Rearrangement — epoxy alcohol isomerization
Payne rearrangement
How the Payne 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: 2,3-Epoxy alcohol, aqueous NaOH or t-BuOK (catalytic to stoichiometric base). Conditions: H₂O/THF or t-BuOH, 0–25 °C. Bonds formed: new epoxide C–O; O–H (new alcohol); bonds broken: old epoxide C–O; O–H (deprotonated). Stereochemistry: Inversion at the attacked carbon (SN2); chirality transferred. Rate and selectivity: Reversible; position set by epoxide substitution; can be trapped by nucleophiles.
At a glance
Step 1: Deprotonation of the hydroxyl
Under basic conditions, a 2,3-epoxy alcohol's free hydroxyl is deprotonated, forming an alkoxide positioned right next to the strained epoxide ring.
Step 2: Intramolecular SN2 opens the epoxide
That alkoxide oxygen attacks the adjacent epoxide carbon intramolecularly — a backside SN2 attack, exactly like any epoxide-opening nucleophile, just tethered in place — displacing the original epoxide oxygen as it opens.
Step 3: Protonation: the epoxide has moved
The result is a new epoxide, shifted over by one carbon, with the original epoxide oxygen now the free hydroxyl — this reversible isomerization lets chemists effectively 'walk' an epoxide down a carbon chain under mild basic conditions.
Abbreviations and terms
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- KOtBu
- Potassium tert-butoxide. Strong, bulky alkoxide base – favours elimination over substitution.
- NaOH
- Sodium hydroxide. Strong aqueous base and nucleophile.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- SN2
- Bimolecular nucleophilic substitution. One concerted step with backside attack and inversion; rate = k[RX][Nu].
How it compares with related reactions
| Payne rearrangement | Bamberger rearrangement | Beckmann rearrangement | |
|---|---|---|---|
| Reagents | 2,3-Epoxy alcohol, aqueous NaOH or t-BuOK (catalytic to stoichiometric base) | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) | H₂SO₄, PCl₅, TsCl or PPA |
| Conditions | H₂O/THF or t-BuOH, 0–25 °C | H₂O, 25–60 °C | Acid, 100–130 °C (industrial: oleum) |
| Stereochemistry | Inversion at the attacked carbon (SN2); chirality transferred | None; para-selective | Group anti to OH migrates, with retention |
| Rate / selectivity | Reversible; position set by epoxide substitution; can be trapped by nucleophiles | 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 Payne rearrangement?
2,3-Epoxy alcohol, aqueous NaOH or t-BuOK (catalytic to stoichiometric base). Conditions: H₂O/THF or t-BuOH, 0–25 °C.
Which bonds are formed and which are broken?
Formed: new epoxide C–O; O–H (new alcohol). Broken: old epoxide C–O; O–H (deprotonated).
What is the stereochemical outcome?
Inversion at the attacked carbon (SN2); chirality transferred
What controls the rate or the selectivity?
Reversible; position set by epoxide substitution; can be trapped by nucleophiles
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti