Rearrangement — epoxide to carbonyl
Meinwald rearrangement
How the Meinwald rearrangement 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: Epoxide (styrene oxide), Lewis acid (BF₃·OEt₂, Mg(II), InCl₃) or Brønsted acid. Conditions: CH₂Cl₂ or benzene, 0–25 °C. Bonds formed: C=O (aldehyde/ketone); C–H (migrated); bonds broken: C–O (more substituted side); migrating C–H. Stereochemistry: Concerted 1,2-shift anti to the leaving C–O: stereospecific. Rate and selectivity: Fast with benzylic or tertiary cation; migratory aptitude: H > aryl > alkyl.
At a glance
Step 1: Lewis acid opens the epoxide
A Lewis or Brønsted acid activates the epoxide oxygen, generating a ring-opened carbocation at the more substituted carbon — the same first step as acid-catalyzed epoxide opening covered much earlier.
Step 2: 1,2-Hydride shift
Instead of an external nucleophile trapping that cation, a hydrogen (or alkyl group) on the adjacent carbon migrates directly onto the cationic center in a concerted 1,2-shift — the same migration logic as the pinacol rearrangement.
Step 3: Collapse to the carbonyl
This positions the positive charge next to the remaining C–O bond, which collapses to a C=O — giving a carbonyl compound (aldehyde or ketone) directly from an epoxide, with the connectivity rearranged rather than simply opened.
Abbreviations and terms
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- BF3
- Boron trifluoride. Lewis acid (usually as the Et₂O complex).
How it compares with related reactions
| Meinwald rearrangement | Bamberger rearrangement | Beckmann rearrangement | |
|---|---|---|---|
| Reagents | Epoxide (styrene oxide), Lewis acid (BF₃·OEt₂, Mg(II), InCl₃) or Brønsted acid | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) | H₂SO₄, PCl₅, TsCl or PPA |
| Conditions | CH₂Cl₂ or benzene, 0–25 °C | H₂O, 25–60 °C | Acid, 100–130 °C (industrial: oleum) |
| Stereochemistry | Concerted 1,2-shift anti to the leaving C–O: stereospecific | None; para-selective | Group anti to OH migrates, with retention |
| Rate / selectivity | Fast with benzylic or tertiary cation; migratory aptitude: H > aryl > alkyl | 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 Meinwald rearrangement?
Epoxide (styrene oxide), Lewis acid (BF₃·OEt₂, Mg(II), InCl₃) or Brønsted acid. Conditions: CH₂Cl₂ or benzene, 0–25 °C.
Which bonds are formed and which are broken?
Formed: C=O (aldehyde/ketone); C–H (migrated). Broken: C–O (more substituted side); migrating C–H.
What is the stereochemical outcome?
Concerted 1,2-shift anti to the leaving C–O: stereospecific
What controls the rate or the selectivity?
Fast with benzylic or tertiary cation; migratory aptitude: H > aryl > alkyl
Study guides that help with this reaction
- Carbonyl chemistry: addition, acyl substitution and enolates
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti