Rearrangement — 1,2-shift
Pinacol rearrangement
How the Pinacol 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: Cat. H₂SO₄ or other strong acid. Conditions: Heat, aqueous or neat acid. Bonds formed: C=O; new C–C (migrated group); bonds broken: C–O (leaving H₂O); migrating C–C. Stereochemistry: Migratory aptitude: aryl, H > tertiary alkyl > methyl. Rate and selectivity: Ionisation to the cation is rate-determining.
At a glance
Step 1: Protonation and loss of water
Acid protonates one hydroxyl of a 1,2-diol; loss of water gives a carbocation, stabilized by the neighboring C–OH group.
Step 2: 1,2-Methyl shift
A group on the adjacent carbon migrates with its bonding electrons in a concerted 1,2-shift, directly onto the cationic carbon.
Step 3: Deprotonation gives the ketone
This positions the positive charge next to the remaining oxygen, which loses a proton to form a C=O — driving the whole rearrangement forward to a stable ketone.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
How it compares with related reactions
| Pinacol rearrangement | Meisenheimer rearrangement | Semipinacol rearrangement | |
|---|---|---|---|
| Reagents | Cat. H₂SO₄ or other strong acid | Allylic tertiary amine N-oxide | Vinyl epoxide with an adjacent tertiary alcohol, Lewis acid (BF₃·OEt₂, TiCl₄ or SnCl₄) |
| Conditions | Heat, aqueous or neat acid | Heat, 25–100 °C (neutral) | CH₂Cl₂, –78 → 0 °C |
| Stereochemistry | Migratory aptitude: aryl, H > tertiary alkyl > methyl | Suprafacial [2,3]; chirality transfer from N-oxide to carbon | Stereospecific, concerted anti 1,2-shift; creates a quaternary centre |
| Rate / selectivity | Ionisation to the cation is rate-determining | Concerted, 5-membered envelope TS; unimolecular | Ionisation to the allylic cation is rate-determining; ring strain relief speeds cyclobutanol expansion |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Pinacol rearrangement?
Cat. H₂SO₄ or other strong acid. Conditions: Heat, aqueous or neat acid.
Which bonds are formed and which are broken?
Formed: C=O; new C–C (migrated group). Broken: C–O (leaving H₂O); migrating C–C.
What is the stereochemical outcome?
Migratory aptitude: aryl, H > tertiary alkyl > methyl
What controls the rate or the selectivity?
Ionisation to the cation is rate-determining
Study guides that help with this reaction
- Pericyclic reactions explained: cycloadditions, electrocyclisations and sigmatropic shifts
- Stereochemistry in mechanisms: inversion, retention, syn and anti