Rearrangement — oxime to amide
Beckmann rearrangement: cyclohexanone oxime → caprolactam
How the Beckmann 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: H₂SO₄, PCl₅, TsCl or PPA. Conditions: Acid, 100–130 °C (industrial: oleum). Bonds formed: C–N (ring expansion), N–C=O amide; bonds broken: N–O, C–C (migrating). Stereochemistry: Group anti to OH migrates, with retention. Rate and selectivity: Migration is rate-limiting; leaving-group ability of N–OX matters.
At a glance
Step 1: Protonate the oxime OH
Acid activates the oxime's OH as a good leaving group, positioned anti to the carbon group that will migrate.
Step 2: Anti C–C bond migrates, water leaves
As the leaving group departs, the anti carbon group migrates from carbon to nitrogen in a concerted 1,2-shift, generating a nitrilium ion.
Step 3: Water adds, tautomerization to lactam
Water adds to the nitrilium ion and tautomerizes to an amide — here, ring expansion produces caprolactam, the monomer used to manufacture nylon-6.
Abbreviations and terms
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- TsCl
- p-Toluenesulfonyl chloride. Turns an alcohol OH into a good leaving group (tosylate).
- H2SO4
- Sulfuric acid. Strong acid and dehydrating agent.
How it compares with related reactions
| Beckmann rearrangement | Bamberger rearrangement | Boekelheide rearrangement | |
|---|---|---|---|
| Reagents | H₂SO₄, PCl₅, TsCl or PPA | N-Phenylhydroxylamine, dilute H₂SO₄ (aq) | 2-Methylpyridine N-oxide, Ac₂O (or TFAA); then aqueous base |
| Conditions | Acid, 100–130 °C (industrial: oleum) | H₂O, 25–60 °C | Ac₂O, reflux (TFAA works at 25 °C) |
| Stereochemistry | Group anti to OH migrates, with retention | None; para-selective | None (achiral product) |
| Rate / selectivity | Migration is rate-limiting; leaving-group ability of N–OX matters | Loss of water to give the nitrenium ion is rate-limiting | N-acylation fast; [3,3] shift rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Beckmann rearrangement?
H₂SO₄, PCl₅, TsCl or PPA. Conditions: Acid, 100–130 °C (industrial: oleum).
Which bonds are formed and which are broken?
Formed: C–N (ring expansion), N–C=O amide. Broken: N–O, C–C (migrating).
What is the stereochemical outcome?
Group anti to OH migrates, with retention
What controls the rate or the selectivity?
Migration is rate-limiting; leaving-group ability of N–OX matters
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