The Beckmann rearrangement involves the use of a strong acid to rearrange an oxime (aldoxime or ketoxime) to an amide. First the oxime hydroxyl group receives a proton from the acid, forming a leaving group. A [1,2]-shift of the R group, which is anti to the leaving group, then promotes its departure, resulting in a nitrilium ion, which reacts with water. Deprotonation of oxygen, protonation of the nitrogen and tautomerization produces the final amide product.

• Forcing conditions (high temperatures (> 130°C) and large amounts of a strong Brӧnsted/Lewis acid) are usually required. Sensitive substrates should be avoided. ^[1]
• Check that the oxime does not isomerize under the reaction conditions, otherwise a mixture of two possible amides is obtained. Reagents p-toluenesulfonyl chloride and phosphorus pentachloride have been found least likely to cause competing isomerization. ^[2]
• Since the hydrogen atom never migrates, N-unsubstituted amides cannot be made. ^[1]

• Reagents: Strong Brӧnsted or Lewis Acid (H₂SO₄)
• Reactant: Oxime
• Product: N-Substituted Amide
• Type of Reaction: Rearrangement

Mechanism

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Related Compounds