triage
- 21Master comparison tables
The 1,2-shift and migratory aptitude (do this first)
Free extractSection G.0 of Part 5, reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
One picture explains Pinacol, Wagner–Meerwein, Beckmann, Hofmann, Curtius and more: a group hops one atom over, into an electron-poor site, carrying its electrons.
In one line: in a 1,2-shift, a group R (a hydrogen, an alkyl group, or an aryl group) moves from one atom to the next-door atom that is short of electrons, taking the two bonding electrons with it. The electron-deficient site is filled, and the deficiency (the +) reappears where R started. Rearrangements happen when the new arrangement is more stable — a better carbocation, an aromatic ring, or a neutral molecule.
Mechanism — Why a 1,2-shift happens (three ideas)
- An empty orbital must be adjacent. The migration terminus is electron-deficient — a carbocation (pinacol, Wagner–Meerwein), an electron-poor N (Beckmann, Hofmann, Curtius, Schmidt), or a carbene (Wolff). No adjacent empty/low-lying orbital → no shift.
- Geometry: antiperiplanar. The migrating σ-bond must be parallel to (aligned with) the empty orbital — roughly anti-periplanar to the leaving group. This alignment lets the bonding electrons overlap the empty orbital in the transition state.
- Driving force: a more stable product. The shift is chosen that gives the more stable cation (3°>2°>1°, or benzylic/allylic), or that leads on to a neutral product (loss of a proton, trapping by water). If the start is already the most stable, no rearrangement occurs.
Which shift: a 2° carbocation with a 3° centre next door Easy
Retention or inversion in the migrating group? Medium
⚠ Common mistakes & exam traps
- A rearrangement only happens if it improves stability. Do not ‘rearrange’ a tertiary cation down to a secondary one.
- Migratory aptitude is context-dependent. The aryl>H>alkyl order is a guide, not a law — in some reactions H migrates faster than aryl; in Beckmann only the group anti to the leaving group migrates regardless of aptitude.
- The group migrates with its bonding pair (nucleophilic migration to an electron-poor centre). Do not draw the electrons going the wrong way.
- Ring-expansion / ring-contraction is just a 1,2-shift where the migrating bond is part of a ring — watch for strained rings that relieve strain by rearranging.
Bridged (non-classical) transition states, stereochemistry of migration, quantitative aptitude, and how Part 5 is organised by the nature of the electron-deficient terminus.
Mechanistically the 1,2-shift is a [1,2]-sigmatropic shift of a σ-bond to an empty (or low-lying) orbital. The transition state is a three-centre, two-electron bridge: the migrating group is partially bonded to both origin and terminus, so the migration is concerted, intramolecular, suprafacial, and proceeds with retention of configuration in the migrating group. Where the bridge is deep enough to be an intermediate (e.g. the phenonium ion, or the 2-norbornyl cation) the species is called non-classical; where it is only a saddle point it is a classical TS. The distinction was the subject of the decades-long non-classical carbocation debate (Winstein vs H. C. Brown).
Stereochemical and labelling signatures (what proves the mechanism)
| Observation | What it demonstrates |
|---|---|
| Retention of configuration in the migrating C | Concerted, bridged migration (group never becomes free) |
| Antiperiplanar migrating bond / leaving group | σ(C–R) must overlap the developing empty orbital |
| Intramolecularity (crossover experiments show no scrambling) | Migration is within one molecule, not via free fragments |
| Only the anti group migrates (Beckmann of oximes; Schmidt of ketones) | Migration is concerted with loss of the leaving group |
| Retention of the migrating group’s chirality in Curtius/Hofmann | R→N migration is concerted, suprafacial |
Migratory aptitude — the finer print
The observed order aryl > H > alkyl reflects the ability of the migrating group to stabilise positive charge in the bridged TS. p-Anisyl (4-MeO-C₆H₄) > p-tolyl > phenyl > p-chlorophenyl > p-nitrophenyl tracks the Hammett σ of the para substituent (EDG accelerate). However, aptitude can invert with mechanism: when migration and ionisation are concerted, the group that is antiperiplanar migrates whatever its intrinsic aptitude; when a discrete cation forms first (stepwise), intrinsic aptitude dominates. Conformational access therefore often beats electronic aptitude.
How Part 5 is organised (by the electron-deficient terminus)
| Terminus (where R migrates TO) | Family | Named reactions |
|---|---|---|
| Carbocation C⁺ | G. Carbocation shifts | Pinacol–pinacolone, Wagner–Meerwein, semipinacol |
| Electron-poor nitrogen (nitrene / N–LG) | H. Migration to nitrogen | Beckmann, Hofmann, Curtius, Lossen, Schmidt |
| Carbene / carbanion carbon | I. Carbene & carbanion | Wolff (+Arndt–Eistert), Favorskii, benzilic acid |
| Concerted 6-electron pericyclic (no charge) | J. [3,3]-sigmatropic | Claisen, Cope (+ oxy-/aza-/Ireland/Johnson) |
| Aromatic ring / ammonium ylide | K. Aromatic & ylide | Fries, Stevens, Sommelet–Hauser, dienone–phenol |
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Framing chapter; primary citations live in each reaction below.
- The 1,2-shift, bridged transition states and migratory aptitude are standard treatments in Clayden, Organic Chemistry, and Carey & Sundberg, Advanced Organic Chemistry, Part A.
Read the rest of Part 5
The remaining 20 sections of this part — The Pinacol–pinacolone rearrangement, The Wagner–Meerwein rearrangement, The Semipinacol rearrangement, The common thread: acyl nitrene / N–LG, and the isocyanate — and all nine parts of Named Reactions are part of ChemVidya Full Access, along with the other books, 55 Study Notes and 6,000+ practice questions.
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