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Beckmann Rearrangement: Oximes to Amides

By Chemca Editorial Team • Last Updated: January 2026 • 8 min read

The Beckmann Rearrangement is an acid-catalyzed rearrangement of an Oxime to a substituted Amide. It is one of the most important methods for synthesizing amides and lactams (cyclic amides) from aldehydes or ketones.

1. General Reaction

A ketoxime reacts with acidic reagents to form an N-substituted amide.

$$ \underbrace{R-C(=N-OH)-R'}_{\text{Ketoxime}} \xrightarrow{H^+ \text{ or } PCl_5} \underbrace{R-CO-NH-R'}_{\text{N-Substituted Amide}} $$

Conditions:

• Substrate: Aldoxime or Ketoxime (prepared from Aldehyde/Ketone + Hydroxylamine).
• Catalyst: Acidic reagents like $H_2SO_4$, $PCl_5$, $SOCl_2$, $H_3PO_4$, or $BF_3$.
• Product: Amide or Lactam (if cyclic).

2. Detailed Mechanism

The reaction involves the formation of a nitrogen-containing cation and water attack.

Step 1: Protonation

The hydroxyl group of the oxime accepts a proton from the acid, converting $-OH$ into a good leaving group ($-OH_2^+$).

$$ R-C(=N-OH)-R' + H^+ \rightleftharpoons R-C(=N-\overset{+}{O}H_2)-R' $$

Step 2: Migration (Rate Determining Step)

The alkyl group located Anti (Trans) to the leaving group migrates from Carbon to Nitrogen. Simultaneously, the water molecule departs. This forms a linear Nitrilium Ion.

$$ R-C(=N-\overset{+}{O}H_2)-R' \rightarrow [R-C \equiv \overset{+}{N}-R'] + H_2O $$

Note: Only the group anti to the -OH migrates.

Step 3: Hydrolysis and Tautomerization

Water attacks the electrophilic carbon of the nitrilium ion. Deprotonation yields an imidic acid (enol form of amide), which tautomerizes to the stable amide.

$$ R-C \equiv N^+-R' + H_2O \rightarrow R-C(OH)=N-R' \rightleftharpoons R-CO-NH-R' $$

3. Stereochemistry (Crucial Rule)

Anti-Migration Rule

The Beckmann rearrangement is Stereospecific. The group that migrates is strictly the one positioned Anti (Trans) to the hydroxyl group in the oxime. Migratory aptitude series (like in Pinacol rearrangement) applies only if the oxime can isomerize under the reaction conditions.

4. Industrial Application: Caprolactam

The most famous application is the synthesis of $\epsilon$-Caprolactam from Cyclohexanone oxime. Caprolactam is the monomer for Nylon-6.

Cyclohexanone Oxime $\xrightarrow{H_2SO_4}$ Caprolactam (7-membered cyclic amide)

5. Beckmann Fragmentation

If the carbocation formed during migration is very stable (e.g., tertiary), the molecule may undergo fragmentation to form a nitrile and a carbocation instead of an amide. This is known as Beckmann Fragmentation (or abnormal Beckmann rearrangement).

Beckmann Quiz

Test your concepts on Rearrangements. 10 MCQs with explanations.

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