Daily answer writing · Tue, 27 Oct, 2026
Beckmann rearrangement: mechanism with acetophenone oxime, and why it is stereospecific
What is the Beckmann rearrangement? Give its mechanism taking acetophenone oxime as the example and name the product. Explain why the rearrangement is said to be stereospecific, and give one industrial application.
Syllabus: Common to CSE Paper II, IFoS Paper II and Geo-Scientist Mains Paper III (organic): molecular rearrangements, migration to electron-deficient nitrogen.
Model answer
Definition. Acid-catalysed conversion of a ketoxime into an N-substituted amide (reagents: conc. $\ce{H2SO4}$, $\ce{PCl5}$, $\ce{SOCl2}$, polyphosphoric acid).
Mechanism (acetophenone oxime)
1. Leaving group made. $\ce{R2C=N-OH + H+ -> R2C=N-OH2+}$ (with $\ce{PCl5}$ the O becomes $\ce{-OPCl4}$).
2. Anti migration (rate step). As water leaves, the phenyl group anti to it shifts from C to N in one concerted step, giving a nitrilium ion:
$$\ce{CH3-C(Ph)=N-OH2+ -> CH3-C#N^+-Ph + H2O}$$
3. Hydration and tautomerism.
$$\ce{CH3-C#N^+-Ph ->[H2O][-H+] CH3-C(OH)=N-Ph -> CH3-CO-NH-Ph}$$
Product: acetanilide (N-phenylethanamide).
Why stereospecific
- Only the group anti to the leaving group migrates, because it can back-side bond to N as the N–O bond breaks. So (E)-acetophenone oxime gives acetanilide; the (Z)-oxime would give N-methylbenzamide, $\ce{PhCONHCH3}$.
- The shift is intramolecular: a chiral migrating group keeps its configuration.
Industrial use
Cyclohexanone oxime $\ce{->[H2SO4]}$ ε-caprolactam, the monomer of nylon-6 (ring expands by one atom).
Conclusion. The Beckmann rearrangement is a concerted anti 1,2-shift to electron-deficient nitrogen, so oxime geometry decides the amide formed.
Where the marks are
- Definition with a reagent (1 mark).
- Mechanism: protonation/activation of OH (1 mark), concerted anti migration to nitrilium ion (2 marks), water attack and tautomerism to amide (1 mark).
- Correct product acetanilide (1 mark).
- Stereospecificity: anti group migrates, (E) vs (Z) give different amides, retention in migrating group (2 marks).
- Caprolactam / nylon-6 (1 mark); conclusion (1 mark).