Overall reaction
Cyclohexanone oxime → (H₂SO₄, heat) ε-caprolactam
An oxime rearranges to an amide; the group anti to OH migrates.
Name reaction · mechanism
Beckmann Rearrangement
In the Beckmann rearrangement a ketoxime is converted into an amide by acid (H₂SO₄, PCl₅, SOCl₂, polyphosphoric acid). The OH is made into a leaving group, and the group anti to it moves from carbon to nitrogen as water leaves, giving a nitrilium ion; water adds and tautomerisation gives the amide.
Mechanism step by step
- Protonation. OH becomes a good leaving group
- Migration. The anti C–C bond moves to N as water leaves
- Water adds. Attack on the nitrilium carbon
- Deprotonation. Imidic acid
- N-Protonation. Tautomerisation begins
- Amide. Loss of the O–H proton gives C=O
Each step is checked electron by electron: the curly arrows give exactly the structures drawn.
Key points
- Stereospecific: the group anti to the leaving group migrates.
- Caprolactam is the monomer of nylon-6 (millions of tonnes a year).
- Other promoters: PCl₅, SOCl₂, TsCl, polyphosphoric acid.
- Aldoximes tend to fragment to nitriles instead.
For JEE and NEET
- Stereospecific: the group anti (trans) to the OH migrates.
- Acetophenone oxime (Ph anti to OH) gives acetanilide.
- Cyclohexanone oxime → caprolactam → nylon-6.
- Intermediate: nitrilium ion, R–C≡N⁺–R′.
FAQ
Which group migrates in the Beckmann rearrangement?
The group anti-periplanar to the leaving group on nitrogen, regardless of its migratory aptitude.
What is the product from acetophenone oxime?
For the usual E-oxime (phenyl anti to OH), N-phenylacetamide (acetanilide).
What is the industrial importance?
Cyclohexanone oxime is rearranged to ε-caprolactam, the monomer of nylon-6.
Draw it in MolDraw
The button above opens the complete figure — overall reaction, numbered mechanism and key points — as ordinary editable structures. Change substituents, recolour atoms, or export a PNG, SVG or chemfig code for notes and slides. The same figure is in the Templates panel under Name reactions.