Carbocation rearrangement
Wagner-Meerwein shift
How the Wagner-Meerwein shift works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: HBr (or ionization of a leaving group). Conditions: Polar medium, 0–25 °C. Bonds formed: C–C (shifted), more stable cation; bonds broken: C–C (migrating group), π C=C. Stereochemistry: Migrating group moves suprafacially with retention. Rate and selectivity: 1,2-Shift very fast (barrier of a few kcal/mol).
At a glance
Step 1: Secondary cation forms
A carbocation forms next to a carbon that carries a hydrogen, alkyl, or aryl group, for example by ionization of a leaving group or by protonation of an alkene. Here a secondary cation sits next to a quaternary carbon.
Step 2: 1,2-Methyl shift
A group on the neighboring carbon shifts with its bonding pair to the cation center in a 1,2-shift. The positive charge moves to the carbon the group left, and it ends up on a tertiary carbon — a more stable cation, so the shift is downhill.
Step 3: Trapping or loss of H⁺
The rearranged cation is trapped by a nucleophile or loses a proton to give an alkene. This shift is the engine of the pinacol and Meinwald rearrangements, and it reshapes ring skeletons in terpene chemistry, including the camphor and norbornyl systems.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- HBr
- Hydrobromic acid. Strong acid; bromide is a good nucleophile.
- SN1
- Unimolecular nucleophilic substitution. Two steps: leaving group departs (rate-determining) to give a carbocation, then nucleophile attacks.
- E1
- Unimolecular elimination. Carbocation forms first, then a base removes a β-H.
How it compares with related reactions
| Wagner-Meerwein shift | Bamberger rearrangement | Beckmann rearrangement | |
|---|---|---|---|
| Reagents | HBr (or ionization of a leaving group) | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) | H₂SO₄, PCl₅, TsCl or PPA |
| Conditions | Polar medium, 0–25 °C | H₂O, 25–60 °C | Acid, 100–130 °C (industrial: oleum) |
| Stereochemistry | Migrating group moves suprafacially with retention | None; para-selective | Group anti to OH migrates, with retention |
| Rate / selectivity | 1,2-Shift very fast (barrier of a few kcal/mol) | Loss of water to give the nitrenium ion is rate-limiting | Migration is rate-limiting; leaving-group ability of N–OX matters |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Wagner-Meerwein shift?
HBr (or ionization of a leaving group). Conditions: Polar medium, 0–25 °C.
Which bonds are formed and which are broken?
Formed: C–C (shifted), more stable cation. Broken: C–C (migrating group), π C=C.
What is the stereochemical outcome?
Migrating group moves suprafacially with retention
What controls the rate or the selectivity?
1,2-Shift very fast (barrier of a few kcal/mol)
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti